U.S. patent application number 09/753146 was filed with the patent office on 2001-09-13 for method and apparatus for detecting a dry fire condition in a water heater.
Invention is credited to Bradenbaugh, Kenneth A..
Application Number | 20010020615 09/753146 |
Document ID | / |
Family ID | 25029360 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010020615 |
Kind Code |
A1 |
Bradenbaugh, Kenneth A. |
September 13, 2001 |
Method and apparatus for detecting a dry fire condition in a water
heater
Abstract
A water heater having a water tank and multiple electric
resistance heating elements extending into the water tank for
heating water in the tank. The water heater includes a proportional
band temperature controller for conducting electric power to the
electric resistance heating elements in bursts. Each burst of
electric power is followed by a period during which the temperature
controller does not conduct power to the electric resistance
heating element. In one embodiment, each burst of electrical power
lasts for about 95% or less of a cycle comprised of one burst of
electric power followed by the period during which the temperature
controller does not conduct electric power. Further, activation of
the heating elements by the controller is carried out in a
sequential or other timed or controlled fashion to permit uniform
heating of the water in the tank.
Inventors: |
Bradenbaugh, Kenneth A.;
(Concord, OH) |
Correspondence
Address: |
MICHAEL BEST & FRIEDRICH, LLP
100 E WISCONSIN AVENUE
MILWAUKEE
WI
53202
US
|
Family ID: |
25029360 |
Appl. No.: |
09/753146 |
Filed: |
January 2, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09753146 |
Jan 2, 2001 |
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09361825 |
Jul 27, 1999 |
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Current U.S.
Class: |
219/497 ;
219/481; 219/501; 219/505 |
Current CPC
Class: |
H05B 1/0283 20130101;
F24H 1/202 20130101; F24H 9/2021 20130101; H05B 3/82 20130101; F24H
9/2007 20130101; G05D 23/1909 20130101 |
Class at
Publication: |
219/497 ;
219/481; 219/501; 219/505 |
International
Class: |
H05B 001/02 |
Claims
What is claimed is:
1. A water heater, comprising: a tank including an inner surface
and being capable of holding water; a heating element mounted on
the tank and having a thermal surface exposed within the inner
surface, the heating element operable to heat the water; and a
control circuit in communication with the heating element, the
control circuit operable to determine whether the heating element
is in contact with the water.
2. A water heater as set forth in claim 1, wherein the controller
includes a microcontroller and a software program for operating the
microcontroller to determine a first resistance of the heating
element, to determine a second resistance of the heating element
after determining the first resistance, and to compare the first
and second resistances to determine whether the heating element is
in contact with the water.
3. A water heater as set forth in claim 2, wherein the software
operates the microcontroller to compare the first and second
resistances by subtracting the second resistance from the first
resistance element to determine whether the difference is greater
than a threshold difference.
4. A water heater as set forth in claim 1, wherein the control
circuit applies a first signal to the heating element, wherein the
control circuit includes a voltage sensing circuit operable to
produce a second signal responsive to the first signal, and wherein
the control circuit includes a voltage amplifier operable to
amplify the second signal resulting in an amplified voltage
signal.
5. A water heater as set forth in claim 4, wherein the control
circuit includes a current sensor operable to produce a third
signal responsive to the first signal, and wherein the control
circuit includes a current amplifier operable to amplify the third
signal resulting in an amplified current signal.
6. A water heater as set forth in claim 5, wherein the control
circuit calculates the resistance by dividing the amplified voltage
current signal by the amplified current signal.
7. A water heater as set forth in claim 1, wherein the control
circuit includes a data latch used for calibrating the voltage and
current amplifiers.
8. A water heater as set forth in claim 2, wherein the heating
element receives a first voltage, wherein a first current flows
through the heating element, wherein the resistance calculating
circuit includes a voltage sensing circuit capable of sensing a
second voltage proportional to the first voltage, and wherein the
resistance calculating circuit includes a current sensing circuit
capable of sensing a second current proportional to the first
current.
9. A heating unit that heats a liquid to a desired temperature, the
liquid being held in a tank, the heating unit comprising: a heating
element having a thermal surface, the heating element being mounted
on the tank such that the thermal surface is capable of being in
thermal contact with the liquid; and a controller in communication
with heating element the controller determining whether the liquid
is in contact with the heating element.
10. A water heater as set forth in claim 9, wherein the controller
includes a microcontroller and a software program for operating the
microcontroller to determine a first resistance of the heating
element, to determine a second resistance of the heating element
after determining the first resistance, and to compare the first
and second resistances to determine whether the heating element is
in contact with the water.
11. A water heater as set forth in claim 9, wherein the software
operates the microcontroller to compare the first and second
resistances by subtracting the second resistance from the first
resistance element to determine whether the difference is greater
than a threshold difference.
12. A water heater as set forth in claim 9, wherein the control
circuit applies a first signal to the heating element, wherein the
control circuit includes a voltage sensing circuit operable to
produce a second signal responsive to the first signal, and wherein
the control circuit includes a voltage amplifier operable to
amplify the second signal resulting in an amplified voltage
signal.
13. A water heater as set forth in claim 12, wherein the control
circuit includes a current sensor operable to produce a third
signal responsive to the first signal, and wherein the control
circuit includes a current amplifier operable to amplify the third
signal resulting in an amplified current signal.
14. A water heater as set forth in claim 12, wherein the control
circuit calculates the resistance by dividing the amplified voltage
current signal by the amplified current signal.
15. A water heater as set forth in claim 12, wherein the control
circuit includes a data latch used for calibrating the voltage and
current amplifiers.
16. A water heater as set forth in claim 10, wherein the heating
element receives a first voltage, wherein a first current flows
through the heating element, wherein the resistance calculating
circuit includes a voltage sensing circuit capable of sensing a
second voltage proportional to the first voltage, and wherein the
resistance calculating circuit includes a current sensing circuit
capable of sensing a second current proportional to the first
current.
17. A method of determining whether water is surrounding a heating
element of a water heater, the method comprising the acts of:
applying a signal to the heating element; measuring a first
resistance of the heating element; measuring a second resistance of
the heating element after measuring the first resistance; and
determining whether the heating element is surrounded by water by
comparing the first heating resistance with the second
resistance.
18. A method as set forth in claim 17, wherein the act of
determining whether the heating element is surrounded by water
includes the acts of: subtracting the first heating resistance from
the second heating resistance to obtain a difference resistance;
and determining whether the difference resistance is greater than a
threshold resistance.
19. A method as set forth in claim 17, wherein the act of
determining whether the heating element is surrounded by water
includes the acts of: subtracting the second heating resistance
from the first heating resistance to obtain a difference
resistance; and determining whether the difference resistance is
greater than a threshold resistance.
20. A method as set forth in claim 17, wherein the act of measuring
a first resistance includes the acts of: obtaining a second signal
proportional to the first signal; applying the second signal to a
first amplifier resulting in a third signal; sensing a current of
the first signal; obtaining a fourth signal proportional to the
sensed current; applying the fourth signal to a second amplifier
resulting in a fifth signal; and calculating the first resistance
in response to the third and fifth signals.
21. A method as set forth in claim 20, wherein the act of measuring
a first resistance includes: obtaining a peak voltage to the
heating element; obtaining a peak current to the heating element;
dividing the peak voltage by the peak current.
22. A method as set forth in claim 20, wherein the act of measuring
a first resistance further includes: calibrating the first and
second amplifiers.
Description
RELATED APPLICATIONS
[0001] This is a continuation-in-part of U.S. Ser. No. 09/361,825,
filed Aug. 17, 1999.
FIELD OF THE INVENTION
[0002] The present invention relates generally to electrical water
heaters. More particularly, the invention relates to methods and
apparatus for determining whether a heating element of a water
heater is surrounded by water.
BACKGROUND OF THE INVENTION
[0003] A storage-type water heater typically comprises a
permanently enclosed water tank, a cylindrical shell coaxial with
and radially spaced apart from the water tank to form an annular
space between the outer wall of the water tank and the inner wall
of the shell, and insulating material in at least a portion of the
annular space for providing thermal insulation to the water tank.
The water tank has various appurtenances such as inlet, outlet and
drain fittings. Additionally, the water heater is provided with a
water heating and temperature control system. The water heating and
temperature control system includes an
electrical-resistance-heating element. The heating element extends
through a fitting in the wall of the water tank such that the
heating element is inside the tank. The heating element is
connected to an electrical power source outside the water tank.
[0004] One of the problems associated with water heaters having
electrical-resistance-heating elements is when the heating element
"dry fires". The term "dry fire" refers to the heating of a heating
element that is not submerged in water. Usually, a dry fire will
destroy or burn-out the heating element in less than a minute.
Therefore, it would be beneficial to determine whether a heating
element is submerged in water before utilizing that element to heat
the water.
SUMMARY OF THE INVENTION
[0005] Accordingly, the invention provides a water heater having a
water heater controller operable to detect when a heating element
is not submerged in water. The controller of the invention
determines whether a heating element is surrounded by water by
applying a voltage to the element and calculates an initial and
subsequent resistance for the element. As the element heats up, the
resistance of the element increases. If the element is in water,
the element reaches equilibrium temperature very quickly. If the
element is "dry", it gets continuously hotter and reaches a high
temperature in a short period of time. Thus, to determine if the
element is surrounded by water, the controller measures a beginning
and ending resistance, and compares the measured resistance values.
If a difference between the two resistance values is small, then
the controller determines that the element is "wet" (i.e.,
surrounded by water). If a difference between the two resistance
values is large, the controller determines that the element is
"dry" (i.e., not surrounded by water). If the controller determines
an element is "dry", then the controller will not use that element
to heat the water.
[0006] The invention further provides a method of determining
whether water is surrounding a heating element of a water heater.
The method includes the acts of applying a signal to the heating
element, measuring a first resistance of the heating element,
measuring a second resistance of the heating element after
measuring the first resistance, and determining whether the heating
element is surrounded by water by comparing the first heating
resistance with the second resistance.
[0007] Other features and advantages of the invention will become
apparent to those skilled in the art upon review of the following
detailed description, claims and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a sectional view of a water heater embodying the
invention, and showing the arrangement of the temperature
controller of the present invention in relation to other components
of the water heater.
[0009] FIG. 2 is an electrical schematic of a temperature
controller embodying the present invention.
[0010] FIG. 3 is a plot of energy usage data of a mechanical
temperature controller of the prior art and a proportional band
temperature controller of the present invention.
[0011] FIG. 4 is a plot of energy consumption rate data of the
mechanical temperature controller of the prior art and the
proportional band temperature controller of the present
invention.
[0012] FIG. 5 is a sectional view of another water heater embodying
the invention and having multiple heating elements.
[0013] FIG. 6 is a sectional view of yet another water heater
embodying the invention and having multiple heating elements.
