U.S. patent application number 11/550619 was filed with the patent office on 2008-05-29 for gas pressure control for warm air furnaces.
This patent application is currently assigned to HONEYWELL INTERNATIONAL INC.. Invention is credited to Michael W. Schultz.
Application Number | 20080124667 11/550619 |
Document ID | / |
Family ID | 39464098 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080124667 |
Kind Code |
A1 |
Schultz; Michael W. |
May 29, 2008 |
GAS PRESSURE CONTROL FOR WARM AIR FURNACES
Abstract
Systems, methods, and controllers for controlling gas-fired
appliances such as warm air furnaces are disclosed. An illustrative
furnace system can include a burner unit in communication with a
combustion air flow conduit and heat exchanger, a variable speed
inducer fan or blower adapted to provide a flow of combustion air
to the burner unit, a furnace controller and motor speed control
unit adapted to regulate the speed of the inducer fan or blower,
and a pneumatically modulated gas valve adapted to variably output
gas pressure to the burner unit based at least in part on the
combustion air flow.
Inventors: |
Schultz; Michael W.; (Elk
River, MN) |
Correspondence
Address: |
HONEYWELL INTERNATIONAL INC.
101 COLUMBIA ROAD, P O BOX 2245
MORRISTOWN
NJ
07962-2245
US
|
Assignee: |
HONEYWELL INTERNATIONAL
INC.
Morristown
NJ
|
Family ID: |
39464098 |
Appl. No.: |
11/550619 |
Filed: |
October 18, 2006 |
Current U.S.
Class: |
431/18 ; 236/10;
236/11 |
Current CPC
Class: |
F23N 5/203 20130101;
F23N 2233/04 20200101; F23N 1/022 20130101; F23N 5/10 20130101;
F23N 2005/181 20130101; F23N 2241/02 20200101 |
Class at
Publication: |
431/18 ; 236/10;
236/11 |
International
Class: |
F24H 9/20 20060101
F24H009/20 |
Claims
1. A furnace system, comprising: a burner unit in communication
with a combustion air flow conduit and a heat exchanger; a variable
speed inducer fan or blower adapted to provide combustion air flow
to the burner unit; a furnace controller and a motor speed control
unit adapted to regulate the speed of the fan or blower; and a
pneumatically modulated gas valve adapted to variably output gas
pressure to the burner unit based at least in part on the
combustion air flow.
2. The system of claim 1, wherein the burner unit includes a gas
manifold and a burner box.
3. The system of claim 1, wherein the furnace controller is in
communication with one or more thermostat units.
4. The system of claim 1, wherein the motor speed control unit is
an integral part of the furnace controller.
5. The system of claim 1, wherein the motor speed control unit is a
separate component from the furnace controller.
6. The system of claim 1, wherein the gas valve is pneumatically
driven via one or more pressure signals received at one or more
locations within the combustion air flow conduit.
7. The system of claim 6, wherein said one or more pressure signals
includes: a first pneumatic signal received from a first conduit in
fluid communication with the input side of the heat exchanger; and
a second pneumatic signal received from a second conduit in fluid
communication with the output side of the heat exchanger.
8. The system of claim 7, further comprising a pneumatic amplifier
coupled to the gas valve, the pneumatic amplifier adapted to
amplify a differential pressure control signal fed to the gas valve
based at least in part on said first and second pneumatic
signals.
9. The system of claim 1, further comprising a sensor or switch for
sensing the rotational speed of the inducer fan or blower.
10. The system of claim 1, further comprising a circuit for
measuring the voltage and/or current within the inducer fan or
blower.
11. The system of claim 1, wherein the furnace system comprises a
warm air furnace system.
12. A controller for regulating a gas-fired appliance, the
gas-fired appliance including a burner unit, a heat exchanger, a
gas valve, and a multi or variable speed fan or blower adapted to
produce a combustion air flow to the burner unit, the controller
comprising: a processor adapted to compute the temperature of the
combustion air flow at the burner unit; and a motor speed control
unit adapted to regulate the speed of the fan or blower based at
least in part on the computed temperature of the combustion air
flow.
13. The controller of claim 12, wherein the controller is adapted
to receive thermostat signals from one or more thermostats.
