U.S. patent application number 11/607729 was filed with the patent office on 2008-06-05 for four-stage high efficiency furnace.
This patent application is currently assigned to Carrier Corporation. Invention is credited to Kevin D. Thompson.
Application Number | 20080127963 11/607729 |
Document ID | / |
Family ID | 39474321 |
Filed Date | 2008-06-05 |
United States Patent
Application |
20080127963 |
Kind Code |
A1 |
Thompson; Kevin D. |
June 5, 2008 |
Four-stage high efficiency furnace
Abstract
A four-stage furnace includes a gas flow control assembly
operable to provide gas to the furnace at one of four firing rates.
The furnace also includes a heat exchanger and an inducer blower
for establishing a flow of combustion gas that generates a
differential pressure across the heat exchanger. A furnace
controller controls the rate at which the gas flow control assembly
provides gas to the furnace and controls the inducer blower to
establish one of four combustion gas flows that corresponds to one
of the four firing rates.
Inventors: |
Thompson; Kevin D.;
(Indianapolis, IN) |
Correspondence
Address: |
KINNEY & LANGE, P.A.
THE KINNEY & LANGE BUILDING, 312 SOUTH THIRD STREET
MINNEAPOLIS
MN
55415-1002
US
|
Assignee: |
Carrier Corporation
Farmington
CT
|
Family ID: |
39474321 |
Appl. No.: |
11/607729 |
Filed: |
December 1, 2006 |
Current U.S.
Class: |
126/116A |
Current CPC
Class: |
F23N 2237/10 20200101;
F23N 2225/06 20200101; F23N 1/062 20130101; F23N 5/184 20130101;
F23N 2241/02 20200101; F24H 3/105 20130101 |
Class at
Publication: |
126/116.A |
International
Class: |
F24H 9/20 20060101
F24H009/20 |
Claims
1. A four-stage furnace comprising: a gas flow control assembly
operable to provide gas to the furnace at one of four firing rates;
a heat exchanger; an inducer blower for establishing a flow of
combustion gases that generates a differential pressure across the
heat exchanger; and a furnace controller for controlling the rate
at which the gas flow control assembly provides gas to the furnace
and for controlling the inducer blower to establish one of four
combustion gas flows that corresponds to one of the four firing
rates.
2. The furnace of claim 1, wherein the gas flow control assembly
comprises: a main gas valve having first and second open states;
and a throttling valve connected in fluidic series with the main
gas valve having at least first and second open states.
3. The furnace of claim 1, and further comprising: a pressure
switch assembly including a first pressure switch and a second
pressure switch, wherein the pressure switch assembly senses
differential pressure across the heat exchanger and generates first
and second pressure signals that vary in accordance with the
differential pressure sensed across the heat exchanger.
4. The furnace of claim 3, wherein the furnace controller controls
the furnace to operate at first and second firing rates as a
function of the first pressure signal from the first pressure
switch, and wherein the furnace controller controls furnace to
operate at third and fourth firing rates as a function of the
second pressure signal from the second pressure switch.
5. The furnace of claim 1, wherein the combustion gas flows
established by the inducer blower for ignition at first and second
firing rates are substantially equal, and wherein the combustion
gas flows established for ignition at third and fourth firing rates
are substantially equal.
6. The furnace of claim 1, wherein the combustion gas flows
established during steady state operation of the furnace at the
four firing rates have different respective magnitudes.
7. The furnace of claim 1, and further comprising: a circulating
air blower that is controlled by the furnace controller for
establishing a flow of circulating air from the furnace.
8. The furnace of claim 7, wherein the circulation air flows
established during steady state operation of the furnace at the
four firing rates have different respective magnitudes.
9. A four-stage furnace including an inducer blower for
establishing a flow of combustion gas, a circulating air blower for
establishing a flow of circulating air, and a heat exchanger, the
furnace comprising: a gas flow control assembly for controlling a
rate at which gas is supplied to the furnace to one of a first
firing rate, a second firing rate, a third firing rate, and a
fourth firing rate; a pressure switch assembly including a first
pressure switch and a second pressure switch, wherein the pressure
switch assembly senses differential pressure across the heat
exchanger and generates pressure signals that vary in accordance
with the differential pressure sensed across the heat exchanger;
and a furnace controller for controlling the rate at which the gas
flow control assembly supplies gas to the furnace, for controlling
speeds of the inducer blower and the circulating air blower, and
for receiving the differential pressure signals from the pressure
switch assembly.