[0014] FIG. 7 is a partial sectional view of the water heater shown
in FIG. 6.
[0015] FIG. 8 is a sectional view of a water heater including a
controller embodying the invention.
[0016] FIG. 9 is an enlarged partial view of the controller shown
in FIG. 8.
[0017] FIG. 10 is a schematic representation of the control circuit
shown in FIG. 8.
[0018] FIG. 11 is an electrical schematic of a power supply for the
control circuit shown in FIG. 10.
[0019] FIG. 12 is an electrical schematic of a zero crossing
detector of the control circuit shown in FIG. 10.
[0020] FIG. 13 is an electrical schematic of a low-voltage reset
circuit of the control circuit shown in FIG. 10.
[0021] FIG. 14 is an electrical schematic of a temperature sensing
circuit of the control circuit shown in FIG. 10.
[0022] FIG. 15 is an electrical schematic of a thermostat of the
control circuit shown in FIG. 10.
[0023] FIG. 16(a) and 16(b) are an electrical schematic of portions
of the control circuit depicted in FIG. 10.
[0024] FIG. 17 is an electrical schematic of an oscillator for the
control circuit shown in FIG. 10.
[0025] FIG. 18 is a flowchart representing a method of controlling
the water heater shown in FIG. 8.
[0026] FIG. 19 is a flowchart representing an exemplary method for
performing a test to determine whether a heating element is
submerged.
[0027] FIGS. 20a, 20b, 20c and 20d are portions of a flowchart
representing an exemplary method of performing the acts of
gathering sensor samples, computing water temperature, computing a
thermostat setting, changing operating mode if necessary, setting a
heating cycle state, and setting a heating priority.
[0028] FIG. 21 is a flowchart representing an eight hundred
microsecond interrupt event.
[0029] Before one embodiment of the invention is explained in
detail, it is to be understood that the invention is not limited in
its application to the details of construction and the arrangements
of the components set forth in the following description or
illustrated in the drawings. The invention is capable of other
embodiments and of being practiced or being carried out in various
ways. Also, it is understood that the phraseology and terminology
used herein is for the purpose of description and should not be
regarded as limiting. The use of "including" and "comprising" and
variations thereof herein is meant to encompass the items listed
thereafter and equivalents thereof as well as additional items. The
use of "consisting of" and variations thereof herein is meant to
encompass only the items listed thereafter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0030] As described above, the use of a proportional band
temperature controller in a water heater having an electrical
resistance heating element has the unexpected advantage of heating
water in the water heater to a preselected set point temperature
while consuming less electric power than is consumed when heating
the same amount of water to the same set point temperature in the
same water heater using a mechanical temperature controller of the
prior art.
[0031] An exemplary proportional band temperature controller is an
electronic device which comprises a water temperature sensing
device (thermistor), a temperature set point device (variable
rheostat), a gated thyristor for switching electric power to the
resistance heating element, and a logic circuit for controlling the
thyristor in response to signals from the water temperature sensing
device and the temperature set point device. The logic circuit
receives a voltage input from the water temperature sensing device
and the temperature set point device which indicates the
differential of the water temperature from the set point
temperature. The logic circuit, in response to the voltage inputs
from the water temperature sensing device and the temperature set
point device, signals the gated thyristor. At large temperature
differentials between the water temperature sensing device and the
temperature set point device, the logic circuit signals the gated
thyristor to conduct electricity during a major portion, about 94%,
of each cycle of the AC current, and signals the gated thyristor to
stop conducting electricity for about 6% of each AC cycle. As the
temperature differential between the water and the set point
narrows, the water temperature enters a proportional control band
where the logic circuit begins to exert more control over the gated
thyristor to limit electric power to the resistance heating
element. As the water temperature enters the proportional control
band, the logic circuit establishes a new control cycle period and
signals the thyristor to conduct electric power for 85% of each
cycle and to stop conducting for 15% of each cycle. As the water
temperature gets closer to the set point temperature the logic
circuit signals the thyristor to conduct for less of each cycle
period. When the water temperature reaches the set point
temperature, the logic circuit closes the thyristor and electric
power is not supplied to the resistance heating element until the
water temperature again falls below the set point temperature. To
prevent undue cycling about the set point temperature, the logic
circuit is set to require the water temperature to drop 5.degree.
to 10.degree. F. below the set point temperature before the
thyristor is again signaled to conduct electric power and heat the
water back to the set point temperature.
[0032] This improvement in the efficiency of heating water in the
water heater using a proportional band temperature controller is
not completely understood. In theory, essentially all the
electrical power supplied to a resistance heating element will be
converted to heat, and that heat should be transferred to the water
surrounding the resistance heating element. The same amount of
electric power should heat the same weight of water the same number
of degrees temperature. As shown in the example below, a water
heater having a proportional band temperature controller requires
about 10% less electrical power to heat a tank of water to a
selected set point temperature than the same water heater having a
mechanical temperature controller of the prior art. The improved
accuracy of a proportional band temperature controller for bringing
water to a set point temperature with little overshoot accounts for
some of the improved efficiency over a mechanical temperature
controller, but does not appear to account for all.
[0033] While not wishing to be bound, I suggest that the
improvement in heating efficiency when using a proportional band
temperature controller arises from physical conditions within the
water tank affecting the transfer of heat from the resistance
heating element to the water. A proportional band temperature
controller conducts electric power to the resistance heating
element in short bursts followed by short periods during which
electric power is not conducted until the water in the water tank
reaches a selected set point temperature. The proportional band
temperature controller accurately stops conducting electric power
to the resistance heating element when the water reaches the set
point temperature. On the other hand, a mechanical temperature
controller of the prior art conducts electric power to the
resistance heating element continuously at full power as the water
is heating. When the water reaches the set point temperature
mechanical characteristics of the bimetallic thermocouple may cause
the mechanical temperature controller to overshoot and heat the
water to a temperature above the set point temperature before it
stops conducting electric power to the resistance heating
element.
[0034] A resistance heating element, as is used in domestic water
heaters, heats in a few seconds to a temperature in the range of
800.degree. F. to 900.degree. F. Water, in contact with such a hot
resistance heating element, may vaporize depending on tank
pressure, may form a layer of vapor around the resistance heating
element and reduce the transfer of heat from the resistance heating
element to the water. With a mechanical temperature controller, the
resistance heating element is so heated and remains at a high
temperature until the bimetallic thermocouple cuts off electric
power. Heat from a resistance heating element controlled by a
mechanical temperature controller may be radiated to the wall of
the water tank, or may be transported by vaporization convection
currents to the top of the water tank where the excess heat is
absorbed in the topmost layer of water which is located away from
the temperature sensing bimetallic thermocouple.
[0035] With a proportional band temperature controller, the
resistance heating element is heated during each burst of electric
power and is cooled by contact with the water during periods
between bursts. This cooling of the resistance heating element
between each burst of electric power reduces the temperature to
which the resistance heating element is raised and reduces the
potential for accumulation of vaporization around the hot
resistance heating element. Consequently, heat transfer from the
resistance heating element to the water is increased. Supplying
electric power to a resistance heating element in a water heater in
discrete short bursts, each burst followed by a period with the
electric power shut off, improves the efficiency of heat transfer
from the resistance heating element to the water in the water
heater.
[0036] Proportional band temperature controllers are well known and
widely used in many commercial applications, including to control
water temperature in such appliances as coffee makers. Proportional
band temperature controllers have not, to my knowledge, been used
to control the temperature of a large volume of water in a storage
water heater.
[0037] FIG. 1 of the drawing shows a sectional view of a water
heater 10 comprising a permanently enclosed water tank 1, a shell
12 surrounding water tank 11, and foam insulation 13 filling the
annular space between water tank 11 and shell 12. Water inlet line
or dip tube 14 and water outlet line 15 enter the top of water tank
11. The water inlet line 14 has an inlet opening 22 for adding cold
water near the bottom of water tank 11. Water outlet line 15 has an
outlet opening 24 for withdrawing hot water from near the top of
water tank 11. Resistance heating element 16 extends through the
wall of water tank 11. The proportional band control circuitry in
control box 17 is connected to resistance heating element 16.
Thermistor 18, in contact with the outer wall of water tank 11 for
sensing the temperature of water in water tank 11, is connected to
the logic circuit by electrical wire 19. Electric A.C. power is
supplied to the gated thyristor through line 20. A customizable
operator interface may be mounted on the outside of the water
heater to permit communication with the control box 17 and provides
security protected access for control of the heating element. The
operator interface may be operable to provide direct or remote
control of the heating element.
[0038] FIG. 2 of the drawings is a schematic drawing of a preferred
proportional band temperature control circuit 100 for heating water
in a water heater according to the method of the present invention.
In FIG. 2, resistance heating element 125 is a 4,500 watt heating
element for heating water in a water heater. Temperature set point
device 101 is a variable rheostat for setting the temperature set
point in the range of about 90.degree. F. to 180.degree. F.
Thermistor 102 is for sensing temperature of water in the water
heater. In an alternative embodiment, a plurality of thermistors
could be placed through the tank to measure water temperature at a
plurality of locations. The output of the thermistors could be
averaged.
[0039] Gated thyristor 103 is a TRIAC, manufactured by Motorola,
Inc., for controlling electric power to resistance heating element
125. Logic chip 104 is a proportional band temperature controller
UAA1016A manufactured by Motorola, Inc. Two hundred forty volt
electric power is supplied to the proportional band temperature
control circuit 100 through lines 105 and 106. Opto-electric
coupler 108, as will be described below, is for controlling the
amount the water temperature must decrease from the set point
temperature before the proportional band temperature control
circuit will reactivate.
[0040] A stabilized supply voltage of about -8 Volts is delivered
to the proportional band temperature control circuit from line 106
through Zener diode 107 and resistor 109 into line 110. Voltage
drops through temperature set point device 101 and temperature
sensor 102 produce a signal voltage at point 111. The signal
voltage is proportional to the temperature difference between the
set point temperature and the sensed water temperature. The sensed
voltage is transmitted via line 112 to one leg of a voltage
comparator 113 within logic chip 104. A reference voltage, the
magnitude of which is determined by voltage drops through resistors
114 and 115, is generated at point 116. A saw tooth voltage,
generated in saw tooth generator 118 in logic chip 104, is imposed
upon the reference voltage at point 119. The reference voltage,
modified by the saw tooth voltage passes via line 117 to the second
leg of voltage comparator 113.
