U.S. patent application number 11/369086 was filed with the patent office on 2006-09-28 for low-cost heat pump water heater.
Invention is credited to Viung C. Mei, John J. Tomlinson.
Application Number | 20060213210 11/369086 |
Document ID | / |
Family ID | 37033822 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060213210 |
Kind Code |
A1 |
Tomlinson; John J. ; et
al. |
September 28, 2006 |
Low-cost heat pump water heater
Abstract
An improved heat pump water heater of the type having a water
tank with an exterior surface and defining a water chamber, a top
on the water tank with at least one opening therethrough in
communication with the water chamber, and a heat pump of the type
having a compressor, the compressor being in fluid communication
with an annular condenser assembly via a first refrigerant conduit,
the annular condenser assembly being in fluid communication with an
expansion device through a second refrigerant conduit, the
expansion device being in fluid communication with an evaporator
through a third refrigerant conduit, the evaporator being in fluid
communication with the compressor through a fourth refrigerant
conduit and control means therefore, the improvement comprising
disposing the annular condenser assembly through the opening in the
top of the water tank and into the water chamber, the water tank
opening further comprising a geometric twisted sleeve positioned to
form a helical insertion pattern in the annular condenser assembly,
the annular condenser assembly further comprising an elongate outer
tube having a closed bottom end and an open and opposed upper end
in flow communication with the first refrigerant conduit, an
elongate return tube disposed within the outer tube and having an
open bottom end and a closed and opposed top end in flow
communication with the second refrigerant conduit, an elongate
capillary tube disposed within the return tube and having an open
bottom end and an open and opposed top end in flow communication
with the second refrigerant conduit, and an air gap disposed
between the capillary tube and the return tube.
Inventors: |
Tomlinson; John J.;
(Knoxville, TN) ; Mei; Viung C.; (Knoxville,
TN) |
Correspondence
Address: |
UT-Battelle, LLC;Office of Intellectual Property
One Bethal Valley Road
4500N, MS-6258
Oak Ridge
TN
37831
US
|
Family ID: |
37033822 |
Appl. No.: |
11/369086 |
Filed: |
March 6, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60664776 |
Mar 24, 2005 |
|
|
|
Current U.S.
Class: |
62/238.6 |
Current CPC
Class: |
F24H 4/04 20130101; F25B
30/02 20130101 |
Class at
Publication: |
062/238.6 |
International
Class: |
F25B 27/00 20060101
F25B027/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with United States Government
support under Contract No. DE-AC05-00OR22725 between the United
States Department of Energy and U. T. Battelle, LLC. The United
States Government has certain rights in this invention.
Claims
1. An improved heat pump water heater of the type having a water
tank with an exterior surface and defining a water chamber, a top
on the water tank with at least one opening therethrough in
communication with the water chamber, and a heat pump of the type
having a compressor, the compressor being in fluid communication
with an annular condenser assembly via a first refrigerant conduit,
the annular condenser assembly being in fluid communication with an
expansion device through a second refrigerant conduit, the
expansion device being in fluid communication with an evaporator
through a third refrigerant conduit, the evaporator being in fluid
communication with the compressor through a fourth refrigerant
conduit and control means therefore, the improvement comprising:
disposing the annular condenser assembly through the opening in the
top of the water tank and into the water chamber, the water tank
opening further comprising a geometric twisted sleeve positioned to
form a helical insertion pattern in the annular condenser assembly,
the annular condenser assembly further comprising: an elongate
outer tube having a closed bottom end and an open and opposed upper
end in flow communication with the first refrigerant conduit, an
elongate return tube disposed within the outer tube and having an
open bottom end and a closed and opposed top end in flow
communication with the second refrigerant conduit, an elongate
capillary tube disposed within the return tube and having an open
bottom end and an open and opposed top end in flow communication
with the second refrigerant conduit, and an air gap disposed
between the capillary tube and the return tube.
2. An improved heat pump water heater as claimed in claim 1 wherein
the first refrigerant conduit, the expansion device, and the second
refrigerant conduit are disposed inside the water tank; the
evaporator, the third refrigerant conduit, the compressor and the
fourth refrigerant conduit and the control means are disposed on
the water tank.
3. An improved heat pump water heater as claimed in claim 1 and
further comprising a housing disposed on the top of the water tank
and containing therein the compressor and evaporator.
4. An improved heat pump water heater as claimed in claim 1 wherein
the annular condenser assembly is constructed of copper.