10. The furnace of claim 9, wherein the gas flow control assembly
comprises: a main gas valve having first and second open states;
and a throttling valve connected in fluidic series with the main
gas valve having at least first and second open states.
11. The furnace of claim 9, wherein the furnace controller controls
the furnace to operate at the first and second firing rates as a
function of the first pressure signal from the first pressure
switch, and wherein the furnace controller controls furnace to
operate at the third and fourth firing rates as a function of the
second pressure signal from the second pressure switch.
12. The furnace of claim 9, wherein a magnitude of the combustion
gas flow established for ignition at the first and second firing
rates are substantially equal, and wherein a magnitude of the
combustion gas flow established for ignition at the third and
fourth firing rates are substantially equal.
13. The furnace of claim 9, wherein the combustion gas flows
established during steady state operation of the furnace at the
first, second, third, and fourth firing rates have different
respective magnitudes.
14. The furnace of claim 9, wherein the circulation air flows
established during steady state operation of the furnace at the
first, second, third, and fourth firing rates have different
respective magnitudes.
15. A furnace comprising: a main gas valve having low open state
and a high open state; a throttling valve connected in fluidic
series with the main gas valve and having a low open state and a
high open state; and a furnace controller for operating the main
gas valve and the throttling valve to supply a gas to the furnace
at one of a low firing rate, a medium-low firing rate, a
medium-high firing rate, and a high firing rate.
16. The furnace of claim 15, wherein the main gas valve and the
throttling valve are in their low open states for the low firing
rate, the main gas valve is in its low open state and the
throttling valve is in its high open state for the medium-low
firing rate, the main gas valve is in its high open state and the
throttling valve is in its low open state for the medium-high
firing rate, and the main gas valve and the throttling valve are in
their high open states for the high firing rate.
17. The furnace of claim 15, and further comprising: a heat
exchanger; and an inducer blower controlled by the furnace
controller for establishing a flow of combustion gas that generates
a differential pressure across the heat exchanger that corresponds
to one of the four firing rates.
18. The furnace of claim 17, wherein the combustion gas flows
established by the inducer blower for ignition at the low and
medium-low firing rates are substantially equal, and wherein the
combustion gas flows established for ignition at the medium-high
and high firing rates are substantially equal.
19. The furnace of claim 17, wherein the combustion gas flows
established during steady state operation of the furnace at the
four firing rates have different respective magnitudes.
20. The furnace of claim 15, and further comprising: a pressure
switch assembly for sensing pressure across the heat exchanger and
including a low pressure switch and a high pressure switch, wherein
the furnace controller controls the furnace to operate at the low
and medium-low firing rates as a function of a pressure signal from
the low pressure switch, and wherein the furnace controller
controls furnace to operate at medium-high and high firing rates as
a function of a pressure signal from the high pressure switch.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to the field of gas furnaces,
and in particular to an induced-draft, gas-fired furnace that is
adapted to operate at any of four different firing rates.
[0002] With a furnace for heating a residential or commercial
space, a thermostat senses when the temperature of an interior
comfort space is below a set temperature. When the temperature
drops below the set temperature, the thermostat provides a call for
heat that turns on a gas burner and, after a delay time, a
circulation air blower. The gas burner injects flame and heated gas
into a heat exchanger, which heats the circulation air that is then
returned to the interior space. An induced combustion fan draws
combustion gases through the heat exchanger and exhausts them into
a vent pipe for discharge to an outside environment. Heating
continues until the thermostat senses that the interior room air
has been heated above the set point, at which time it opens and
ends the call for heat.
[0003] Multi-stage furnaces have gas burners that operate at
different flow rates, ranging from a high flow rate (i.e., high
fire) to varying levels of partial flow rates. The high fire mode
is employed when there is a high demand for heating, such as when
the partial flow rates fail to increase the interior room air
temperature above the set point in an allotted time or when
specifically commanded by the thermostat. The partial flow rates
are employed when there is a lower demand for heat, and the gas
burners provide a corresponding level of fire proportionate to the
demand for heat.