[0041] The saw tooth voltage imposed upon the reference voltage
causes the voltage at the second leg of voltage comparator 113 to
vary, in a saw tooth pattern, over a cycle of about 0.85 seconds
from a minimum to a maximum voltage. In voltage comparator 113, the
signal voltage at the first leg is compared to the modified
reference voltage at the second leg. The comparison result is
transmitted via line 120 to logic circuit 121. In logic circuit
121, a signal is generated for passing via line 122, amplifier 123
and line 124 for controlling thyristor 103. When the signal voltage
at the first leg of comparator 113 is greater than the maximum
value of the reference voltage at the second leg of comparator 113,
the signal to thyristor 103 is to conduct and allow electric power
to flow through resistance heating element 125 for heating water in
the water tank. Logic chip 104 is arranged such that the signal in
line 124 causes thyristor 103 to conduct electricity for 96% of
each AC current cycle and stop conducting for 4% of each current
cycle.
[0042] The signal voltage at the first leg of voltage comparator
113 will fall to a value less than the maximum value of the
reference voltage at the second leg of voltage comparator 113 as
the water temperature sensed by temperature sensor 102 approaches
the set point temperature selected on set point temperature device
101. When the signal voltage is in the range between the maximum
value of the reference voltage and the average of the reference
voltage value, the temperature control circuit 100 is within the
proportional band control range. Thus, when the signal voltage is
greater than the value of the reference voltage at the second leg
of the voltage comparator, logic circuit 121 signals amplifier 123
to signal thyristor 103 to conduct electric power to resistance
heating element 125. Then, as the saw tooth voltage causes the
reference voltage at the second leg of voltage comparator to
increase to a value greater than the value of the signal voltage at
the first leg of the voltage comparator, logic circuit 121 signals
amplifier 123 to signal thyristor 103 to stop conducting electric
power to resistance heating element 125. As the signal voltage at
the first leg of voltage comparator approaches closer to the
average value of the reference voltage at the second leg of voltage
comparator 113, thyristor 103 is not conducting for greater
percentages of each cycle of the generated saw tooth voltage. When
the water temperature sensed by temperature sensor 102 is equal to
the set point temperature of temperature set point device 101 the
signal voltage at the first leg of voltage comparator 113 will
equal the average reference voltage value at the second leg of
voltage comparator 113 and logic circuit 121 signals amplifier 123
to turn off thyristor 103, shutting off electric power to
resistance heating element 125. Thyristor 103 remains in the
non-conducting state until the water temperature sensed by
temperature sensor 102 falls below the set point temperature by a
preset amount, as is described below.
[0043] The signal voltage at the first leg of voltage comparator
113 and the reference voltage at the second leg of voltage
comparator 113 must have values which allow logic circuit 121 to
produce a signal to amplifier 123 which will properly control
thyristor 103 to heat the water to the desired temperature.
Temperature set point device 101 is a variable rheostat the
resistance of which may be adjusted manually for changing the set
point temperature. Temperature sensor 102 is a thermistor in which
the resistance decreases as the sensed temperature of the water
increases. The values of resistors 126 and 127 are selected such
that the signal voltage at point 111 will be proportional to the
difference between the set point temperature and the sensed water
temperature. The reference voltage at point 116 is determined by
the value of resistors 114 and 115, and the magnitude of the saw
tooth voltage imposed upon the reference voltage at point 119 is
determined by the values of resistors 128 and 129. The values for
these resistors must be adjusted to accommodate the characteristics
of the particular temperature set point device 101, temperature
sensor 102 and logic chip 104 selected for the proportional band
temperature control circuit 100.
[0044] As described above, opto-electric coupler 108 is included in
proportional band temperature control circuit 100 to prevent undue
cycling of thyristor 103 when the sensed water temperature is at
about the set point temperature. When the sensed water temperature
equals the set point temperature, logic circuit 121 signals
amplifier 123 to cut off thyristor 103 and stop conduction of
electric power to resistance heating element 125. Without
opto-coupler 108, when the sensed water temperature drops a small
amount, for example, less than 1.degree. C., below the set point
temperature, logic circuit 121 will signal amplifier 123 to open
thyristor 103 and conduct electric power to resistance heating
element 125 until the sensed water temperature is again heated to
the set point temperature. This action results in rapidly turning
thyristor 103 off and on, to control the sensed water temperature
as closely as possible to the set point temperature.
[0045] Opto-electric coupler 108, connected electrically across
resistance heating element 125 by lines 130 and 131, operates to
make the sensed temperature appear to be about 5.degree. C. higher
than it actually is when electric current is flowing through
resistance heating element 125. So, when the water temperature
sensed by temperature sensor 102 reaches the set point temperature,
thyristor 103 is stopped from conducting electric current through
resistance heating element 125 and opto-electric coupler 108. With
no current flowing through opto-electric coupler 108, the signal
voltage at point 111 is determined by voltage drop through
temperature sensor 102 and voltage drop through set point device
101, resistor 126, and resistor 127. Resistor 127 produces a
voltage drop equivalent to the voltage drop caused by about a
5.degree. C. temperature change in the sensed temperature.
[0046] Consequently, the sensed temperature appears to be about
5.degree. C. higher than it actually is, and the sensed temperature
must drop an additional 5.degree. C. before the signal voltage at
the first leg of voltage comparator 113 will indicate that the
sensed temperature is below the set point temperature. When voltage
comparator 113 signals logic circuit 121 that the sensed
temperature is below the set point temperature, logic circuit 121
signals amplifier 123 to open thyristor 103 and allow electric
current to flow through resistance heating element 125. With
electric current flowing through resistance heating element 125,
electric current flows through opto-electric coupler 108 via lines
130 and 131. With electric current flowing through opto-electric
coupler 108, resistor 127 is bypassed and the 5.degree. C. bias to
the apparent sensed water temperature is removed. Logic circuit 121
then signals amplifier 123 to open thyristor 103 until the sensed
water temperature again reaches the set point temperature. This
action of opto-electric coupler 108 allows the sensed temperature
to fall about 5.degree. C. below the set point temperature before
thyristor 103 again conducts electric power through resistance
heating element 125, and allows the sensed water temperature to be
heated to the set point temperature before electric power is cut
off from resistance heating element 125. This action prevents
cycling of electric current through resistance heating element 125
when the sensed water temperature is at about the set point
temperature.
[0047] In an alternative embodiment, the temperature control
circuit 100 could include a programmable real time clock wherein
peak or off-peak energy demand periods or vacation operation cycles
could be programmed into the control cycle for the heating element.
Additionally, a pressure sensor, temperature sensor, mineral
deposit sensor and/or sensor for detecting the presence of water
could be added. The control circuit would be programmed to
disconnect power from the water heater and/or the heating element
when predetermined conditions or limits are detected. Further, the
control circuit could include means for automatically adjusting the
set point in response to various conditions such as amount of water
used, or whether it is a peak or off-peak energy demand period.
EXAMPLE
[0048] In a first example, an electric water heater having a 4,500
Watt resistance heating element was operated for heating water from
60.degree. F. to 120.degree. F. using 240 Volt AC current. In a
first run, a commercially available bimetallic thermostat, as
described in the introduction to this application, was used to
sense the water temperature and control electric current to the
resistance heating element. In a second run, the proportional band
temperature control circuit, as shown in FIG. 2 and described in
this application, was used to sense the water temperature and
control flow of electric current to the resistance heating element.
Results of the two comparative runs are shown in FIG. 3 of the
drawings.
[0049] For Run 1, tension on a bimetallic thermostat was adjusted
with a threaded stud such that the bimetallic thermostat would snap
from a flat configuration to a domed configuration at a set point
temperature of 120.degree. F. The bimetallic thermostat was placed
in contact with the outer wall of the water heater water tank at a
position about three inches above the electric resistance heating
element. The bimetallic thermostat was connected, via an insulating
rod, to an electric switch in a line supplying electric power to
the resistance heating element. The water tank was filled with
60.degree. F. water and the electric power connected to the line
supplying the resistance heating element. The bimetallic thermostat
remained in a flat position and the electric switch was closed.
Electric current passed through the resistance heating element at a
rate of 19.7 amperes for about 27 minutes until the water was
heated to about 122.degree. F. The bimetallic thermostat then
snapped into a domed shape, activating the switch to cut off
electric current to the resistance heating element. A graph of
water temperature versus time for this first run is shown in FIG.
3.
[0050] For Run 2, a proportional band temperature control circuit,
as shown in FIG. 2 and described above in this application, was
used. The temperature set point device 101 was calibrated for a set
point of 120.degree. F., and the thermistor temperature sensing
device 102 was attached to the water tank about three inches above
the resistance heating element 125. Thyristor 103 was connected to
resistance heating element 125. The water tank of the water heater
was drained and refilled with 60.degree. F. water and the
proportional band temperature control circuit 100 was connected to
the electric power main. The proportional band temperature control
circuit 100 initially supplied 18.8 amperes of electricity to the
resistance heating element 125, i.e. about 95% of the amperes
supplied by the mechanical thermostat of Run 1. After about four
minutes (at 68.degree. F.), the proportional band temperature
control circuit 100 reduced the electricity supplied to resistance
heating element 125 to 18.6 amperes, i.e. about 91% of the amperes
supplied by the mechanical thermostat of Run 1. After about 21
minutes (at 104.degree. F.), the sensed water temperature entered
the proportional band temperature range and the proportional band
temperature control circuit 100 began to slowly reduce electric
current to resistance heating element 125, until after 27 minutes
the sensed water temperature reached the set point temperature and
the proportional band temperature circuit 100 shut off electric
current to the resistance heating element 125.
[0051] Inspection of FIG. 3 shows that the same amount of water was
heated to substantially the same temperature in the same amount of
time in Run 1 and Run 2. However, in Run 1, 19.7 amperes of
electricity were required and in Run 2, only about 18.6 amperes of
electricity were required over the heating period. That is, heating
water in a water heater equipped with the proportional band
temperature control circuit of the present invention, which
supplies electricity to the resistance heating element 125 in short
bursts followed by short periods with electricity shut off,
requires about 9% less electric power than heating the same amount
of water to the same temperature in the same water heater, but
using a mechanical temperature controller. This is an unexpected
result.
[0052] The pulsing of current to the load by the proportional band
temperature control circuit permits the water temperature to
minutely rise and fall rapidly in response to the applied current.
A brief interruption of current applied to the heater element each
cycle allows for a more efficient transfer of radiation energy to
the water from the heater element.
[0053] As a second example, a test was performed to determine the
actual amount of energy a consumer would use during a typical hot
water heater operating cycle. Referring to FIG. 4, the actual
kilowatt hours (kWh) is plotted versus time for a mechanical
thermostat and an electronic thermostat including proportional band
control logic.
[0054] FIG. 4 illustrates that during a typical heating cycle,
approximately 3% less energy is being used as a direct result of
using the proportional band control logic. It is possible that this
percentage could be increased to approximately 5-5.5% by changing
the conduction angle of the triac's firing quadrants, without
adversely affecting the performance of the water heater.