5. An improved heat pump water heater as claimed in claim 1 wherein
the annular condenser assembly is constructed of a first metal and
a second metal which is capable of corroding at a rate greater than
the rate of corrosion of the first metal.
6. An improved heat pump water heater as claimed in claim 5 wherein
the second metal is selected from the group consisting of aluminum,
magnesium or zinc.
7. An improved heat pump water heater as claimed in claim 1 wherein
the annular condenser assembly is a double walled helical
device.
8. An improved heat pump water heater as claimed in claim 1 wherein
the coefficient of performance is in the range of approximately 1.7
to 1.8.
9. An improved heat pump water heater as claimed in claim 1 wherein
the control means further comprises a control module having a
voltage divider network to energize heating sources.
10. An improved heat pump water heater as claimed in claim 9 having
three heating sources.
11. A method of constructing an improved heat pump water heater of
the type having a water tank formed of a first metal and defining a
water chamber, a top on the water tank with at least one opening
therethrough for an anode rod to be disposed within the water
chamber, and a heat pump of the type having a compressor, the
compressor being in fluid communication with an annular condenser
assembly via a first refrigerant conduit, the annular condenser
assembly being in fluid communication with an expansion device
through a second refrigerant conduit, the expansion device being in
fluid communication with an evaporator through a third refrigerant
conduit, the evaporator being in fluid communication with the
compressor through a fourth refrigerant conduit and control means
therefore, the comprising the steps of: a. removing the anode rod
from the water tank; and b. inserting the annular condenser
assembly through the opening in the top of the water tank and into
the water chamber, the water tank opening further comprising a
geometric twisted sleeve positioned to form a helical insertion
pattern in the annular condenser assembly, the annular condenser
assembly further comprising an elongate outer tube having a closed
bottom end and an open and opposed upper end in flow communication
with the first refrigerant conduit, an elongate return tube
disposed within the outer tube and having an open bottom end and a
closed and opposed top end in flow communication with the second
refrigerant conduit, an elongate capillary tube disposed within the
return tube and having an open bottom end and an open and opposed
top end in flow communication with the second refrigerant conduit,
and an air gap disposed between the capillary tube and the return
tube.
12. A method as claimed in claim 11 wherein the annular condenser
assembly is constructed of copper.
13. A method as claimed in claim 11 wherein the annular condenser
assembly is constructed of a first metal and a second metal which
is capable of corroding at a rate greater than the rate of
corrosion of the first metal.
14. A method as claimed in claim 13 wherein the second metal is
selected from the group consisting of aluminum, magnesium or
zinc.
15. A method as claimed in claim 11 wherein the annular condenser
assembly is a double walled helical device.
16. A method as claimed in claim 11 wherein the coefficient of
performance is in the range of approximately 1.7 to 1.8.
17. A method as claimed in claim 11 wherein the control means
further comprises a control module having a voltage divider network
to energize multiple heating sources.
18. A method as claimed in claim 17 further comprising three
heating sources.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application 60/664,776 filed Mar. 24, 2005, and is herein
incorporated by reference. This application is also related to U.S.
Pat. No. 6,233,958, issued May 22, 2001, herein incorporated by
reference in its entirety.
FIELD OF THE INVENTION
[0003] This invention is in the general field of water heaters and
specifically teaches a low-cost embodiment for a heat pump water
heater.
BACKGROUND OF THE INVENTION
[0004] U.S. Pat. No. 5,946,927 teaches a "drop-in" heat pump water
heater (HPWH), and ECR International produces a HPWH based around
this patent. U.S. Pat. No. 6,233,958 teaches a HPWH with bayonet
condenser. Residential U.S.-made HPWHs are manufactured and sold by
AERS (Atlanta, Ga.) and Nyle Special Products (Bangon, Me.);
however the small market that was present is drying up due to high
costs based on old technologies. HPWHs are also manufactured and
sold in Asia and Australia at low volumes due to high first
costs.
[0005] After much work on the HPWH, the U.S. Department of Energy
(DOE) has concluded that high costs are the principal market
barrier for the HPWH, and DOE has established a goal of $500 for a
50-gallon residential HPWH. A preferred way of attaining this goal
is taught in this invention.