14. The controller of claim 12, wherein the controller is adapted
to receive a speed signal from the fan or blower.
15. The controller of claim 12, wherein the controller is adapted
to receive a voltage or current signal from the fan or blower.
16. A method of controlling a gas-fired appliance including a
burner unit, a heat exchanger, a gas valve, a multi or variable
speed inducer fan or blower adapted to produce a combustion air
flow to the burner unit, and a heated air blower adapted to provide
heated air to one or more warm air ducts, the method comprising the
steps of: receiving a heat request signal and activating the fan or
blower to provide a combustion air flow to the burner unit;
activating the gas valve to provide fuel to the burner unit and
igniting the air/fuel mixture within the burner unit; adjusting the
speed of the inducer fan and/or heated air blower based at least in
part on heat demand signals received from one or more thermostats;
sensing or measuring the mass air flow of the inducer fan and/or
heated air blower and calculating the supply air temperature fed to
the warm air ducts; and adjusting the speed of the inducer fan or
blower to modulate the gas pressure outputted by the gas valve.
17. The method of claim 16, further comprising the step of sensing
the presence of a flame within the burner unit after said step of
igniting the burner unit.
18. The method of claim 16, wherein said step of sensing or
measuring the speed of the inducer fan or blower is accomplished
via a sensor or switch.
19. The method of claim 16, wherein said step of sensing or
measuring the speed of the inducer fan or blower is accomplished by
measuring the voltage and/or current of the inducer fan or
blower.
20. The method of claim 16, wherein said step of adjusting the
speed of the inducer fan or blower based on heat demand signals
received from one or more thermostats is accomplished with a motor
speed control unit.
Description
FIELD
[0001] The present invention relates generally to the field of
gas-fired appliances. More specifically, the present invention
pertains to systems, methods, and controllers for regulating gas
pressure to gas-fired appliances such as warm air furnaces.
BACKGROUND
[0002] Warm air furnaces are frequently used in homes and office
buildings to heat intake air received through return ducts and
distribute heated air through warm air supply ducts. Such furnaces
typically include a circulation fan or blower that directs cold air
from the return ducts across a heat exchanger having metal surfaces
that act to heat the air to an elevated temperature. An ignition
element such as an AC hot surface ignition (HSI) element or direct
spark igniter may be provided as part of a gas burner unit for
heating the metal surfaces of the heat exchanger. The air heated by
the heat exchanger can be discharged into the warm air ducts via
the circulation fan or blower, which produces a positive airflow
within the ducts. In some designs, a separate inducer fan or blower
can be used to remove exhaust gasses resulting from the combustion
process through an exhaust vent.
[0003] In a conventional warm air furnace system, gas valves are
typically used to regulate gas pressure supplied to the burner unit
at specific limits established by the manufacturer and/or by
industry standard. Such gas valves can be used, for example, to
establish an upper gas flow limit to prevent over-combustion or
fuel-rich combustion within the appliance, or to establish a lower
limit to prevent combustion when the supply of gas is insufficient
to permit proper operation of the appliance. In some cases, the gas
valve regulates gas pressure independent of the inducer fan. This
may permit the inducer fan to be overdriven to overcome a blocked
vent or to compensate for pressure drops due to long vent lengths
without exceeding the maximum firing rate of the appliance.
[0004] In some designs, the gas valve may be used to modulate the
gas firing rate within a particular range in order to vary the
amount of heating provided by the appliance. Modulation of the gas
firing rate may be accomplished, for example, via pneumatic signals
received from the inducer fan, or via electrical signals from a
controller tasked to control the gas valve. While such techniques
are generally capable of modulating the gas firing rate, such
modulation is usually accomplished via control signals that are
independent from the control of the combustion air flow produced by
the inducer fan. In some two-stage furnaces, for example, the gas
valve may output gas pressure at two different firing rates based
on control signals that are independent of the actual combustion
air flow produced by the inducer fan. Since the gas control is
usually separate from the combustion air control, the delivery of a
constant gas/air mixture to the burner unit may be difficult or
infeasible over the entire range of firing rate.