[0055] Additionally, by limiting the current to the heater element
using proportional band control logic and by supplying the current
to the heater in pulses, gradually coasting to the temperature set
point without overshooting the desired temperature offers an
additional 15% energy reduction.
[0056] The combination of current modulation and preventing the
overshooting of the temperature set points offers the consumer a
combined energy savings of nearly 10% over the cost of operation of
a similar heater using a bimetal mechanical thermostat.
[0057] Overheating water past a reasonable temperature of
125.degree. F.-130.degree. F. generally wastes energy. A typical
two inch thick layer of insulation loses its capacity to
effectively retain heat at temperatures above 130.degree. F. or so.
This energy loss in standby mode is wasteful and potentially causes
the heater to cycle more often than necessary.
[0058] The proportional band control circuit of the present
invention prevents overshooting and allows the water temperature to
drop only 10.degree. F. or so to cycle only the needed difference
to return the water temperature to a desired setpoint.
[0059] An additional advantage to the proportional band control
circuit is its suitability for a flammable vapor environment. For
example, such an environment may exist in a garage, workshop, or
basement storage area wherein solvents, gasoline, propane or other
highly flammable or explosive vapors are present. Mechanical
thermostats and contact type switching devices can arc when an
electrical contact is made or broken, depending on the amount of
current being switched. The electrical arc can ignite a flammable
vapor if the vapor is sufficiently volatile. In contrast, the
proportional band control circuit is totally solid state, has no
moving parts, and would not ignite flammable vapors.
[0060] While implementing proportional band control as described
above is advantageous, even greater heating efficiency can be
achieved in a water heater with multiple, controlled heating
elements. An exemplary water heater 150 with such elements is shown
in FIG. 5, and is the subject of U.S. patent Ser. No. 09/361,825,
entitled PROPORTIONAL BAND TEMPERATURE CONTROL FOR MULTIPLE HEATING
ELEMENTS, filed Aug. 17, 1999, which is incorporated herein by
reference. The water heater 150 shares many common elements with
the water heater 10, and common elements are designated with the
same reference numerals in FIGS. 1 and 5. However, unlike the water
heater 10, the water heater 150 has multiple heating elements 16
and 16'. Heating element 16 is located in the lower portion of the
tank and heating element 16' is located in the upper portion of the
tank. The heating element 16' may be controlled by control
circuitry stored in a control box 17' which receives input from a
thermistor or temperature sensor 18' through a communication link
19', such as an electrical wire. Alternatively, although not shown,
the sensor 18' and heating element 16' could communicate with
control circuitry stored in the control box 17 and just one
controller rather than multiple circuits could be employed.
Communication between the sensor 18' and heating element 16' could
be accomplished through a communication link (not shown) running
physically parallel to line 20. In the case of controlling two
heating elements with a single controller, the control circuitry in
box 17 might take the form of a programmable microprocessor. Of
course, more than two heating elements could be installed in the
water heater 150 and controlled by such a controller, if
desired.
[0061] Regardless of the exact control circuitry used, or whether a
single controller or multiple controllers are implemented, the
heating elements in FIG. 5 are activated sequentially or at some
predetermined frequency or fashion so that heat energy being
transferred to the tank 150 is distributed in a balanced or uniform
manner. Thus, for example, the heating element 16 might be active
for a first period of time T1 during which power is supplied to it
in the pulsed or multiple-burst manner described above.
Subsequently, the element 16' might be activated in a pulsed manner
for a period of time T2. Times T1 and T2 may or may not be of
equivalent lengths, and may or may not overlap one another
depending on the specific heating application and conditions.
Moreover, feedback mechanisms employing the temperature sensors 18
and 18' may be used to trigger activation of the specific heating
elements depending upon the temperature sensed in the upper and
lower portions of the tank 11.
[0062] Whatever specific sequencing is employed, the use of a
proportional band temperature controller to control multiple
elements in a water heater helps to avoid uneven heating of the
water in the tank. Uneven heating generally occurs in conventional
heating systems where the bulk of water heating is accomplished
with a heating element positioned near the bottom of the heater
tank. This configuration often results in the creation of
"stacking," where water that is heated rises to the top of the tank
and becomes super-heated, while non-uniform temperature strata are
formed in the lower portion of the tank. To make matters worse, the
heat accumulation at the top of the tank tends to rapidly dissipate
because the insulation 13 in the tank cannot effectively retain the
high energy heat from the super heated water. With sequential pulse
or burst heating of water as described herein, water in the tank 11
is more uniformly heated. This reduces the occurrence of hot or
cold spots in the strata from the top to the bottom of the tank.
The creation of super heated water is also reduced and efficiency
is increased.
[0063] The sequencing described above may also be combined with
controlled introduction of cold water through an outlet or conduit
155 of a mixing valve 157 positioned in the dip tube 14. The valve
157 may be controlled through a communication link V.sub.I/O
coupled to the control circuitry in box 17' or, alternatively (and
not shown), the circuitry in box 17 when it is configured to
control multiple heating elements. Thus, for example, if super
heating is sensed by the sensor 18' in the upper portion of the
tank, an amount of cold water may be introduced into the top
portion of the tank 11 through the outlet 155 to lower the
temperature of the heated water.
[0064] Yet another water heater 160 embodying the invention is
shown in FIG. 6. The water heater 160 shares many common elements
with the water heaters 10 and 150, and common elements are
designated with the same reference numerals in FIGS. 1, 5 and 6.
For the embodiment shown in FIG. 6, the water tank 160 defines a
volume 165 having an approximately upper two-thirds volume 170 and
an approximately lower one-third volume 175. The inlet opening 22
is disposed in the lower one-third volume 175 and introduces cold
water into the tank 11. The outlet opening 24 is disposed within
the upper two-thirds volume 170.
[0065] As shown in FIG. 6, both heating elements 16 and 16' extend
into the lower one-third volume 175 of the tank 11. The heating
elements 16 and 16' are controlled by control circuitry stored in
control box 17 which receives input from temperature sensors 18 and
18'. Alternatively, the water heater 160 may include more than one
control box, may include more than two heating elements and may
include more than two temperature sensors.
[0066] Similar to what was disclosed for water heater 150, the
heating elements 16 and 16' are activated sequentially or at some
predetermined frequency or fashion so that heat is transferred to
the tank 11 in a balanced or uniform manner. Additionally, heating
elements 16 and 16' are preferably activated by controller 17
utilizing proportion band control techniques.
[0067] In the preferred embodiment of water heater 160, the heating
elements 16 and 16' are arranged in a plane 180 substantially
orthogonal to the longitudinal axis 185 of the tank 11 (i.e., in a
substantially "horizontal" plane) (see FIG. 7). However, the
heating elements 16 and 16' may be place in any other configuration
in the approximately lower one-third volume 175 as long as both
elements are in the approximately lower one-third volume 175 (See
FIG. 6). Also, if additional heating elements are used, they too
are located in the approximately lower one-third volume 210.
[0068] Typically, water heaters of the prior art rarely utilize the
upper heating element. The upper heating element is typically
active only when the water heater is first installed, when the
water heater has not been used for a long period of time, or when a
large amount of hot water has been extracted from the tank in a
short period of time. Except for these rare occurrences, the upper
heating element of the prior art is rarely used. Thus, most of the
water heated over the life of the unit is heated using only the
lower element. The use of only the lower element is energy
inefficient, requires a large period of time for recovery of the
water temperature to set point temperatures, and often requires a
large reserve storage tank of heated water to insure that an
adequate supply of hot water is present when needed. The water
heater 160 overcomes the above-described deficiencies by placing
the second heating element 16' in the approximately lower one-third
volume 175 of the tank 11. Arranging the elements 16 and 16' this
way and controlling the operation of the elements 16 and 16' by
generating sequential pulses having proportional band control
allows the water heater 160 to utilize more efficient water heating
strategies. This results in the elements 16 and 16' having an
improved effective transfer of heat energy to the water.
Furthermore, elements 16 and 16' more evenly distribute watt
densities, which reduces vaporization losses. Consequently, the
water heater 160 has a faster recovery time while using less energy
than conventional heaters of the prior art. Moreover, the water
heater 160 can have a more compact tank size for comparable hot
water demands than the prior art.
[0069] FIG. 8 illustrates another water heater 200 embodying the
invention. Water heater 200 includes a permanently enclosed water
tank 205, a shell 210 surrounding water tank 205, and foam
insulation 210 filling the annular space between the water tank 205
and the shell 210. The water tank 205 has an outer surface 206.
Water inlet line or dip tube 215 and water outlet line 220 enter
the top of water tank 205. The water inlet line 215 has an inlet
opening 225 for adding cold water near the bottom of water tank
205. Water outlet line 220 has an outlet opening 230 for
withdrawing hot water from near the top of water tank 205.
[0070] The water heater 200 further includes a first resistance
heating element 235 and a second resistance heating element 240
extending through the wall of the water tank 205. It is envisioned
that the heating elements 235 and 240 may be placed anywhere within
the tank 205 and may be of any particular shape. However,
preferably, the first and second heating elements 235 and 240 are
in a lower one-third volume of the tank 200, and are in a plane
substantially orthogonal to a longitudinal axis (similar to FIG.
7). In addition, although the invention will be described with two
heating elements 235 and 240, the water heater 200 may include
additional heating elements or may contain just one heating element
235. For example, a commercial tank water heater (as compared to a
residential tank water heater) may contain as many as fifteen
heating elements.
[0071] The water heater 200 includes a first water temperature
sensor 245 and a second water temperature sensor 250. Both water
temperatures sensors 245 and 250 are mounted on the outer surface
206 of water tank 205. The water temperature sensors 245 and 250
are preferably thermistors and are thermodynamically coupled to the
water in the water tank 205. Preferably, the water temperature
sensor 250 is located on a lower half of the tank 205 and the
temperature sensor 245 is located on an upper half of the tank 205.
However, it is envisioned that the water temperature sensors 245
and 250 may be mounted on the same half of the tank 205.
Additionally, the water heater 200 may include additional
temperature sensors or may contain only one temperature sensor
245.
[0072] The water heater 200 may include an ambient or room
temperature sensor 255. The ambient temperature sensor 255 is
located external to the water heater 200, but is located within the
surrounding environment of the water heater 200 and senses the
temperature of the surrounding environment of the water heater 200.
Of course, the water heater 200 may include additional ambient
temperature sensors and may include other sensors (e.g., a water
consistency sensor).
[0073] The water heater 200 includes a proportional band controller
or control unit 260 electrically connected to the first and second
heating elements 235 and 240, the first and second water
temperature sensors 245 and 250, and ambient temperature sensor
255. In general terms, the controller 260 receives a
two-hundred-forty volt alternating current (AC) signal from power
line 265; modulates a first and second proportional band signal
provided to the first and second heating elements 245 and 250,
respectively; receives a first and second water temperature signals
from the first and second temperature sensors 245 and 250; and
receives an ambient temperature signal from ambient sensor 255.