[0006] There are basically three types of residential HPWHs. One is
the desuperheater which is connected to the heat pump system that
is used for house cooling and heating. The desuperheater takes part
of the heat from the compressor discharge gas and uses it for
domestic water heating. One problem with a desuperheater HPWH is
that the house space conditioning load is usually not in sync with
the water heating load. That is, when hot water is needed, the
house might not need cooling or heating, the desuperheater is
therefore inactive, and water heating is provided by inefficient,
backup electric resistance water heaters. For this reason, field
data show that desuperheaters provide only 20-30% of all hot water
needs. A second type of HPWH is a dedicated stand alone unit that
pumps water from the water tank and heats it using a small,
dedicated heat pump. This type of HPWH is bulky, requires a water
pump, and its costs tend to be high (typically $1000, or more). The
third type is a HPWH mounted on the top of the water tank to form a
single, integrated unit. Integrated HPWHs heat water in a number of
ways: some use small pumps to circulate water between an external
condenser and the tank itself while others heat the exterior of the
tank letting the warm tank transfer heat to the water that is
contained inside. Finally, HPWHs that use an immersed condenser are
taught; however, the immersed condenser requires a large hole be
present on the top of the water tank through which the condenser
assembly can be installed. A problem with all of these approaches
is a market one: the costs are simply too high and as a result, the
market for the HPWH is vanishing. Most of the HPWHs mentioned
require a tank that is special in some way and therefore expensive.
The wraparound condenser design (probably the best recent attempt
at a residential HPWH) begins its manufacture with a bare metal
tank, wrapping a heat exchanger coil around the tank, affixing
temperature sensors to the tank, insulating the wrapped tank,
shrouding it with a metal cover, and finally installing a small
heat pump on the top. The major water heater manufacturers, who
dominate the water heating distribution chain, have shown no
interest in the wraparound design nor in any other HPWH for that
matter. They are tank manufacturers, and moving into any field
beyond that is simply too involved and expensive. Other attempts at
a residential HPWH have faired even worse. Today, none of the major
water heater manufacturers who produce conventional electric water
heaters by the millions are interested in the current HPWH design.
Yet, if the market for a HPWH is to materialize, the major tank
manufacturers must be players. Needed then is a simple, low-cost
HPWH design based on (1) use of a conventional insulated electric
water heater--everywhere available; (2) a pre-charged HPWH
design--one that does not require the services of refrigeration
trades for installation; (3) HPWH controls that work with the
existing controls for the conventional electric water heater and
simply plug together; and (4) an overall design approach in which
the HPWH and electric resistance tank can be "married" in the
factory with little additional labor and no additional
manufacturing processes. Only in this way, using the invention
herein, will the economics for the heat pump water heater be
favorable enough to support a reasonable market.
BRIEF DESCRIPTION OF THE INVENTION
[0007] An improved heat pump water heater of the type having a
water tank with an exterior surface and defining a water chamber, a
top on the water tank with at least one opening therethrough in
communication with the water chamber, and a heat pump of the type
having a compressor, the compressor being in fluid communication
with an annular condenser assembly via a first refrigerant conduit,
the annular condenser assembly being in fluid communication with an
expansion device through a second refrigerant conduit, the
expansion device being in fluid communication with an evaporator
through a third refrigerant conduit, the evaporator being in fluid
communication with the compressor through a fourth refrigerant
conduit and control means therefore, the improvement comprising
disposing the annular condenser assembly through the opening in the
top of the water tank and into the water chamber, the water tank
opening further comprising a geometric twisted sleeve positioned to
form a helical insertion pattern in the annular condenser assembly,
the annular condenser assembly further comprising an elongate outer
tube having a closed bottom end and an open and opposed upper end
in flow communication with the first refrigerant conduit, an
elongate return tube disposed within the outer tube and having an
open bottom end and a closed and opposed top end in flow
communication with the second refrigerant conduit, an elongate
capillary tube disposed within the return tube and having an open
bottom end and an open and opposed top end in flow communication
with the second refrigerant conduit, and an air gap disposed
between the capillary tube and the return tube.
[0008] The invention comprises a low-cost type HPWH with the heat
pump unit mounted on top of a conventional hot water storage tank.
Low-cost connotes turning an inexpensive, conventional
resistance-type electric water heater into an HPWH with no changes
to the tank or to the existing controls.
[0009] The invention addresses the need for a low-cost HPWH by
using a conventional, insulated electric water heater and controls
as presently produced and found throughout the United States.
[0010] The invention teaches a method for installing a condenser
with a large surface area through the small threaded opening at the
top of the water heater.