[0004] The gas burners can be actuated into the various flow rate
modes based on the states of combustion pressure switches in the
furnace. Combustion pressure switches, which sense the negative
pressure in the furnace combustion chamber, serve to turn the
burners on only if the inducer fan is bringing enough combustion
air in to support the level of fire provided by the burners. In
conventional furnace systems, the furnace control is designed to
have the same number of pressure switch inputs as the number of
operating modes supported. Thus, a change in the number of
operating modes in the furnace typically requires a change to the
control circuitry of the furnace.
BRIEF SUMMARY OF THE INVENTION
[0005] The subject invention is directed to a four-stage furnace
including a gas flow control assembly operable to provide gas to
the furnace at one of four firing rates. The furnace also includes
a heat exchanger and an inducer blower for establishing a flow of
combustion air that generates a differential pressure across the
heat exchanger. A furnace controller controls the rate at which the
gas flow control assembly provides gas to the furnace and controls
the inducer blower to establish one of four combustion air flows
that corresponds to one of the four firing rates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a perspective, cutaway view of a two-stage
furnace.
[0007] FIG. 2 is a schematic diagram of the gas flow control
portion of a furnace for four-stage operation.
[0008] FIG. 3 is a graph of the heat exchanger differential
pressure and the circulating airflow of a four-stage furnace versus
time during high, medium-high, medium-low, and low stages of
operation.
DETAILED DESCRIPTION
[0009] A four-stage furnace constructed in accordance with the
present invention comprises adaptations of a similar conventional
two-stage furnace. Accordingly, the following description will
first discuss the structure and operation of a two-stage furnace
that is known in the art, and then discuss how the structure and
operation of a four-stage furnace that is constructed in accordance
with the present invention differs from the conventional two-stage
furnace.
[0010] FIG. 1 is a perspective cutaway view of a conventional
two-stage condensing furnace 10. Furnace 10 includes burner
assembly 12, burner box 14, air supply duct 16, gas valve 18,
primary heat exchanger 20, condensing heat exchanger 24, condensate
collector box 26, exhaust vent 28, induced draft blower 30, inducer
motor 32, thermostat 34, low pressure switch 42, high pressure
switch 44, pressure tubes 46 and 48, blower 50, blower motor 52,
and furnace control 54.
[0011] Burner assembly 12 is located within burner box 14 and is
supplied with air via air supply duct 16. The gases produced by
combustion within burner box 14 flow through a heat exchanger
assembly, which includes primary or non-condensing heat exchanger
20, secondary or condensing heat exchanger 24, and condensate
collector box 26. The gases are then vented to the atmosphere
through exhaust vent 28. The flow of these gases, herein called
combustion gases, is maintained by induced draft blower 30, which
is driven by inducer motor 32. Inducer motor 32 is driven in
response to speed control signals that are generated by furnace
control 54, in response to the states of low pressure switch 42 and
high pressure switch 44, and in response to call-for-heat signals
received from thermostat 34 in the space to be heated.
[0012] Fuel gas is supplied to burner assembly 12 through gas valve
18, and is ignited by an igniter assembly (not shown). Gas valve 18
may comprise a conventional, solenoid operated two-stage gas valve
which has a closed state, a high open state associated with the
operation of furnace 10 at its high firing rate, and a low open
state associated with the operation of furnace 10 at its low firing
rate.
[0013] Air from the space to be heated is drawn into furnace 10 by
blower 50, which is driven by blower motor 52 in response to speed
control signals that are generated by furnace control 54. The
discharge air from the blower 50, herein called circulating air,
passes over condensing heat exchanger 24 and primary heat exchanger
20 in a counterflow relationship to the flow of combustion gases,
before being directed to the space to be heated through a duct
system (not shown). While the present invention is described with
regard to condensing furnaces (i.e., furnaces that use heat
exchanger assemblies that include primary and secondary heat
exchangers), it will be appreciated that the concepts of the
present invention are also applicable to non-condensing furnaces
(i.e., furnaces that have heat exchanger assemblies with only a
single heat exchanger unit).