[0074] As shown in FIG. 9, the controller 260 includes a housing
267 having a visual display area 270 and a user entry area 275. The
visual display area 270 includes a plurality of light-emitting
diodes (LEDs). The LEDs include a first element LED2, a second
element LED3, a system LED4, a heat LED5, an alert LED6 and a power
LED7. Power LED7 is preferably a red LED and lights any time the
electronics are active (i.e., "on"). System LED4 is preferably
green and is used to indicate the overall status of the system.
During normal operation, the system LED4 blinks approximately one
blink per second. The fact that the system LED4 is blinking
regularly indicates that the water heater is working properly. Heat
LED5 blinks in unison with the system LED4 when the controller 260
is in a "heating" mode (i.e., the water heater is heating the water
to a desired). First element LED2 and second element LED3 activate
whenever the respective heating elements are active. Alert LED6 and
heating LED5 are in the same package. Alert LED6 works in
conjunction with the system LED4 to indicate the status of the
water heater 200.
[0075] During normal operation, if the controller 260 is in a
"Stand-by" mode (i.e., the temperature of the water is equal to or
greater than the desired water temperature), only the system LED4
blinks. If the controller 260 is in the heating mode, the
controller 260 blinks the system LED4 and the heating LED5 in
unison. If for any reason there is an error state, then the heating
LED5 changes to the Alert LED6, which is red. During the error
state, the system LED4 blinks an error code indicating the type of
error. Of course, other LEDs can be added, and any of the disclosed
LEDs can be removed or modified. Additionally, an audible speaker
can be included to provide audible indication, or the information
provided by the LEDs can be communicated by other visual indicators
(e.g., a liquid crystal display).
[0076] The user entry area 280 includes an entry dial 283 for a
user to enter a desired water temperature. The entry dial 283
includes an off position (i.e., the water heater 200 is "off"), a
vacation position, and a plurality of positions between a low or
cold water temperature and a high or hot water temperature. If the
entry dial 285 is in the vacation position, then the controller is
in a "vacation" mode. The "vacation" mode heats the water to a
preset temperature lower than the normal temperature range of the
water heater. Alternatively, the user entry area 275 may include
other possible devices for entering a desired water temperature
state including a plurality of push buttons with a digital LCD
display. Of course, the visual display area 275 and the user entry
area 280 may be mounted in a second control box located remotely
from the water heater 20 (i.e., not mounted on the water heater
20). The second control box in communication with the controller
260 either through a hard-wired connection, or through RF or other
appropriate communication scheme.
[0077] The controller 260 includes a control circuit 285, which is
schematically represented in FIG. 10. In general terms, the control
circuit 285 includes a power supply 290, a zero crossing detector
295, a low-voltage reset circuit 300, a temperature sensing circuit
305, a thermostat circuit 310, an LED control circuit 312, a
microcontroller U1, a memory unit 315, a first driving circuit 320,
a second driving circuit 325, and a dry fire circuit 330.
[0078] As shown in FIG. 10, the power supply 290 receives a
high-voltage AC signal (e.g., AcIn=240 VAC) from line 260 (FIG. 8)
and creates a low voltage AC signal (e.g., AcOut=9 VAC), an
unregulated direct current (DC) signal (e.g., V-SNS=5 VDC), and a
regulated direct-current signal (e.g., Vcc=5 VDC). An exemplary
power supply 290 is shown in greater detail in FIG. 11.
[0079] As shown in FIG. 11, the power supply 290 includes a
transformer T2 having a primary coil and a secondary coil for
transforming the high-voltage AC signal (AcIn) to the low-voltage
AC signal (AcOut). The resulting low-voltage AC signal (AcOut) is
provided to the zero-crossing detector 295 (FIG. 10) and to a
switch S1, which is a single-throw, single pole (SPST) switch
connected to the high side of the secondary coil. When the switch
S1 is closed, the control circuit 285 is active.
[0080] The power supply further includes a full-wave bridge
rectifier D8, a capacitor C26, a zener diode D9, a voltage
regulator U9, and capacitors CU1, CU2, CU4, CU7 and CU8. The bridge
rectifier D8 rectifies the low-voltage AC signal (AcOut) and the
capacitor C26 filters the signal resulting in the unregulated DC
signal (VSNS). The zener diode D9 caps the unregulated DC signal
(VSNS) and protects the input of the voltage regulator U9 from
short-term, over-voltage transients. The voltage regulator U9
regulates the voltage to a Vcc signal of five volts and each of the
capacitors CU1, CU2, CU4, CU7 and CU8 on voltage regulator U9 are
decoupling capacitors dedicated to a respective integrated circuit.
For example, capacitor CU1 is a decoupling capacitor for integrated
circuit U1.
[0081] Referring back to FIG. 10, the power supply 290 provides the
low voltage AC signal (AcOut) to zero-crossing detector 295. An
exemplary zero-crossing detector 295 is shown in greater detail in
FIG. 12. Zero-crossing detector 295 provides an output signal
(ZeroCross) which indicates each time the detector 295 detects that
the low voltage signal (AcOut) has changed plurality. The
zero-crossing detector 295 includes resistors R55, R61 and R53,
capacitor C21, diode D1, and transistor Q8. The resistor R55
receives the low-voltage AC signal (AcOut). The diode D1, capacitor
C21, and resistor R61 are connected in parallel with one end
connected to resistor R55 and the base of transistor Q8 and the
other end connected to the emitter of transistor Q8. Resistor R53
has one end connected to Vcc and the other end connected to the
collector of transistor Q8. The zero-crossing signal (ZeroCross) is
generated at the collector of transistor Q8. As the AC voltage
changes polarity, Q8 goes back and forth between the off state and
saturation, generating a series of pulses having a front edge. The
front edge of each pulse corresponds to a zero crossing.
[0082] Referring back to FIG. 10, the control circuit 285 includes
a low-voltage reset circuit 300. An exemplary low-voltage reset
circuit 300 is shown in greater detail in FIG. 13. The low voltage
reset circuit includes an integrated circuit U3, which is
preferably a Motorola MC34064P-5 (although other circuits may be
used) connected to a capacitor C18, and resistors R45 and R46. The
integrated circuit U3 provides an under voltage reset protection
signal to the microcontroller U1. In the event that power should
fail or "brown" out, integrated circuit U3 causes the
microcontroller U1 to reset. Preferably, this occurs as soon as the
requested DC signal drops below four and one-half volts. The
low-voltage reset circuit ensures that the control circuit 285
safely operates and does not malfunction due to low-line power.
[0083] Referring back to FIG. 10, the control circuit 285 includes
a temperature sensing circuit 305. The temperature sensing circuit
305 in combination with first and second water temperature sensors
245 and 250 transmits a water temperature for the water heater 200
to the microcontroller. As shown in greater detail in FIG. 14, the
temperature sensing circuit includes resistors R70 and R71, and
thermistors RT1 and RT2, which have a negative temperature
coefficient. Resistor R70 and thermistor RT1 form a first voltage
divider resulting in a first temperature signal (First-Sensor), and
resistor R71 and thermistor RT2 form a second voltage divider
resulting in a second temperature signal (Second-Sensor). Since the
first and second voltage dividers are preferably the same, only the
first voltage divider will be discussed in detail. As the
temperature on the outside of the water tank 205 increases, the
resistance in the thermistor RT1 decreases causing the output
voltage (First-Sensor) to increase. The voltage (First-Sensor) is
read by an analog-to-digital (A/D) converter in microcontroller U1
resulting in an eight-bit number. The eight-bit number is used as
an index to a lookup table that has a plurality of corresponding
sensed temperatures. Based on the eight-bit number, a sensed
temperature results.
[0084] As the water inside the tank 205 increases in temperature,
there is an increasing error in what the temperature sensor 245 or
250 senses. That is, the thermal conductive path from the water
through the material of the water tank 205 has a lag time
differential. To correct this, the sensed temperature value read
from the lookup table is "corrected" by a linear equation. The
corrected temperature is used in making water heating decisions by
the microcontroller U1.
[0085] Referring back to FIG. 10, the control circuit includes a
thermostat 310. As shown in greater detail in FIG. 15, the
thermostat is a potentiometer R65 wired as a voltage divider and
having a resistance range (e.g., 20 kOhms). The output signal of
the voltage divider (Thermostat) is converted to an eight-bit
number by the microcontroller U1 and then scaled to produce a
set-point temperature value. The set-point temperature value is the
temperature to which the water will be heated.
[0086] Referring back to FIG. 10, the control circuit 285 includes
an LED control circuit 312. The LED control circuit 312 controls
the activation of the light-emitting diodes LED2, LED3, LED4, LED5,
LED6 and LED7. As shown in greater detail in FIG. 16(a), the LED
controller 312 includes resistors R56, R57, R58, R59, R60, R47,
R48, R49, R50, R51 and R52, and transistors Q3, Q4, Q5, Q6 and Q7.
When switch S1 (FIG. 11) is closed, the power supply 290 generates
a regulated low-voltage DC signal (Vcc) that is provided to LED7
and resistor R52. The provided low-voltage regulated DC signal
(Vcc) lights LED7. For controlling LED2, LED3, LED4, LED5 and LED6,
a five-bit signal is provided to resistors R56, R57, R58, R59 and
R60. If any of the bits are high, a low-voltage DC signal is
provided to the respective resistor R56, R57, R58, R59 or R60
resulting in a base current sufficient to allow current flow
through the respective transistor Q3, Q4, Q5, Q6 or Q7. The current
flows from Vcc through the transistor Q3, Q4, Q5, Q6 or Q7, through
the respective light emitting diode LED2, LED3, LED4, LED5 or LED6,
to ground.
[0087] Referring back to FIG. 10, the control circuit includes a
microcontroller or processor U1 and a memory unit 315. The
microcontroller U1, which is also shown in FIG. 16(a), is
preferably a 28-pin Motorola MC68HC705P6A (although other
microcontrollers may be used). The microcontroller U1 includes an
eight-bit input/output port (pins 3-10), a three-bit serial
interface (pins 11-13), a four-bit analog to digital converter
(pins 15-19), memory for storing a software program that operates
the microcontroller, and two pins (pins 26 and 27) for receiving a
signal from an oscillator 317 (FIG. 17). The memory unit 315
includes a two hundred fifty six byte Electrically Erasable
Programmable Read Only Memory (EEPROM) chip U4. The EEPROM U4 is
used to store configuration data, such as water heater construction
specifics (e.g., operating voltage, tank water capacity,
resistances of various elements, etc.), user usage pattern data,
element type data, and other related data. With the EEPROM data and
real-time sensory data (e.g., the sensed temperature of the first
and second water temperature sensors 245 and 250), the micro
controller U1 implements a software program to control the heating
elements to heat and maintain water temperature. In addition, the
software program includes at least one subroutine to determine
whether water is surrounding each heating element.