[0005] In some systems, supply air temperature and pressure sensors
are employed to sense the combustion air flow produced by the
inducer fan. Typically, the temperature and pressure sensors will
sense the supply air fed to the burner box, which can then be used
by the controller to compute mass flow through the combustion side
of the furnace. In some designs, a mass flow sensor may also be
used in lieu of, the temperature and pressure sensors to compute
mass flow.
[0006] The addition of these sensors require additional power to
operate the furnace, decreasing overall power efficiency. In some
cases, the performance of these sensors can degrade over time,
causing the furnace to operate at a lower efficiency or to
shut-down due to a system fault. The complexity associated with
installing these sensors can also increase the level of skill and
time required to install and service the furnace system.
SUMMARY
[0007] The present invention pertains to systems, methods, and
controllers for controlling gas-fired appliances such as warm air
furnaces. A furnace system in accordance with an illustrative
embodiment can include a burner unit in communication with a
combustion air flow conduit and heat exchanger, a variable speed
inducer fan or blower adapted to provide combustion air flow to the
burner unit, a furnace controller and motor speed control unit
adapted to regulate the speed of the fan or blower, and a
pneumatically modulated gas valve adapted to variably output gas
pressure to the burner unit based at least in part on the
combustion air flow.
[0008] The furnace controller can include a processor adapted to
compute the combustion mass air flow at the burner unit, and a
motor speed control unit adapted to regulate the speed of the fan
or blower based at least in part on the computed air mass flow. In
some embodiments, the motor speed control unit can comprise a
separate unit from the furnace controller. In other embodiments,
the motor speed control unit can be a part of the furnace
controller. During operation, the furnace controller can be
configured to receive heat demand signals from one or more
thermostats that can be utilized by the motor speed control unit to
either increase or decrease the combustion air flow in order to
modulate the gas valve.
[0009] An illustrative method of controlling the gas-fired
appliance can include the steps of receiving a heat request signal
and activating the inducer fan or blower to produce a combustion
air flow at the burner unit. Once the combustion air flow is
initiated, the gas valve can be activated to provide fuel to the
burner unit, which can then be ignited via an ignition element. To
modulate the gas pressure fed to the burner unit, the speed of the
inducer fan or blower can be adjusted based on the heat request
signals. During operation, the rotational speed of the inducer fan
or blower can be sensed via a sensor or switch, or alternatively
the voltage or current to the inducer fan or blower motor can be
measured in order to determine the supply air mass flow. Using the
computed supply air mass flow, the speed of the inducer fan or
blower can then be adjusted upwardly or downwardly in order to
modulate the gas pressure outputted by the gas valve.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a diagrammatic view showing a conventional warm
air furnace system;
[0011] FIG. 2 is a diagrammatic view showing a warm air furnace
system in accordance with an illustrative embodiment;
[0012] FIG. 3 is a diagrammatic view showing several illustrative
inputs and outputs to the furnace controller of FIG. 2;
[0013] FIG. 4 is a diagrammatic view showing several illustrative
inputs and outputs to an alternative furnace system having a
separate furnace controller and motor speed control unit;
[0014] FIG. 5 is a flow chart showing an illustrative method of
operating the furnace system of FIG. 2;
[0015] FIG. 6 is a flow chart showing another illustrative method
of operating the furnace system of FIG. 2; and
[0016] FIG. 7 is a graph showing the change in combustion air
pressure as a function of gas valve output pressure for the
illustrative furnace system of FIG. 2.
DETAILED DESCRIPTION
[0017] The following description should be read with reference to
the drawings, in which like elements in different drawings are
numbered in like fashion. The drawings, which are not necessarily
to scale, depict selected embodiments and are not intended to limit
the scope of the invention. Although examples of furnace systems
methods, and controllers are illustrated in the various views,
those skilled in the art will recognize that many of the examples
provided have suitable alternatives that can be utilized. While the
furnace systems and methods are described with respect to warm air
furnaces, it should be understood that the systems and methods
described herein could be applied to the control of other gas-fired
appliances, if desired. Examples of other gas-fired appliances that
can be controlled can include, but are not limited to, water
heaters, fireplace inserts, gas stoves, gas clothes dryers, gas
grills, or any other such device where gas control is desired.