[0014] In two-stage furnace 10, inducer motor 32 and blower motor
52 operate at a low speed when the furnace is operating at its low
firing rate (low stage operation) and at a high speed when the
furnace is operating at its high firing rate (high stage
operation). Motors 32 and 52 may be motors that are designed to
operate at a continuously variable speed, and to operate at their
low and high speeds in response to speed control signals generated
by furnace control 54. Furnace control 54 may control the steady
state low and high operating speeds of motors 32 and 52 and the
times and the rates or torques at which they accelerate to and
decelerate from these operating speeds.
[0015] The combustion efficiency of an induced-draft gas-fired
furnace is optimized by maintaining the proper ratio of the gas
input rate and the combustion air flow rate. Generally, the ideal
ratio is offset somewhat for safety purposes by providing for
slightly more combustion air (i.e., excess air) than that required
for optimum combustion efficiency. In furnace 10, the excess air
level is kept within acceptable limits in part by low and high
pressure switches 42 and 44, respectively, which cause inducer
motor 32 to run at speeds that are related to the differential
pressure across the heat exchanger assembly. Low and high pressure
switches 42 and 44 are connected to burner box 14 through pressure
tube 46 to sense the pressure at the inlet of primary heat
exchanger 20, and are connected to collector box 26 through a
pressure tube 48 to sense a pressure at the outlet of secondary
heat exchanger 24.
[0016] When thermostat 34 provides a call-for-heat signal to
furnace control 54 and furnace control 54 determines that furnace
10 is to operate at its low firing rate, furnace control 54
accelerates inducer motor 32 until it attains a pre-ignition steady
state speed corresponding to a heat exchanger differential pressure
that is sufficient to actuate low pressure switch 42, but not high
pressure switch 44. When this differential pressure has been
sustained for a preset time, gas valve 18 assumes its low open
state. Under this condition, gas valve 18 supplies gas at the low
firing rate to burner assembly 12, which ignites the gas and begins
heating the combustion gases passing through the heat exchange
assembly. This heating initiates a change in the density of the
combustion gases which, in turn, causes an increase in the
differential pressure across the heat exchange assembly. The speed
of inducer motor 32 is then reduced until it attains a steady state
speed value that corresponds to a heat exchanger differential
pressure that is somewhat lower than its pre-ignition value. After
reducing the speed of inducer motor 32, furnace control 54 provides
a signal that causes blower motor 52 to accelerate until it reaches
a steady state speed that corresponds to a circulating airflow at
which furnace 10 is designed to operate at low stage.
[0017] Similarly, when thermostat 34 provides a call-for-heat
signal to furnace control 54 and furnace control 54 determines that
furnace 10 is to operate at its high firing rate, furnace control
54 accelerates inducer motor 32 until it attains a pre-ignition
steady state speed that corresponds to a heat exchanger
differential pressure that is sufficient to actuate both low
pressure switch 42 and high pressure switch 44. When this
differential pressure has been sustained for a preset time, gas
valve 18 assumes its high open state. Under this condition, gas
valve 18 supplies gas at the high firing rate to burner assembly
12, which ignites the gas and begins heating the combustion gases
passing through the heat exchanger assembly. This heating initiates
a change in the density of the combustion gases which, in turn,
causes an increase in the differential pressure across the heat
exchange assembly. The speed of inducer motor 32 is then increased
to attain a steady state speed value that corresponds to a heat
exchanger differential pressure that is somewhat higher than its
pre-ignition value. After increasing the speed of inducer motor 32,
furnace control 54 causes blower motor 52 to accelerate to a steady
state speed value that corresponds to the circulating airflow value
at which furnace 10 is designed to operate.
[0018] In order to reduce the operating cost of furnace 10 by
improving its annual fuel utilization efficiency (AFUE), the
combustion gas flow for furnace 10 may be adapted to provide for
intermediate stages of operation between the low and high stages of
operation. In accordance with the present invention, furnace 10 is
modified to provide four-stages of operation including low,
medium-low, medium-high, and high states of operation. As will be
described in more detail below, this may be accomplished by
adapting pressure switches 42 and 44 for use in association with
two stages of operation each, adding a throttling valve to the gas
control portion of furnace 10 to provide an additional gas
regulating component, and programming furnace control 54 to provide
control for four stages of operation.