[0088] Referring back to FIG. 10, the control circuit includes a
first driving circuit 320 and a second driving circuit 325 that
control the power being provided to the first and second heating
elements 235 and 240, respectively. The driving circuits are
identical and, thus, only driving circuit 320 will be discussed in
detail. As shown in FIG. 16(b), the first driving circuit 320
includes resistors R66 and R86 a triac Q1, and an opto coupled
zero-cross triac driver U5. The triac driver U5 is gate driven as
determined by gate pulses being received from the output of the
microcontroller U1. A pulse train is generated by the
microcontroller U1, which determines the power levels being
delivered to the heating element 235 (FIG. 10). For example, the
microcontroller U1 may provide a pulse train to the triac driver U5
resulting in a sixty-six percent power transfer (i.e., sixty-six
percent of the available power is transferred to the heating
element), or may provide a pulse train to the triac driver U5
resulting in a forty percent power transfer. The triac driver U5 is
coupled to the zero-crossing detector 295 to insure that the triac
turns completely off when the set point temperature is reached.
Without the use of driver U5, the triac Q1 could remain partially
open in a conduction state and potentially effect the reliability
of the control circuit 285.
[0089] Referring back to FIG. 10, the control circuit includes a
dry fire circuit 330. As shown in greater detail in FIGS. 16(a) and
16(b), the dry fire circuit 330 includes data latch U2 (16(a)), a
first resistor ladder 335 (16(a)), a second resistor ladder 340
(16(a)), a voltage sensing amplifier 345 (16(b)), a current sensing
amplifier 350 (16(b)), resistors R90, R91, R92, R97, R98 and R100
(all in 16(b)), transistors Q9 and Q10 (both in 16(b)), a current
sensor T1 (16(b)), and a resistor R44 (16(b)). The data latch U2 is
preferably a Motorola 74HC374 data latch (other data latches may be
used) and is used to hold a five-bit data word that controls the
first and second resistor ladders 335 and 340. The first resistor
ladder 335 generates a voltage that is used as a reference by the
voltage sensing amplifier 345. Once this reference voltage has been
set or calibrated, the data latch U2 is used to control the second
resistor ladder 340 to generate a voltage that is used as a
reference by the current sensing amplifier 350. The latch also
holds three additional data bits. The first data bit (bit 7),
controls one of the display LEDs; the second data bit (bit 6),
selects the EEPROM; and the third data bit (bit 5), enables
communication with off-board testing equipment. The current sensor
T1 and the resistor R44 create a voltage that is proportional to
the current being provided to the heating elements. Transistors Q9
and Q10 select which amplifier is currently providing a signal to
the microcontroller U1.
[0090] The basis for the "DryFire" test is the measurement of the
peak voltage and peak current on an "almost" cycle by cycle basis.
The reason that the measurement is not exactly cycle-by-cycle is
that the voltage is measured after it has been rectified and
filtered. Changes in the AC line voltage manifest as changes in the
rectified DC voltage. Because of the time constant of the capacitor
C26, with the resistance in the secondary windings of the power
transformer, voltage and current samples are taken on a
cycle-by-cycle basis and stored in a buffer. When the buffer is
full, the voltage samples are examined to determine whether the
voltage was stable during the time period it took to fill the
buffers. If the variance is within acceptable limits, the voltage
and current samples are average and a simple resistance calculation
is performed (i.e., R=V/I).
[0091] When the manufacturer assembles the water heater 200, the
manufacturer programs into the memory unit 315 the components used
for assembly of the water heater 200, the capacity of the water
tank 205, and/or product information about particular components of
the water heater 200. For example, the manufacturer may program one
or more tank characteristics and/or one or more element
characteristics into the memory unit. The tank characteristics may
include, but are not limited to, tank diameter, tank height, tank
storage capacity, etc. The tank characteristics determine heating
convection current flow patterns within the tank 205 that create
different temperature water strata layers in the tank 205. The
element characteristics may include, but are not limited to, number
of elements, element type, voltage of an element, physical location
of an element (e.g., upper and lower, or side-by-side), element
watt density, etc. The element characteristics help to provide
information on how effectively the elements 235 and 240 will heat
the water.
[0092] In addition, some of the tank or element characteristics can
be determined by the microcontroller U1. For example, the
microcontroller can calculate an element wattage for a particular
element by applying a voltage to the element and calculating a
resistance for the element over time.
[0093] Preferably, all of the water heater tank characteristics and
element characteristics are programmed into the memory unit 315.
Based on the variables and characteristics, the microcontroller U1
obtains from a lookup table a code specific to the water heater
200. The software of the microcontroller U1 creates a heating
strategy for the water heater 200 based in part on the water heater
code (discussed below). The microcontroller U1 can update the water
heater code if it senses that an element has been replaced or if a
repairperson reprograms the data stored in the memory unit 315.
Additionally, although the manufacturer programs each variable or
characteristic into the memory unit 315, it is envisioned that the
manufacturer can directly program the code into the memory unit
315.
[0094] Because there are a diversity of tank characteristics and
elements used in the manufacture and construction of electric water
heaters, one heating strategy alone is unable to account for the
numerous constructions. Instead, the software assigns a code to the
water heater 200 based on the variables and characteristics of the
water heater 200. The variables and characteristics define a water
heater signature and, when used with a water heater usage pattern,
create a more reliable effective and energy efficient water
heater.
[0095] In operation of water heater 200 and referring now to FIG.
18, a user "turns-on" the water heater 200 (act 500) by turning the
thermostat 310 clockwise from the off position. This closes switch
S1. Upon closing switch S1, the power supply 290 generates the
low-voltage AC signal (AcOut), the unrectified DC signal (V-SNS)
and the rectified DC signal (Vcc). Once the power source generates
a Vcc greater than four and one-half volts, the low voltage reset
300 brings the microcontroller U1 out of reset. If at any time the
voltage drops below four and one-half volts (e.g., a user turns the
system off, a "black-out" occurs, or a "brown-out" occurs), the low
voltage reset 300 provides a signal to the microcontroller U1
resetting the microcontroller U1.
[0096] At act 505, after the microcontroller U1 comes out of reset,
the software initializes the microcontroller U1. The software
resets all variables to their default values, and resets all
outputs to their respective default states.
[0097] At act 510, the microcontroller performs a "DryFire" test.
The term "DryFire" refers to the heating of a heating element 235
or 240 that is not submerged in water. Usually, a "Dry Fire" will
destroy or burn-out the heating element 235 or 240 in less than a
minute. The control circuit 285 performs the "DryFire" test to
determine whether the heating element is surrounded by water.
[0098] In general terms, the control circuit 285 performs the
"DryFire" test by measuring the peak current and the peak voltage
being applied to each heating element 235 and 240 and making a
resistance calculation based on the measurement. For example, by
applying a voltage to one of the heating elements 235 or 240 for a
specific period of time and measuring the resistance at the
beginning and end of the test period, the status of the heating
element 235 or 240 can be determined. As the element 235 or 240
heats up, its resistance increases. If the element is in water, the
element reaches equilibrium (i.e., a steady temperature and
resistance), very quickly. Conversely, if the element 235 or 240 is
"dry", it continues heating and reaches high temperatures (and
resistances) in a very short time. At the end of the test, the
beginning and ending resistances are compared. For a "wet" element,
the difference between the beginning and ending resistances is
small, while for a "dry" element, the difference between the
beginning and ending resistances is many times larger than when the
element is wet.
[0099] In addition, by varying the length of the DryFire test, the
watt density of the heating element 235 or 240 can be accurately
measured. Based on the watt density, the microcontroller U1 can
update the water heater code.
[0100] An exemplary method for performing the DryFire test is shown
in FIG. 19. At act 605, the microcontroller U1 deactivates all the
LEDs during the DryFire test. Deactivating the LEDs ensures that
the blinking of the LEDs does not affect the test. At act 610, the
software sets an element number indicating the first heating
element 235 is being tested. At act 615, the software sets the
operating mode for the microcontroller U1 to a DryFire mode which
informs all subroutines that the microcontroller U1 is performing a
DryFire test. At act 620, the software clears all DryFire error
flags. The DryFire error flags indicate whether the most recent
DryFire test (if one occurred) resulted in an error. For example,
if the previous DryFire test resulted in an error flag
corresponding to the first element being "dry", then the
microcontroller U1 resets the error flag pending the results of the
current test.
[0101] At act 625, the microcontroller U1 calibrates the voltage
amplifier 345. Before any voltage samples can be taken for DryFire
calculations, the voltage amplifier 345 must be calibrated using a
variable reference voltage generated by data latch U2 and resistor
ladder 335. To accomplish this calibration, the microcontroller U1
first selects the output of the voltage sensing circuit by driving
Q10 into saturation (Q9 is off). The reference voltage (V-REF) is
then set to its highest value. Next, the reference voltage (V-REF)
is incrementally reduced until the output of the voltage amplifier
(Dry-Out) reaches a predetermined value. The reference voltage is
then left at this value.
[0102] For example, V-SNS is a non-regulated DC signal having a
steady-state component and a small "alternating current" component.
Any increases or decreases in the signal being provided to the
transformer (AcIn) will reflect in the small "AC" component of
V-SNS signal. In order for the microcontroller U1 to notice any
changes of significance, the voltage amplifier 345 amplifies small
"AC" component changes. If, for example, the steady state is 2.0
volts, any reference voltage (V-REF) feeding resistor R88 (FIG.
16(b)) above 2.0 volts will result in no amplification taking place
and the output of the amplifier will be zero. If the reference
voltage (V-REF) is below 2.0 volts, amplification will take place.
The reference voltage (V-REF) is adjusted so the output of U7B is
somewhere in the middle of its output swing (e.g., 0-3.5 volts).
The microcontroller U1 continues to reduce the reference voltage
(V-REF) in steps until a desired output is reached (e.g., reference
voltage is equal to 1.5 volts). Thus, any changes in the line
voltage are exaggerated by a factor equal to the gain of U7B.
[0103] At act 630, microcontroller U1 calibrates the current
amplifier 350. As with the voltage amplifier 345, the second stage,
U8B (FIG. 16(b)), must be calibrated before sampling can begin. The
current sensing circuit is selected by driving Q9 into saturation
(Q10 is off) and then incrementally adjusting the reference voltage
(I-REF) similar to the reference voltage (V-REF).
[0104] At act 635, the software determines whether the voltage and
current amplifiers 345 and 350 were properly calibrated. If there
was an error in the calibration, then the software sets a
calibration error flag(s) (act 640) to a positive result and
proceeds to act 660. If the calibration did not result in any
errors, then the microcontroller U1 proceeds to act 645.