Typically, such appliances utilize fuels such as natural gas or
liquid propane gas as the primary fuel source, although other
liquid and/or gas fuel sources may be provided depending on the
type of appliance to be controlled.
[0018] Referring now to FIG. 1, a diagrammatic view showing a
conventional warm air furnace (WAF) system 10 will now be
described. As shown in FIG. 1, gas supplied via a gas valve 12 is
fed to a gas manifold 14, which distributes gas to the burners of a
burner box 16. Combusted air discharged from the burner box 16 can
then be fed to the combustion side 18 of a heat exchanger 20, which
transfers heat to a second side 22 for heating the warm air ducts
24 of a heated air space 26 such as a home or office building. An
inducer fan or blower 28 coupled to the combustion side 18 of the
heat exchanger 20 can be configured to draw in air through an air
supply (e.g. an intake vent), which can be used for the combustion
of fuel within the burner box 12. As indicated by arrow 30, the
combustion air discharged from the heat exchanger 20 can then be
exhausted via an exhaust vent 32.
[0019] The inducer fan 28 can be configured to produce a positive
airflow through the heat exchanger 20 forcing the combusted air
within the burner box 16 to be discharged through the exhaust vent
28. A pressure switch 34 can be attached to the combustion side of
the heat exchanger 20 at the input of the inducer fan 28 to sense
the pressure of combustion air flow present on the combustion side
of the furnace. The pressure signals from the pressure switch 34
can be fed to a controller 40 that can be used to enable the gas
valve 12 and initiate ignition.
[0020] On the non-combustion side 22 of the heat exchanger 20, a
heated air blower or fan 36 blows heated air through a separate
path in the heat exchanger 20 into the warm air ducts 24, the
heated air space 26, and back through cold air return ducts 38. One
or more thermostats 42 located in the heated air space 26 may
provide input back to the controller 40. The feedback from the
thermostats 42 may be in the form of temperature set-points
inputted by an occupant of the space 26.
[0021] During operation, a supply of gas can be fed to the gas
valve 12, which, in turn, outputs a metered gas pressure to the gas
manifold 14 for combustion in the burner box 16. The fuel fed to
the burner box 16 can then be ignited via an AC hot surface
ignition element, direct spark igniter, or other suitable ignition
element 44. A flame sensor 48 can be employed to provide an
indication when a flame is present. The flame sensor 48 signals and
signals from a flame rollout switch 46 can be inputted to the
controller 40, which can be configured to shut down the gas valve
12 upon the occurrence of a fault condition. A thermal limit sensor
50 can be used to sense the temperature within the heat exchanger
20, which can be used by the controller 40 to shut down or limit
the gas supplied to the burner box 16 via the gas valve 12 or to
change the speed of the inducer fan 28 or heated air blower 36 in
order to reduce the heat exchanger temperature.
[0022] FIG. 2 is a diagrammatic view showing a warm air furnace
(WAF) system 52 in accordance with an illustrative embodiment of
the present invention. Furnace system 52 can be configured similar
to furnace system 10 described in FIG. 1, including a gas valve 54,
a gas manifold 56, and a burner box 58. Combusted air discharged
from the burner box 58 can be fed to the combustion side 60 of a
heat exchanger 62, which can be configured to transfer heat to a
second side 64 thereof to provide heat to the warm air ducts 66 of
a heated air space 68 such as a home or office building. An inducer
fan or blower 70 coupled to the combustion side 60 of the heat
exchanger 62 can be configured to draw in air through an air supply
such as an intake vent or duct for use in combustion of fuel at the
burner box 58. Combusted air 74 discharged from the heat exchanger
62 can be exhausted from the home or office building via an exhaust
vent 72.
[0023] On the non-combustion side 64 of the heat exchanger 62, a
heated air fan or blower 76 can be configured to blow heated air
through a separate path in the heat exchanger 62, similar to that
described above with respect to furnace system 10. In the
illustrative embodiment of FIG. 2, a number of thermostats 78
located in the heated air space 68 can provide input commands to a
furnace controller 80. In some embodiments, for example, one or
more thermostats 78 can be utilized to program temperature
set-points and/or set-point schedules in order to control the
temperature within the heated air space 68. The controller 80 can
be configured to provide signals back to the thermostats 78 to
provide the occupant with status information on the operation of
the furnace system 52. Examples of such status information can
include, but is not limited to, an indication of whether the
furnace is currently on or off, a fault or error message indicating
if one or more of the components of the furnace needs servicing
and/or maintenance, a message regarding the last time the furnace
system was serviced, etc.