[0011] The invention comprises a simple method (geometric twisted
sleeve) for arranging the internal condenser by adjusting the
number of coils, pitch, radius and location within the tank to
affect an optimal, efficient design.
[0012] The invention comprises a linear condenser design that
avoids loss of refrigerant subcooling as the refrigerant travels
upward along its path out of the storage tank.
[0013] The invention comprises a linear condenser design that
incorporates the expansion device into the condenser itself forming
into a single element the two functions of an immersed condenser
and an expansion device.
[0014] The invention comprises a HPWH control system that uses to
the greatest extent all of the conventional controls of the
baseline resistance water heater, and applies them in a unique way
as part of a HPWH control system that can heat water using
resistance heaters or using the heat pump.
[0015] The invention can be applied to a new water heater by a
manufacturer, retrofitted to an existing water heater, and applied
to a gas water heater resulting in a dual-fuel, efficient water
heater for both residential and commercial buildings.
[0016] One clear application is to residential electric water
heating. The annual operating costs for a conventional residential
water heater are about $350. The HPWH would cut these costs in
half. Applied to a gas water heater, the HPWH technology would
provide a "dual-fuel" capability. And finally, since the HPWH has
an air-source evaporator, the HPWH provides some cooling and
dehumidification to the space where is the HPWH is located.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic of a HPWH with an internal condenser
coil.
[0018] FIG. 2 is a diagram showing the condenser coil inserted and
formed through a sleeve having an expansion device.
[0019] FIG. 3 is a graph showing the field test data from a
previous HPWH with an immersed condenser.
[0020] FIG. 4 is a schematic of a HPWH control system.
[0021] FIG. 5 is a schematic of a HPWH with the condenser wrapped
around the tank wall.
DETAILED DESCRIPTION OF THE INVENTION
[0022] One of the biggest challenges to be met is finding a way to
install a large surface area condenser into the conventional tank
through one of the small pipe fittings (3/4'' FPT) that is already
on the top of all water heaters. This is somewhat akin to building
a ship in a bottle with a small neck. The invention overcomes this
problem by installing a short, geometric twisted sleeve through the
small diameter top fitting in the top of the tank, and then pushing
a long condenser assembly through the open sleeve. As the linear
condenser is pushed through the sleeve, it forms itself into a
helix inside the tank. The radius, pitch and location of the helix
inside the tank is pre-determined by the geometry of the sleeve.
Calculations show that a 5/16-in diameter condenser can be pushed
through a 3/4-inch, sleeved hole in a tank, and that a 50-foot long
condenser section, 5/16-in diameter, will form a helical condenser
inside the tank providing more than 4 sq. ft. of condenser surface
inside of a conventional water tank that is only 3.5 feet long. The
sleeve, having served its purpose, remains in the tank. A major
advantage of this design is that a small manufactured package
consisting of an air-source heat pump and linear condenser assembly
can be simply mated to a conventional, insulated hot water tank to
make an HPWH.
[0023] A second constraint is loss of performance when a linear
condenser, designed as a assembly, is used to heat water in a tank.
Conventional immersed condenser art takes the form of a probe or
"U-tube" design in which the condenser's inlet and exit are at the
same location; usually at the top of the tank where the water is
the hottest. This means that as superheated refrigerant from the
compressor enters the condenser and passes downward towards the end
of the condenser at the bottom of the tank, it condenses and
becomes subcooled. The refrigerant then returns from the bottom of
the condenser, passing Upwards and out of the condenser at the top
of the tank. The problem is that the hot water at the top of the
tank heats the refrigerant so that it loses its subcooling that it
attained at the bottom of the tank. As the refrigerant, having lost
its subcooling continues to the expansion device, refrigerant
metering and control are lost, the operation of the cycle becomes
unstable and the performance and efficiency of the system drop. To
overcome this problem, the condenser in the invention has a unique
feature that ensures that the refrigerant returning from the bottom
of the tank is not reheated by the hot water at the top of the
tank. This is done by using an annular condenser design that allows
the refrigerant to return from the bottom of the tank (from the end
of the condenser assembly) through a small, return tube that is
insulated from the outer annulus. The design is such that an air
space is formed in the return tube, and the air space effectively
insulates the cooler refrigerant returning from the bottom of the
tank from the hot condensing refrigerant passing downwards through
the annulus of the assembly. The invention takes the condenser
design one step further. Since the small, central return tube of
the condenser provides no heating function, it is used as the
expansion device in the cycle, that is, it serves as a capillary
tube to meter low temperature, low pressure refrigerant directly
into the evaporator of the cycle. Therefore, the linear condenser
assembly is a condenser on it's outside where it is always adjacent
to the water to be heated, and is an expansion device along the
central tube. This simplifies the cycle, reduces its costs and
improving reliability as compared to HPWHs that use active
thermostatic expansion valves.