[0105] At act 645, the microcontroller U1 performs a DryFire test
for the first element 235. For the test, instantaneous voltages and
currents are measured at their peak values. This is accomplished by
sampling the signal from the voltage and current amplifying
circuits 345 and 350 (Dry-Out) relative to a zero crossing of the
low-voltage AC signal (AcOut). At the appropriate zero crossing, a
timer is started for each of the amplifying circuits 340 and 350. A
time-out variable is used to take the voltage or current samples at
a predetermined time period with respect to the zero crossing when
the voltage and current waveforms are at their peak. The
instantaneous voltage and current samples are each loaded into
separate buffers within the microcontroller U1. When the buffers
are full, the data is analyzed to determine if the line voltage has
been stable during the sampling period. If the sampled voltage is
stable, an average voltage and current is computed, and a
resistance calculation is made. Calculations continue in this
manner for the duration of the DryFire test. At the end of the
test, the beginning and ending resistance values are subtracted to
find out how much the resistance has changed over the course of the
test. The basis of the test is not the actual value of resistance
(which is different for each type of heating element), but the
difference in resistances from the beginning of the test to the end
of the test.
[0106] At act 650, the microcontroller U1 determines whether the
first element 235 is dry. If the calculated resistance difference
is greater than a set resistance change value (which may vary
depending upon the heating element used) then the microcontroller
U1 determines that the element is not surrounded by water (i.e.,
"dry") and proceeds to act 655. If the microcontroller U1
determines that the calculated resistance change is equal to or
less than a set resistance change value, then the microcontroller
U1 determines that the element is surrounded by water and proceeds
to act 660.
[0107] At act 655, the software sets a first element error flag to
a positive result. A positive first element error flag informs
subsequent subroutines that the first element 235 is not surrounded
by water. Consequently, later subroutines will not use this element
to heat the water. The microcontroller U1 will also set a ReCheck
timer to 180 minutes. The ReCheck timer will decrease in time until
it reaches zero minutes. When the ReCheck timer reaches zero, the
microcontroller U1 will perform another DryFire test on that
element.
[0108] At act 660, the microcontroller U1 sets the element number
to the second element. At act 665, the microcontroller U1 repeats
acts 625, 630, 635, 640, 645, 650 and 655 for the second element to
determine whether the second element is dry. If the microcontroller
U2 determines the second element is dry, it will set a second
element error flag to a positive result. Of course, if the water
heater includes more than two heating elements, then the
microcontroller U2 performs a dry test for the remaining elements.
Additionally, if the water heater contains only one heating
element, then the microcontroller U2 will not perform acts 660 or
665.
[0109] Referring back to FIG. 18, at act 515, the software
determines whether a "ReCheck" timeout is greater than zero. The
ReCheck timeout is a timer (e.g., twenty milliseconds) used by the
software to inform the software when to sample the temperature
sensors 245, 250 and 255, and create or modify a heating strategy
for heating the water contained within the water heater 200. If the
ReCheck timeout is greater than zero, then the software proceeds to
act 520. If the ReCheck timeout is less than or equal to zero, then
the software proceeds to act 525.
[0110] At act 520, the microcontroller U1 "blinks" the system LED4,
the heat LED5 and the alert LED6. That is, the software performs a
subroutine that activates appropriate LEDs depending on the mode
the software is in or if an error flag has occurred. For example,
during normal operations, microcontroller 305 generates a signal
resulting in the system LED4 to blink on and off. If the software
is in a heating mode (discussed below), then the heat LED5 blinks
in unison with the system LED4. If the software has a positive
error flag, the alert LED6 works in conjunction with the system
LED4 to indicate the status of the water heater 200 to an operator
or repairperson.
[0111] If the ReCheck timeout is less than or equal to zero, then
the microcontroller U1 proceeds to Act 525. In general terms, the
microcontroller U1 samples temperature sensor samples (act 525),
computes a water temperature (act 530), computes the thermostat
setting (act 535), establishes an operating mode (act 540), sets a
heating cycle state (act 545), and sets a heating priority (act
550). An exemplary method implementing acts 525, 530, 535, 540, 545
and 550 is shown in FIG. 18. In addition, the microcontroller U1
stores data for creating a usage history (act 555) and blinks the
LEDs (560).
[0112] At act 705 (FIG. 20(a)), the microcontroller U1 samples
temperature sensor 245 and loads a resulting first voltage into the
software for processing. At act 710, the microcontroller U1 samples
temperature sensor 250 and loads a resulting second voltage into
the software for processing. At act 715, the microcontroller U1
converts the first and second sampled voltages to a first and
second sensed temperatures, respectively, using a temperature
lookup table. The look-up table contains a plurality of voltage
ranges having a respective associated temperature. For example, if
the first temperatures sensor generates a 2.1 volt signal, the
associated temperature may be 110 degrees fahrenheit. The look-up
table can vary depending on the sensor used. After obtaining the
first and second sensed temperatures, the software modifies the
sensed temperatures to take into account any lag time in obtaining
the temperature. That is, as the water inside the tank 205
increases in temperature, there is an increasing error in what the
temperature sensor 245 or 250 senses. The thermal conductive path
from the water through the material of the water tank 205 has a lag
time differential. To correct this, the temperature values read
from the lookup table are "corrected" for the lag. The corrected
first and second temperatures are used in making water heating
decisions by the software.
[0113] At act 720, the microcontroller U1 loads or samples a signal
from the thermostat 310. If the microcontroller U1 determines that
the thermostat voltage corresponds to the thermostat being in off
position (act 725), then the software sets an operating mode equal
to an off state (act 730) and returns to act 555 of FIG. 18. For
example, if the thermostat voltage is less than 0.1 volts, then the
software determines the thermostat is in an off position and turns
off the controller 260. If the thermostat voltage is greater than a
voltage corresponding to an off position (act 725), then the
software proceeds to act 735.
[0114] At act 735, the software determines whether the operating
mode was previously set to off (i.e., the system was just turned
on). If the operating mode was previously off, then the software
changes the operating mode to "stand-by" (act 740). As will be
discussed in more detail below, when the water heater 200 is in a
stand-by mode, the controller 260 is not increasing the temperature
of the water. If the operating mode is in a mode other than the off
operating mode, then the software proceeds to act 745.
[0115] At act 745, the software compares the thermostat voltage
with a set voltage representing the vacation position of the
thermostat. For example, if the thermostat voltage is less than 0.7
volts, then the software determines that the thermostat is set to
the vacation position and proceeds to act 750. If the thermostat
voltage is greater than 0.7 volts, then the software determines
that a user has set the water heater to a desired temperature and
proceeds to act 755.
[0116] At act 750, the software sets the set point temperature
equal to a vacation temperature (e.g., 90 degrees Fahrenheit). The
vacation temperature may be a manufacturer-determined value, or may
be preset by a user. After setting the set-point temperature, the
software proceeds to act 760 (FIG. 20(b)).
[0117] At act 755 (FIG. 20(b), the software computes a set point
temperature based on the sampled thermostat voltage. The
microcontroller U1 preferably uses a second lookup table, but may
alternatively use a formula based on the input voltage.
[0118] At act 760, the software computes a heater-on temperature.
The heater-on temperature is the temperature at which one or more
elements receive a power signal. The heater-on temperature is the
set-point temperature minus a hysteresis temperature. The
hysterisis temperature is the number of degrees fahrenheit (e.g.,
10 degrees fahrenheit) that the water temperature drops below the
set-point temperature before heating occurs. Thus, by calculating a
heater-on temperature, the microcontroller U1 avoids "under
cycling".
[0119] At act 765, the software determines whether the operating
mode is in a "stand-by" mode or a "heating" mode. If the operating
mode is set to stand-by, the software proceeds to act 770. If the
operating mode is set to heating, then the software proceeds to act
775.
[0120] At act 770, the software determines whether the lower-tank
temperature (from temperature sensor 250) is less than or equal to
the heater-on temperature. If the lower-tank temperature is less
than or equal to the heater-on temperature, then the software
determines that the water should be heated and proceeds to act 780.
If the lower-tank temperature is greater than the heater-on
temperature, then the software determines that the water should not
be heated and proceeds to act 800.
[0121] At act 780, the software sets the operating mode to the
heating made indicating that the water should be heated. After
setting the operating mode to heating, the software resets all
operating state variables and timeouts for another heating cycle
(act 785). For example, the software resets the ReCheck timeout
(e.g., to twenty milliseconds.) If, at act 765, the software
determines the operating mode is set to heating, the software
proceeds to act 775. At act 775, the software determines whether
the lower tank temperature is greater than or equal to the set
point temperature. If the lower tank temperature is greater than or
equal to the set point temperature, then the software determines
that the water should continue to be heated, and therefore stays in
the heating mode and proceeds to act 800. If the lower tank
temperature is less then the set point temperature, than the
software determines that the water has been properly heated and
proceeds to act 785.
[0122] At act 785, the software changes the operating mode to
stand-by (i.e., indicating that the water temperature no longer
should increase). At act 790, the software determines whether the
first heating element 235 is surrounded by water (this is assuming
the first element is above the second 235). If the first heating
element 235 is not surrounded by water (i.e., the element is dry),
then the software sets the ReCheck timeout variable to two minutes
(act 795). By changing the length of the ReCheck timeout variable,
the software allows the water tank to fill with more water before
heating with the first element. Of course, the amount of time the
software sets the ReCheck timeout variable to can vary, and a
specific value is not required for purposes of the invention to
work. If the first element does have water surrounding the element
(i.e., a wet state has resulted), then the software proceeds to act
800.
[0123] At act 800 (see FIG. 20(c)), the software determines whether
a temperature slope calculation period has elapsed. If the period
has elapsed, then the software resets the timer and computes a
temperature slope (act 805). Computing the temperature slope allows
the determination of whether a water draw is occurring. At regular
intervals (e.g., 90 seconds), the most recent temperature sample of
the tank is compared with previous samples stored in the memory
unit (315). Based on the temperature values, a temperature slope or
rate of change of temperature is calculated for the water. If the
user is drawing water, a large negative slope value will result
informing the software that a draw of water is in progress.
[0124] At act 810, the software sets a duty cycle that determines
the amount of power to be transferred to each heating element. The
amount of power varies depending on the temperature of the water
and the water heater code for the water heater 200. In addition,
the amount of power may take into account a water heater usage
pattern (which is stored in the memory unit 315), the ambient
temperature, a water consistency value, or other information.
[0125] For act 810, the software obtains from the memory unit 315
the water heater code and past records of data stored by the water
heater. The past records are stored each time the software
completes act 555 (FIG. 18), and each record includes the time of
day, duration of past heatings, rate of change (slope) in water
temperature decline and rise, and may additionally include other
information such as ambient room temperature. As the controller 260
heats the water, it looks into the memory unit 315 for recorded
information of similar circumstances during the same time period in
previous days and/or weeks. If it appears that the user is using
about the same amount of water during any given period then the
water will be heated at a standard rate for the water heater code
that will satisfy the anticipated consumption rate of heated water.