[0024] The furnace controller 80 can include a motor speed control
unit 82 capable of varying the speed of the inducer fan 70. The
inducer fan 70 can comprise a multi-speed or variable speed fan or
blower capable of adjusting the combustion air flow between either
a number of discrete airflow positions or variably within a range
of airflow positions. In certain embodiments, for example, the
inducer fan 70 can vary the combustion air flow 74 through the
combustion side 60 of the furnace between an infinite number of
positions within the speed range of the fan 70, allowing the
furnace to draw in supply air into the burner box 58 and heat
exchanger 62 at a variable rate. In some embodiments, the motor
speed controller unit 82 can also vary the rate at which the heated
air fan or blower 76 discharges heated air into the warm air ducts
66.
[0025] Although the furnace controller 80 depicted in FIG. 2 is
equipped with an on-board motor speed control unit 82 for
controlling the inducer fan 70 and/or heated air fan or blower 76,
the furnace system 52 can alternatively employ a motor speed
controller separate from the furnace controller 80. For example,
the motor speed controller 82 could be provided as a part of the
inducer fan 70, or as a stand-alone unit in communication with the
furnace controller 80 and inducer fan 70.
[0026] In the illustrative embodiment of FIG. 2, the gas valve 54
is pneumatically driven via pressure signals received from the
input and output sides 84,86 of the heat exchanger 62. A first
pneumatic conduit 88 in fluid communication with the input side 84
of the heat exchanger 62, for example, can be used to provide a
first, relatively-low pneumatic negative pressure signal for the
gas valve 54. A second pneumatic conduit 90 in fluid communication
with the output side 86 of the heat exchanger 62, in turn, can be
used to provide a second, relatively-high pneumatic negative
pressure signal for the gas valve 54. During operation, the
differential pressure between the first and second pneumatic
pressure signals can be used to modulate the firing rate outputted
by the gas valve 54 in order to adjust the air/fuel ratio within
the burner box 58.
[0027] In some embodiments, and as shown in FIG. 2, the pneumatic
conduits 88,90 can be coupled to a pneumatic amplifier 92, which
amplifies a differential pressure control signal 94 fed to the gas
valve 54. Although an amplifier 92 can be employed to adjust the
gain of the control signal 94, it should be understood that the gas
valve 54 can be configured to operate without such amplifier 92, if
desired. In addition, while the differential pressure control
signal 94 can be developed by the pressure drop of combustion air
across the heat exchanger 62, other locations such across the
inducer fan 70 or at the input to the burner box 58 could also be
used to provide the desired pressure signals. In some cases,
modulation of the gas valve 54 can be accomplished via electrical
signals received from the furnace controller 80 or from some other
component, if desired.
[0028] In use, gas supplied to the gas manifold 56 and burner box
58 is automatically modulated based on the pressure differential of
the combustion air across the heat exchanger 62. If, for example,
the combustion air flow through the heat exchanger 62 is increased,
the corresponding increase in pressure differential between the
pneumatic conduits 88,90 causes the gas valve 54 to increase the
firing rate in order to maintain a particular air/fuel ratio at the
burner box 58. If, conversely, the combustion air flow through the
heat exchanger 62 is decreased, the corresponding decrease in
pressure differential between the pneumatic conduits 88,90 causes
the gas valve 54 to decrease the firing rate. Typically, the gas
firing rate outputted by the gas valve 54 will be linear with
respect to the combustion air flow produced by operation of the
inducer fan 70, although other non-linear configurations are
possible.
[0029] The pressure metered fuel outputted from the gas valve 54
can be fed to the gas manifold 56, which injects the fuel into the
burner box 58 for combustion. An ignition element 96 such as an AC
hot surface ignition element, direct spark igniter, or other
suitable igniter can then activated via the controller 80 to ignite
the air/fuel mixture within the burner box 58. If desired, a flame
rollout switch 98 and flame sensor 100 can be used by the
controller 80 to monitor the presence of a flame within the burner
box 58.