[0019] FIG. 2 is a schematic view of gas flow portion 60 of a
furnace that is configured for four-stage operation. Similar
components between gas flow portion 60 and the gas flow portion of
furnace 10 (FIG. 1) are labeled with like numbers, including gas
valve 18, inducer motor 32, thermostat 34, low pressure switch 42,
high pressure switch 44, blower motor 52, and furnace control 54.
Gas flow portion 60 also includes throttling valve relay 62
(including switch 62a and solenoid 62b), gas valve relay 66
(including switch 66a and solenoid 66b), and throttling valve
68.
[0020] The operation of gas control portion 60 is monitored and
controlled by furnace control 54, which includes control CPU 58
which has connection pins, labeled P1, P2, P3, P4, P5, P6, P7, P8,
P9, and P10, to provide signals to and receive signals from the
components of gas flow portion 60. Thermostat 34 is connected to
pin P1 to communicate with control CPU 58, and power is supplied
from a 24-VAC transformer secondary to thermostat 34 and to pin P2
of control CPU 58. Relay solenoids 62b and 66b are connected to
pins P4 and P8, respectively, to receive energizing signals from
control CPU 58. The pole of single pole, single throw relay switch
62a is connected to pin P3, and the normally closed contact side of
relay switch 62a is connected to throttling valve 68. The poles of
single pole, single throw pressure switches 42 and 44 are also
connected to pin P3 of control CPU 58. The output contact of low
pressure switch 42 is connected to pin P7 to provide an ON/OFF
pressure signal to control CPU 58. The pole of relay switch 66a is
also connected to pin P7, and the normally open output contact of
relay switch 66a is connected to pin P6 and to main and redundant
solenoids 70 and 72 of gas valve 18. The output of high pressure
switch 44 is connected to pin P5 and to high-fire solenoid 74 of
gas valve 18. Throttling valve 68 is connected between the output
contact of normally closed relay switch 62a and ground. Control CPU
58 provides control signals to inducer motor 32 and blower motor 52
via pins P9 and P10, respectively. It should be noted that the
schematic in FIG. 2 only shows the connectivity of components of
gas flow portion 60 in the furnace, and components from other
portions of a heating, ventilation, and air conditioning (HVAC)
system may also be connected to and controlled by furnace control
54. However, these components are omitted from FIG. 2 for
clarity.
[0021] In order to provide four stages of operation, pressure
switches 42 and 44 are each associated with two stages of
operation. More specifically, pressure switch 42 is used in
establishing and maintaining low and medium-low stages of operation
of the furnace, while pressure switch 44 is used in establishing
and maintaining medium-high and high stages of operation. To
accomplish this, pressure switch 42 is configured to actuate when
inducer motor 32 reaches a speed that establishes a differential
pressure across the heat exchanger corresponding to low and
medium-low stage operation. Pressure switch 44 is configured to
actuate when inducer motor 32 reaches a speed that establishes a
differential pressure across the heat exchanger corresponding to
medium-high and high stage operation, but not at differential
pressures corresponding to low and medium-low stage operation. When
actuated, pressure switches 42 and 44 provide signals via pins P7
and P5, respectively, to control CPU 58.
[0022] Throttling valve 68 may comprise a multi-stage throttling
valve having at least a first, high open state that provides a low
resistance to the flow of gas, and a second, low open state that
provides a relatively high resistance to the flow of gas.
Throttling valve 68 is disposed in fluidic series between burner
box 14 and gas valve 18 (FIG. 1). When the solenoid of throttling
valve 68 is de-energized, it is in its low open state, and when the
solenoid of throttling valve 68 is energized, it is in its high
open state. The open state of throttling valve 68 is a function of
the state of throttling valve relay 62, which is controlled by
control CPU 58 using a time based staging algorithm that determines
staging based on the duration of the call-for-heat signals provided
by thermostat 34. As will be described in more detail below,
control CPU 58 controls the open states of gas valve 18 and
throttling valve 68 to provide four firing rates corresponding to
four stages of operation.
[0023] Control CPU 58 is programmed to control inducer motor 32 and
blower motor 52, in conjunction with gas valve 18 and throttling
valve 68, to provide four stages of operation in the furnace. FIG.