If the stored data would indicate that there may be no further
usage after the present heating cycle, the water then will be
heated very slowly at a lower duty cycle to minimize energy
consumption. If there is an abrupt and rapid decline (i.e.,
negative temperature slope) in water temperature, the software will
calculate a new duty cycle according to the present usage condition
of the water heater. As usage patterns change the old records will
be modified to reflect the current operating conditions. For the
preferred embodiment, the base line formula in considering what
minimum water temperature flow rates will be acceptable is a
minimum recovery equal to ten gallons per hour at sixty degree
Fahrenheit temperature rise.
[0126] With this formula, product code information and usage
records, the power input ratios versus temperature rate change are
used in determining heating strategies. The strategies provide
input power levels to meet or exceed the minimum recovery rate,
while keeping energy efficiency to a maximum. As conditions change
in usage patterns the strategy is modified to maintain the minimum
recovery standard.
[0127] For example, a standard heating strategy for a first water
heater code having a first element wattage will differ when
compared to a heating strategy for a second water heater code
having a second element wattage. Two exemplary heating strategies
for the second element 240 are shown in Tables 1 and 2.
1TABLE 1 Water Heating Strategy for a First Heater Code Power or
Duty Cycle of the Water Temperature Second Element <115.degree.
F. 100% 115.degree. F. to 120.degree. F. 66% 120.degree. F. to
125.degree. F. 57% 125.degree. F. to 130.degree. F. 50% 130.degree.
F. to 135.degree. F. 40% 135.degree. F. > 20%
[0128]
2TABLE 2 Water Heating Strategy for a Second Heater Code Power or
Duty Cycle of the Water Temperature Second Element <115.degree.
F. 100% 115.degree. F. to 120.degree. F. 80% 120.degree. F. to
125.degree. F. 66% 125.degree. F. to 130.degree. F. 50% 130.degree.
F. to 135.degree. F. 40% 135.degree. F. > 20%
[0129] For water heater 200, the duty cycle or power applied to the
heating elements 235 or 240 is based at least in part on the sensed
water temperature and the water heater code. The concept of a
heating strategy dependent on a water heater code is unlike the
method of heating water for water heaters 10 and 150. For water
heaters 10 and 150, the duty cycle or power applied to the heating
elements 16 and/or 16' is based on the difference between the
sensed water temperature and the desired water temperature.
However, it has been determined that increasing the power to an
element submerged in water at a given water temperature may not
result in an optimum water temperature gain when compared to the
power input. For example, assuming all other conditions are the
same, it has been determined that more heat can be transferred from
an element to water when the water is at a cooler temperature. As
the water temperature increases, less power needs to be provided to
the heating element 235 or 240 regardless of the difference between
the sensed temperature and the desired temperature (i.e., the
excess power will not result in an optimum transfer when compared
to the power input). Therefore, the software does not need to take
into account the difference between the desired temperature and the
sensed temperature for heating the water. But it is envisioned that
under some circumstances (e.g., the usage pattern changes resulting
in the water needing to be heated as fast as possible without a
concern for efficiency) that a heating strategy may want to include
a difference measurement.
[0130] At act 815, the software determines the "draw down" state.
The draw down state indicates whether a user is currently drawing
water and at what rate the user is drawing the water. The draw down
state has four values: "tank is heating", "draw-down-one",
"draw-down-two", and "recovering". If the draw down state is
"tank-is-heating", then the software proceeds to act 820. If the
draw down state is "draw-down-one", then the software proceeds to
act 825. If the draw down state is "recovering", then the software
proceeds to act 830. If the draw down state is "draw-down-two",
then the software proceeds to act 835.
[0131] At act 820, the software determines whether the temperature
slope is less than or equal to a threshold for a draw down. For
example, if the calculated temperature slope is less than ten
degrees Fahrenheit then the software determines a draw down is in
progress and sets the draw down state to "draw-down-one" (act 840).
If the temperature slope is greater than the draw down threshold
then the software determines a draw is not in progress and proceeds
to act 870.
[0132] If the draw down state is currently "draw-down-one", then
the water heater had previously been in a draw down (i.e., a user
is using hot water). At act 825, the software determines whether
the temperature slope is positive. If the temperature slope is
positive, then the software determines that the water heater is
recovering and sets the draw down state to recovering (act 845). If
the temperature slope is still negative, then the software
determines the water heater is still in a draw down and proceeds to
act 870.
[0133] If the draw down state is currently set to "recovering",
then the water heater is recovering from a draw down. At act 830,
the software determines whether there has been another draw down
(i.e., the temperature slope is less than or equal to the threshold
for a draw down). If there was another draw down, then the software
sets the draw down state to "draw-down-two" (act 850). If the
software determines the water heater is still recovering, the
program proceeds to act 870.
[0134] At act 835, the software determines whether the lower tank
temperature is greater than or equal to a heater-on temperature. If
the lower tank temperature is greater than or equal to a heater-on
temperature, then the software sets the draw down state to
recovering and resets the temperature slope. If the lower tank
temperature is less than the heater-on temperature, then the
microcontroller U1 sets the duty cycle to full power (act 760). Of
course, other duty cycles can be used depending upon the particular
water heater and environmental circumstances.
[0135] At act 870, the software determines the heating priority for
the water heater. If the heating priority is "fifty-fifty"
(discussed below), then the software sets the duty cycle to full
power (act 875) regardless of the water temperature. Of course,
other duty cycles can be used depending upon the particular water
heater and environmental circumstances. If the heating priority is
not in the fifty-fifty mode, then the software proceeds to act 880
(FIG. 20(d)).
[0136] At act 880, the software selects a case based on the
previously determined heating priority. The heating priority is
used for determining which elements are active. For example, if the
first element is an upper element and the second element is a lower
element (similar to FIG. 5), then under certain conditions both
elements may be used. For this arrangement, if both elements are
being used, then the heating priority will be fifty-fifty. If only
one element is used, then the heating priority is zero-one-hundred.
Alternatively, if the elements are in a substantially horizontal
plane, both elements may be used in a fifty-fifty arrangement (vs.
only one element being used) to heat the water.
[0137] At act 885, the software determines if the upper tank
temperature has fallen (i.e. the temperature slope of the upper
element is less than or equal to a threshold). If the upper tank
temperature has fallen, then the software sets the heating priority
to "fifty-fifty" (act 887), resulting in both elements heating the
water. If the upper tank temperature has not fallen, then the
software proceeds to act 555 (FIG. 16).
[0138] At act 890, the software determines whether the upper tank
temperature has recovered (i.e., the temperature slope of the upper
element is greater than a threshold). If the upper temperature tank
has recovered, then the software sets the priority to
"zero-one-hundred" (act 895), resulting in only the second element
240 heating the water. If the upper tank temperature has not
recovered, then the software proceeds to act 555 (FIG. 16).
[0139] Every eight hundred microseconds, the software performs a
timer interrupt event. The timer interrupt is used as a time base
for various timeouts (e.g., the "ReCheck" timeout). During each
interrupt, the microcontroller's timer is reset and the timeout
variables are decreased if their value is still greater than zero.
Once a timeout value reaches zero, the associated routine can be
performed at that time, or can be performed during the main loop.
As shown in FIG. 21, at act 905, the software resets the timer for
the next scheduled interrupt. At act 910, the software services
timeouts (i.e., decrease each timeout) and delays variables. At act
915, the software executes event-related routines as required. At
act 920, the software returns from the interrupt to the act it was
previously implementing.
[0140] Every time the signal (AcOutHI) crosses zero volts, the
micro controller U1 performs a zero crossing event interrupt. When
transistor Q8 (FIG. 12) turns on, it goes into saturation causing a
falling edge that generates an interrupt to the microcontroller U1.
The falling edge is used as a reference edge for activating triacs
Q1 and Q2 (FIG. 16(b)). When the reference edge occurs, the timer
interrupt (FIG. 21) is adjusted so that it will correspond exactly
to when a zero crossing occurs. In this way, the zero crossing
interrupt fires the triacs at precisely the right time.
[0141] To control the power transmitted to the heating elements 235
and 240, the microcontroller U1 generates an output signal
(first-element or second-element) which is provided to the
zero-cross triac drivers U5 and U6, respectively. The zero-cross
triac drivers U5 and U6 in combination with triacs Q1 and Q2
control the high-voltage AC signal (AcIn) being provided to the
heating elements 235 and 240.
[0142] For controlling the power transmitted to the heating
elements 235 and 240, triac Q1 or Q2 is fired for a sequence of
four sequential half AC cycles. The triac Q1 or Q2 fired is based
on the heating priority and the status at the software relative to
the heating cycle. For example, if the heating priority is
"zero-one-hundred", then only one triac Q2 will be fired.
Alternatively, if the heating priority is "fifty-fifty" and the
heating elements 235 and 240 are being fired sequentially, then the
software includes a variable specifying which heating element 235
or 240 is being activated. After firing a sequence of four
sequential AC half cycles, the software delays firing, i.e. does
not fire the triac Q1 or Q2 for a number of cycles. The number of
cycles the triac Q1 or Q2 does not fire is determined by the amount
of power to be transmitted to the heating elements 235 or 240. For
example, if 100% power is to be transmitted, then the software will
not delay the firing at all. If 50% power is to be transmitted,
then the software will delay the firing of the triac Q1 or Q2 for
four half AC cycles. Table 3 discloses an exemplary power transfer
table.
3TABLE 3 Lookup Table for Various Duty Cycles based on an Initial
Four Cycle Firing Delay (half-cycle) Power Transfer 0 half cycle
delay 100% Power 1 half cycle delay 80% Power 2 half cycle delay
66% Power 3 half cycle delay 57% Power 4 half cycle delay 50% Power
6 half cycle delay 40% Power 16 half cycle delay 20% Power
[0143] Of course, other half cycle delays can be used and the
initial four cycle firing can vary to obtain different power
transfer ratios.
[0144] While particular embodiments of the invention have been
shown and described herein, changes and modifications may be made
without departing from the spirit and scope of the invention. For
example, logic chips other than the Motorola UAA1016A logic chip
may be used to control the on-off cycle of thyristor 103. Also, a
temperature sensing device other than the thermistor used as
temperature sensing device 102 may be employed. Also, a thyristor
other than a Motorola TRIAC may be used as thyristor 103 and
multiple heating elements and other alternative control circuits,
as noted above, may be utilized. Therefore, no limitation of the
invention is intended other than limitations contained in the
appended claims.
[0145] Various other features and advantages of the invention are
set forth in the following claims.
* * * * *