[0030] The motor speed control unit 82 can be configured to control
the firing rate of the gas valve 54 at a desired value or within a
range of values by adjusting the rotational speed of the inducer
fan 70. The motor speed control unit 82 can include a
microprocessor that calculates the air flow (CFM) based at least in
part by sensing the fan speed and/or by measuring the motor voltage
and/or current within the inducer fan 70. For example, in some
embodiments the voltage and/or current used to operate the inducer
fan motor can be measured and then correlated with a conversion
factor or map stored within the motor speed control unit 82 in
order to compute the combustion air flow produced by the inducer
fan 70. From this calculation, the heat input to the heat exchanger
62 can then be determined, and based on the heat transfer
properties of the system, can be used to determine the supply air
temperature.
[0031] By sensing and computing the supply air temperature via
feedback signals received from the inducer fan 70 and/or the heated
air blower 76, the furnace system 52 obviates the need for
additional sensors such as thermal sensors, mass flow sensors,
and/or pressure sensors in the combustion air flow or
non-combustion air flow path. With respect to the furnace system 10
described above with respect to FIG. 1, for example, the ability to
compute the supply temperature via feedback from the inducer fan 70
and/or heated air blower 36 obviates the need for a supply air
temperature sensor. In some cases, the elimination of this sensor
may reduce the complexity associated with installation of the
furnace system 52, and may reduce power consumption and/or the
occurrence of sensor faults.
[0032] FIG. 3 is a diagrammatic view showing several illustrative
inputs and outputs to the furnace controller 80 of FIG. 2. As shown
in FIG. 3, the furnace controller 80 can be configured to receive
as inputs 102 a thermostat signal 104, a flame sensor signal 106, a
fan speed signal 108, and a fan voltage/current signal 110. The
thermostat signal 104 can include set-points values received from
the thermostats as well as other status and operational
information. When a flame sensor is employed, the flame sensor
signal 106 can be fed to the controller 80 to permit the controller
80 to shut-off the supply of gas fed to the burner box in case a
flame is not present or is insufficient. For example, an off signal
received from the flame sensor can cause the controller 80 to
shut-off the supply of gas fed to the gas valve until at such point
the ignition element can be configured to reestablish ignition.
[0033] The fan speed signal 108 can be utilized by the on-board
motor speed control unit 82 compute the temperature of the supply
air fed to the burner box based on the combustion air flow, as
discussed above. The fan speed signal 108 can be sensed, for
example, via a sensor (e.g. a Hall effect sensor, reed switch,
magnetic sensor, optical sensor, etc.) in order to compute the
combustion air flow produced by the inducer fan or blower wheel. In
some embodiments, for example, rotational speed of the inducer fan
can be determined via a sensor or switch located adjacent the
blower wheel used in some fan or blower configurations. The manner
in which the speed signal 108 is obtained will differ, however,
depending on the type of fan configuration employed. From the fan
speed signal 108, the controller 80 can be configured to compute
the supply air temperature from the heat transfer properties of the
heat exchanger.
[0034] A fan voltage/current signal 110 can also be received in
addition to, or in lieu of, the fan speed signal 108 for computing
the combustion air flow through the combustion side of the furnace
system. In some embodiments, for example, the fan voltage/current
signal 110 can be determined by directly measuring the power drop
across a resistive element (e.g. a high-precision resistor) coupled
to the fan motor or by other methods such as via a resistive bridge
circuit. As with the fan speed signal 108, the fan voltage/current
signal 110 can be used to compute the heat provided to the heat
exchanger, which, in turn, can be used to compute the supply air
temperature.
[0035] As indicated generally by reference number 112, the furnace
controller 80 can be configured to receive one or more other
signals for controlling other aspects of the furnace system.
Examples of other types of signals 112 can include actuator signals
from other furnace components such as any dampers or shut-off
valves as well as power signals from the other furnace components.
It should be understood that the types of signals fed to the
controller 80 will typically depend on the type of gas-power
appliance being controlled.