3 is a graph of the heat exchanger differential pressure (HXDP) and
the circulating airflow, expressed in cubic feet per minute (Blower
CFM), in a four-stage furnace as controlled by control CPU 58. The
left-side vertical axis of FIG. 3 may also be regarded as showing
the speed of inducer motor 32, since the magnitude of the HXDP is
related to the speed of inducer motor 32. The HXDP and Blower CFM
are plotted versus the time elapsed since the call-for-heat signal
was received from thermostat 34. The operation of a furnace
including gas control portion 60 will now be described for each of
the four stages of operation with reference to FIG. 3.
[0024] When thermostat 34 provides a call-for-heat signal to
furnace control 54 and control CPU 58 determines that the furnace
should operate at its low or medium-low stage of operation, control
CPU 58 then accelerates inducer motor 32 until it attains a
pre-ignition steady state speed corresponding to a heat exchanger
differential pressure, HXDP-ML/L-1, that is sufficient to actuate
low pressure switch 42, but not high pressure switch 44. This
provides power at the pole of relay switch 66a.
[0025] When differential pressure HXDP-MUL-1 has been sustained for
a preset time, gas valve 18 and throttling valve 68 assume states
that correspond to the medium-low firing rate for ignition. The
medium-low firing rate is used for ignition of both the low and
medium-low firing rates because ignition at the low firing rate may
not be possible for ignition (but is sufficient to support
combustion after ignition). To provide the medium-low firing rate,
control CPU 58 energizes solenoid coil 66b to close relay switch
66a. When relay switch 66a is closed, power is provided to main and
redundant solenoids 70 and 72, which causes gas valve 18 to assume
its low open state. In addition, control CPU 58 keeps relay
solenoid 62b de-energized, which maintains switch 62a in its
normally closed state and energizes the solenoid of throttling
valve 68. The combination of gas valve 18 in its low open state and
throttling valve 68 in its high open state provides the medium-low
firing rate. In one embodiment, gas is supplied at the medium-low
firing rate at 60% of the high firing rate.
[0026] Gas valve 18 and throttling valve 68 supply gas at the
medium-low firing rate to burner assembly 12, which ignites the gas
and begins heating the combustion gases passing through the heat
exchange assembly. This heating initiates a change in the density
of the combustion gases that, in turn, causes an increase in the
differential pressure across the heat exchanger assembly, as shown
in FIG. 3. At this time, for a medium-low call for heat, control
CPU 58 maintains gas valve 18 and throttling valve 68 to continue
to provide gas at the medium-low firing rate. For a low call for
heat, control CPU 58 energizes relay solenoid 62b to open relay
switch 62a and de-energize the solenoid of throttling valve 68. The
causes throttling valve 68 to assume its low open state which, in
combination with the low open state of gas valve 18, provides the
low firing rate. In one embodiment, gas is supplied at the low
firing rate at 45% of the high firing rate.
[0027] For both medium-low and low firing rates, the speed of
inducer motor 32 is then reduced until it attains a steady state
speed value that corresponds to a heat exchanger differential
pressure, HXDP-ML/L-2 that is somewhat lower than its pre-ignition
value. For the medium-low firing rate, this heat exchanger
differential pressure is maintained until operation of the furnace
is terminated or until control CPU 58 determines that it needs to
operate at another stage. For the low firing rate, the speed of
inducer motor 32 is again reduced to its low stage steady state
speed to provide a heat exchanger differential pressure, HXDP-L-3,
corresponding to low stage operation of the furnace. The heat
exchanger differential pressure HXDP-L-3 for low stage operation is
still sufficient to maintain the closed state of pressure switch
42.
[0028] Control CPU 58 then provides a signal that causes blower
motor 52 to accelerate until it reaches a steady state speed to
provide a circulating airflow corresponding to the stage of
operation. In particular, for low stage operation, blower motor 52
is accelerated to provide a circulating airflow BCFM-L at steady
state speed, and for medium-low stage operation, blower motor 52 is
accelerated to provide a circulating airflow BCFM-ML at steady
state speed.