[0024] The third issue facing a low-cost HPWH that is designed
around the use of a conventional, insulated, electric storage water
heater with heating elements, is a low-cost method for controlling
the operation of the HPWH. HPWH controls need to respond to tank
temperatures and to compressor operating conditions. If the bottom
of the tank becomes cool due to a hot water draw, the HPWH needs to
turn on and operate until the bottom of the tank reaches a
setpoint. If the top of the tank becomes cool due to an extended
hot water draw, the upper heating element in the tank needs to turn
on. And if the compressor finds itself operating outside of an
acceptable envelope of discharge or suction temperatures, controls
need to change the HPWH's operation, and if acceptable compressor
operating conditions cannot be attained, controls must turn off the
compressor and return the system to a conventional control strategy
in which the lower tank heating element supplemented by the upper
element provides the heating. The invention retains and uses the
upper and lower heating elements and thermostats that are present
in a conventional electric water heater as well as two temperature
sensors on the suction and discharge sides of the compressor and an
ambient temperature sensor to perform the control functions that
are needed. The invention avoids any need for additional sensors,
controls and wiring to be applied to the conventional water heater
to turn it into a HPWH. Therefore, this invention could be
manufactured by a company skilled in vapor compression
refrigeration fabrication, shipped to a water heater company (with
no skills in vapor compression equipment), connected together with
very little labor, and shipped by the water heater company as a new
product into their existing distribution network. Therefore, it
will be easier for water heater tank manufacturers to accept this
type of HPWH and incorporate it into existing product lines.
[0025] FIG. 1 is a simple schematic of the HPWH with its internal
annular condenser coil 1. The HPWH portion 3 at the top absorbs
heat from the surrounding air and, together with the heat generated
by compressor operation, heats the water in the tank 2 via the
annular condenser coil 1. The compressor, evaporator, fan, linear
condenser, expansion device and controls are packaged as a single
assembly. The annular condenser coil 1 can be made from copper
tubing, and it is possible that for corrosion protection, the
exterior of the condenser assembly could be coated with the same
material as used in the sacrificial anode of the water heater.
Anodes of metals such as aluminum, magnesium, or zinc are sometimes
installed in water heaters and other tanks to control corrosion of
the tank. The introduction of the anode creates a galvanic cell in
which the magnesium or zinc will go into solution (be corroded)
more quickly than the metal of tank thereby imparting a cathodic
(negative) charge to the tank metal(s) and preventing tank
corrosion. This corroding of the anode metal is called "the
sacrifice of the anode."
[0026] FIG. 2 shows a geometric twisted condenser sleeve 21 that is
installed first in the conventional insulated tank 22. A double
walled, annular condenser assembly 23 with the return tube 30 along
the central axis of the annular condenser assembly 23 is then be
inserted into the tank 22 through the sleeve 21. Due to the shape
of the sleeve 21, the inserted annular condenser assembly 23 takes
a helical shape as it is pushed into the tank. The pitch, diameter
and position of the helix inside the tank is determined by the
geometry of the sleeve. Experience has shown that locating an
immersed condenser in the bottom 1/3 to 1/2 of the tank works best,
heating the cooler water near the bottom of the tank. A large
condenser area can be produced by a helix of the largest diameter
and smallest pitch subject to limitations of the tank itself.
Experience will tell whether it is necessary to remove temporarily
all obstructions on the inside of the tank (e.g. the elements, dip
tube, anode rods) during insertion of the condenser.