[0036] The outputs 114 of the controller 80 can include a
thermostat signal 116 for communicating with each thermostat, a
gas-shut-off signal 118 for controlling the supply of gas to the
gas valve, and an igniter signal 120 for ignition of fuel within
the burner box. An inducer fan speed signal 122 outputted to the
inducer fan can be provided to control the speed of the fan to
either increase or decrease the combustion air flow. A heated air
blower speed signal 124, in turn, can be outputted to the heated
air fan or blower to control the operational times and/or speed of
the heated air discharged into the warm air ducts. As indicated
generally by reference number 126, the controller 80 can also be
configured to output one or more other signals, if desired.
[0037] FIG. 4 is a diagrammatic view showing several illustrative
inputs and outputs to an alternative furnace system having a
separate furnace controller 128 and a motor speed control unit 130.
The inputs 132 to the furnace controller 128 can be similar to that
discussed above with respect to FIG. 3, including the thermostat
signal 104, the flame sensor signal 106, as well as other signals
112. The outputs 134 to the furnace controller 128, in turn, can
include the thermostat signal 116, the gas shut-off signal 118, the
igniter signal 120, as well as other signals 126.
[0038] As illustrated diagrammatically in FIG. 4, the motor speed
control unit 130 can comprise a separate unit from the furnace
controller 128. In certain embodiments, for example, the motor
speed control unit 130 can be a part of the inducer fan, or a
separate component in communication with the furnace controller 128
and inducer fan. The motor speed control unit 130 can communicate
with the furnace controller 128 via a communications bus 136. In
some embodiments, for example, the motor speed control unit 130 can
be configured to communicate with the furnace controller 128 over
an ENVIRACOM platform developed by Honeywell, Inc. It should be
understood, however, that the motor speed control unit 130 can be
configured to communicate using a wide range of other platforms
and/or standards, as desired.
[0039] FIG. 5 is a flow chart showing an illustrative method 138 of
operating the warm-air furnace system of FIG. 2. Beginning at block
140, a heat request signal from one or more of the thermostats 78
(e.g. from a user adjusting the temperature setpoint upwardly) can
cause the furnace controller 80 to activate the inducer fan 70,
causing the fan 70 to discharge combustion air through the exhaust
vent 72. The initial speed of the inducer fan 70 can be set based
on the inputted temperature set-point received at the thermostat
78, or can be predetermined via software and/or hardware within the
motor speed control unit 82. During this period, the ignition
element 96 can be heated to a temperature sufficient for ignition
of the burner elements within the burner box 58. In those gas-fired
appliances employing an AC hot surface ignition element, for
example, an AC line voltage of either 120 VAC or 24 VAC can be
applied to heat the element to a temperature sufficient to cause
ignition.
[0040] Once the inducer fan 70 is at its proper ignition speed and
the ignition element 96 is at the proper ignition temperature, the
controller 80 may then power the gas valve 54, as indicated
generally by block 142, forcing metered fuel into the burner box 58
for combustion. Upon activation, the ignition element 96 may ignite
the fuel causing a flame to develop, which can then be sensed via
the flame sensor 100, as indicated generally by block 144. After
the heat exchanger 62 warms for a predetermined period of time
(e.g. 15 to 30 seconds), the heated air fan or blower 76 can then
be activated to direct cold air across the heat exchanger 62 and
into the warm air ducts 66, as indicated generally by block
146.
[0041] Once ignition is proven, the ignition element 96 can then be
deactivated and the controller 80 tasked to adjust the speed of the
inducer fan 70 to meet the heat demand set-points received by the
thermostats 78, as indicated generally by block 148. The furnace
controller 80 can be configured to sense and/or measure the speed
of the inducer fan 70, as indicated generally by block 150. Sensing
of the inducer fan speed can be accomplished, for example, with a
sensor, switch, or other suitable means for sensing rotation of the
blower wheel or other component of the inducer fan 70.
[0042] In an alternative method 158 depicted in FIG. 6, the furnace
controller 80 can be configured to sense the voltage and/or current
within the inducer fan motor, which can also be used by the
controller 80 to compute the supply air temperature to the burner
box 58. Method 158 may be similar to that of FIG. 5, with like
steps labeled in like fashion in the drawings. As indicated
generally by block 160, however, the furnace controller 80 can be
configured to measure the voltage/current of the inducer fan motor
in order to determine the combustion air flow. The measurement of
the voltage and/or current within the inducer motor can be
accomplished, for example, by measuring the voltage or current drop
across a reference resistor, or using an electrical bridge circuit
such as a Wheatstone bridge.