[0029] When thermostat 34 provides a call-for-heat signal to
control CPU 58 and control CPU 58 determines that the furnace is to
operate at its medium-high or high stage of operation, control CPU
58 then accelerates inducer motor 32 until it attains a
pre-ignition steady state speed corresponding to a heat exchanger
differential pressure, HXDP-H/MH-1, that is sufficient to actuate
both low pressure switch 42 and high pressure switch 44. This
provides power at the pole of relay switch 66a and energizes
high-fire solenoid 74.
[0030] When differential pressure HXDP-H/MH-1 has been sustained
for a preset time, gas valve 18 and throttling valve 36 assume
states that correspond to the high firing rate for ignition. To
provide the high firing rate, furnace control 54 energizes solenoid
coil 66b to close relay switch 66a. When relay switch 66a is
closed, power is provided to main and redundant solenoids 70 and
72, which, in conjunction with the energized state of high-fire
solenoid 74, causes gas valve 18 to assume its high open state. In
addition, control CPU 54 keeps relay solenoid 62b de-energized,
which maintains switch 62a in its closed state and energizes the
solenoid of throttling valve 68. The combination of gas valve 18 in
its high open state and throttling valve 68 in its high open state
provides the high firing rate.
[0031] Gas valve 18 and throttling valve 68 supply gas at the high
firing rate to burner assembly 12, which ignites the gas and begins
heating the combustion air passing through the heat exchanger
assembly. This heating initiates a change in the density of the
combustion air that, in turn, causes an increase in the
differential pressure across the heat exchanger assembly, as shown
in FIG. 3. At this time, for a high call for heat, control CPU 58
maintains gas valve 18 and throttling valve 68 to continue to
provide gas at the high firing rate, and inducer motor 32 is
accelerated until it attains a steady state speed corresponding to
a heat exchanger differential pressure of HXDP-H-2.
[0032] For a medium-high call for heat, furnace control 54
energizes relay solenoid 62b to open relay switch 62a and
de-energize the solenoid of throttling valve 68. The causes
throttling valve 68 to assume its low open state which, in
combination with the high open state of gas valve 18, provides the
medium-high firing rate. In one embodiment, gas is supplied at the
medium-high firing rate at 75% of the high firing rate. Inducer
motor 32 is then decelerated until it to attains a steady state
speed corresponding to a heat exchanger differential pressure of
HXDP-MH-2.
[0033] Control CPU 58 then provides a signal that causes blower
motor 52 to accelerate until it reaches a steady state speed to
provide a circulating airflow corresponding to the stage of
operation. In particular, for medium-high stage operation, blower
motor 52 is accelerated to provide a circulating airflow BCFM-MH at
steady state speed, and for high stage operation, blower motor 52
is accelerated to provide a circulating airflow BCFM-H at steady
state speed.
[0034] A two-stage furnace modified as described above to provide
four stages of operation offers several advantages. For example, by
allowing the gas control portion of a two-stage furnace to be
adapted to provide for intermediate stages of operation, the
operating cost of the furnace is reduced without requiring a
hardware redesign of the furnace control circuit board or the
entire furnace unit. In addition, since the four-stage furnace will
ordinarily spend most of its time operating at low stage, it will
operate at a lower average noise level and provide a higher average
thermal comfort level. Furthermore, since the amount of electrical
power used during low stage operation is lower than that of other
stages of operation, the furnace will operate at a reduced
electrical operating cost.
[0035] In summary, the subject invention is directed to a
four-stage furnace including a gas flow control assembly operable
to provide gas to the furnace at one of four firing rates. The
furnace also includes a heat exchanger and an inducer blower for
establishing a flow of combustion air that generates a differential
pressure across the heat exchanger. A furnace controller controls
the rate at which the gas flow control assembly provides gas to the
furnace and controls the inducer blower to establish one of four
combustion air flows that corresponds to one of the four firing
rates. By allowing the gas control portion of a two-stage furnace
to be adapted to provide for four stages of operation, the
operating cost of the furnace is reduced without requiring a
hardware redesign of the furnace control circuit board or the
entire furnace unit.
[0036] Although the present invention has been described with
reference to examples and preferred embodiments, workers skilled in
the art will recognize that changes may be made in form and detail
without departing from the spirit and scope of the invention.
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