[0027] Details of the annular condenser assembly 23 are shown in
FIG. 2. The condenser assembly is double-walled for safety since
the water in the tank is considered potable. The exterior of the
return tube 30, and the inside of the outer tube 24 of the
double-wall configuration ensures that a path exists to allow any
refrigerant leaks to exit the condenser and tank without contacting
the potable water. Experience has shown that good thermal contact
between the outer tube 24 and return tube 30 can be provided by
hydraulically expanding the return tube 30 against the outer tube
24 during manufacture. This process also leaves a path for the
refrigerant 26 to escape in the event of a leak. FIG. 2 also
illustrates condenser detail that shows the refrigerant capillary
tube 29 along the center of the return tube 30 with a reducer 28
mounted to the end of the capillary tube 29. By making the
capillary tube 29 small, two objectives are reached: (1) an air gap
25 is formed next to the return tube 30, and this provides a high
degree of thermal insulation between the annular condensing portion
and the refrigerant return portion of the assembly, and (2) by
proper design, the capillary tube 29 serves wholly or partially as
the expansion device so that as the refrigerant 26 exits the
assembly, it is at the low temperature and low pressure needed for
the evaporator.
[0028] FIG. 3 shows the field test data from a previous HPWH with
an immersed condenser. These data show that the HPWH is twice as
efficient as a conventional electric resistance heating water
heater, with a coefficient of performance (COP) energy factor of
around 1.7 to 1.8. It is expected that the invention will perform
even better due to the greater heat transfer surface from the use
of longer tubes.
[0029] In order to be low cost and energy efficient, the HPWH
controls must be as simple as possible. FIG. 4 shows the controls
of the HPWH and how they take advantage of existing electric water
heater controls. The existing electric water heater controls
consist of an upper element 45 and its thermostat 42, and a lower
element 44 and its thermostat 43. As shown in FIG. 4, the lower
element thermostat 43 is slaved to the upper element thermostat 42
so that if the upper portion of the tank is cool, the upper
thermostat 42 activates the upper element 45 and deactivates the
lower element 44 irrespective of the lower thermostat 43 position.
This means that only one heating element can be active at any one
time. The power for a typical water heating element is 4500 W with
a voltage input of 240 VAC. Based in this, it can be shown that the
resistance of this typical heating element is about 12.8 ohms
(small). From FIG. 4, it can be seen that the control module 48
provided as part of the HPWH system is powered by the 240 VAC;
other inputs to the control module 48 are from temperature sensors
that measure the evaporator temperature 40, the compressor
discharge temperature 50, the ambient temperature 51 and safetys 41
such as a condensate switch to turn the compressor off in the event
of an imminent evaporator condensate pan overflow.
[0030] Conventional (resistance heating) operation: If any one of
the control conditions needed for operation of the HPWH is not
satisfied, no voltage is applied between terminals A and B of relay
49, and the contacts remain closed. In this case, the HPWH behaves
as a conventional resistance water heater with the upper and lower
thermostats controlling the heat input to the water tank by the
elements.
[0031] HPWH operation: If all conditions for HPWH operation (e.g.
compressor temperatures and ambient temperatures) are within a
nominal range, the control module 48 applies a control voltage
(e.g. 24 VAC) between terminals A and B. This activates relay 49
thereby opening its contact, and the HPWH is capable of operating.
By design, the control module 48 has high input impedance between
terminals A and C. Therefore, with either thermostat 42 or 43
"made", the series connection between terminals A-C of the control
module 48 and either heating element forms a voltage divider
network. Assume, for example, that the input impedance between
module terminals A-C is 130 ohms. Then with the lower thermostat 43
"made", the voltage between terminals A-C is 218 VAC. With the
lower thermostat satisfied (open), the A-C voltage rises to 240
VAC. The control module 48 uses this difference (or change) to
operate the HPWH in response to the lower thermostat. By installing
upper and lower elements of two different power inputs (e.g. a
3000-W lower element and a 4500-W upper element), the A-C voltage
would have three levels to energize three heat sources. If the A-C
impedance were 130 ohms as before, a 240 VAC signal between
terminals A and C would indicate that both thermostats are
satisfied (the compressor and fans can be turned off); a 210 VAC
signal between terminals A and C would indicate that the lower
thermostat is not satisfied (but the upper one is satisfied)
suggesting that the tank is calling for heating at the bottom; and
a 218 VAC signal between terminals A and C would indicate that the
upper thermostat is not satisfied, but the lower one is satisfied.
The ability to use the existing thermostats and the resistance
elements to discriminate how the tank is to be heated is part of
the uniqueness of this invention. The values chosen for the
examples above are arbitrary and serve only to illustrate the
idea.
[0032] Finally, it will be understood that the preferred embodiment
has been disclosed by way of example, and that other modifications
may occur to those skilled in the art without departing from the
scope and spirit of the appended claims.
* * * * *