[0043] From the sensed speed at block 150 in FIG. 5, or from
voltage and/or current measurements made at block 160 in FIG. 6,
the furnace controller 80 can then calculate the supply air
temperature to the burner box 58, as indicated generally by block
152. Calculation of the supply air temperature can be accomplished,
for example, using conversion factors or maps based at least in
part on the heat transfer characteristics of the heat exchanger 62,
the air flow characteristics of the inducer fan 70, and the
dimensions of the combustion air flow conduit.
[0044] Once the supply air temperature has been computed at block
152, the furnace controller 80 may next adjust the speed of the
inducer fan 70 in order to achieve the temperature set-point
received by the thermostats 78, as indicated generally by block
154. If, for example, the controller 80 determines that an increase
in air flow is necessary based on the calculated temperature of the
supply air fed to the heat exchanger 62, the controller 80 can
increase the rotational speed of the inducer fan 70. Conversely, if
the controller 80 determines that a decrease in air flow is
necessary based on the calculated supply air temperature, the
controller 80 can decrease the rotational speed of the inducer fan
70.
[0045] As the controller 80 adjusts the speed of the inducer fan 70
either upwardly or downwardly depending on the heating demand, the
combustion air flow will likewise fluctuate causing a change in air
pressure across the heat exchanger 62. This change in pressure can
then be sensed by the gas valve 54 via the pneumatic conduits
88,90. As indicated generally by block 156, the gas valve 54 can
then modulate the fuel fed to the burner box 58 based on these
pressure signals. The process of sensing and/or measuring the speed
of the inducer fan 70 or the voltage/current of the inducer fan
motor, computing the supply air temperature, and then adjusting the
speed of the inducer fan 70 based on the calculated supply air
temperature in order to modulate the gas valve can then be
repeated, as necessary, to achieve or maintain the desired
temperature set-point.
[0046] FIG. 7 is a graph 162 showing the change in combustion air
pressure .DELTA.P.sub.air as a function of gas valve output
pressure P.sub.g for the illustrative furnace system 52 of FIG. 2.
Beginning at point 164, when a sufficient pressure differential
.DELTA.P.sub.air between the pneumatic conduits 88,90 is sensed,
the gas valve 54 can be configured to open and output gas pressure
to the burner box 58. In some embodiments, the pressure
differential .DELTA.P.sub.air at which the gas valve 54 opens can
be adjusted by a negative offset 166 so that the gas valve 54 is
not opened until a minimum amount of combustion air flow is
present. Such offset, for example, can be utilized to prevent the
gas valve 54 from opening unless a sufficient flow of combustion
air is present at the burner box 58.
[0047] Once the gas valve 54 is initially opened at point 164, the
gas pressure P.sub.g outputted by the gas valve 54 increases in
proportion to the pressure change .DELTA.P.sub.air produced by the
pressure signals received from the pneumatic conduits 88,90, as
illustrated generally by ramp 168. In those embodiments employing
an amplifier 92, the slope of the ramp 168 will typically be
greater due to the amplification of the pressure differential
.DELTA.P.sub.air fed to the gas valve 54.
[0048] In some embodiments, the gas valve 54 can be equipped with a
high-fire pressure regulator in order to limit the gas pressure
outputted from the gas valve 54 once it reaches a particular point
170 along the ramp 124. When a high-fire pressure regulator is
employed, and as illustrated generally by line 172, the gas
pressure P.sub.g outputted by the gas valve 54 will not exceed a
maximum gas pressure P.sub.g(max), thus preventing over-combustion
at the burner box 58.
[0049] Having thus described the several embodiments of the present
invention, those of skill in the art will readily appreciate that
other embodiments may be made and used which fall within the scope
of the claims attached hereto. Numerous advantages of the invention
covered by this document have been set forth in the foregoing
description. It will be understood that this disclosure is, in many
respects, only illustrative. Changes can be made with respect to
various elements described herein without exceeding the scope of
the invention.
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