Modulating furnace with two-speed draft inducer

• US 5,732,691 A
• Filed: 10/30/1996
• Issued: 03/31/1998
• Est. Priority Date: 10/30/1996
• Status: Expired due to Term

• Current Assignee: Rheem Manufacturing Company (Paloma Co., Ltd.)
• Original Assignee: Rheem Manufacturing Company (Paloma Co., Ltd.)
• Inventors: Maiello, Dennis R., Willbanks, Scott A.
• Primary Examiner(s): Dority, Carroll B.
• Application Number:
  US08/752,371
• Time in Patent Office: 517 Days
• Field of Search: 126/110 R, 126/116 A
• US Class Current: 126/116.00A
• CPC Class Codes: F23N 1/022 using electronic means
• CPC Class Codes: F23N 2005/181 using detectors sensitive to rate of flow of air
• CPC Class Codes: F23N 2223/08 Microprocessor; Microcomputer
• CPC Class Codes: F23N 2225/04 pressure
• CPC Class Codes: F23N 2233/04 with variable speed
• CPC Class Codes: F23N 2235/16 variable flow or proportional valves

Abstract

A modulating, forced draft, fuel-fired air heating furnace is provided with a two-speed draft inducer fan, and a fuel valve which is fully modulatable between a maximum firing rate and a low firing rate of approximately forty percent thereof. Incorporated into the furnace control system are normally closed low and high fire pressure-electric switches which sense and are sequentially closed by increasingly negative pressure in the draft inducer fan. Upon a call for heat from a thermostat located in the conditioned space served by the furnace, the draft inducer fan is energized at its high speed setting, and a signal is sent to the fuel valve to set it at its full firing rate flow when opened by an ignition switch portion of the control system. The control system functions to open the fuel valve at this maximum flow setting, and permit light-off of the burner, only if (1) both of the low and high fire pressure-electric switches are closed, and (2) the draft inducer fan is operating at its high speed setting. In this manner, in addition to providing a wide range of furnace heating output modulation, reliable burner ignition is facilitated and heat exchanger warm-up corrosion is reduced due to the full firing rate start-up of the furnace. Moreover, the incorporation of the two-speed draft inducer fan substantially improves the overall fuel efficiency of the furnace.

Claims

1. A fuel-fired air heating furnace comprising:

a heat exchanger through which hot combustion gas may be flowed;

a two-speed draft inducer fan operatively connected to said heat exchanger and being selectively drivable at a low speed and a high speed;

an air blower operative to flow air to be heated across said heat exchanger;

a burner operative to receive fuel from a source thereof, burn the received fuel, and flow resulting hot combustion gas into the interior of said heat exchanger;

a fuel supply valve coupled to said burner and selectively operable in either a low fire mode or a high fire mode,

said fuel supply valve, in said low fire mode thereof, being operative to deliver a fixed minimum flow rate of fuel from said source thereof to said burner,

said fuel supply valve, in said high fire mode thereof, being operative to deliver, from said fuel source to said burner, a fuel flow which is modulatable from said minimum flow rate of fuel to a predetermined maximum flow rate of fuel; and

a control system including (1) normally open low and high fire switches operative to be respectively closed by the successive generation of first and second predetermined pressures within said draft inducer fan, and (2) an ignition circuit, responsive to the generation of any heating demand signal from a conditioned space served by said furnace, for initiating operation of said draft inducer fan at said high speed thereof, setting said fuel supply valve to said high fire mode thereof, and then lighting said burner if and only if each of said low and high fire switches is closed, said control system being further operative, after lighting said burner and in response to the magnitude of the heating demand signal, to automatically (1) maintain the operation of said draft inducer fan at said high speed thereof, and modulate said fuel supply valve in said high fire mode thereof between said predetermined minimum and maximum fuel flow rates thereof, or (2) cause said draft inducer fan to be driven at said low speed thereof and operate said fuel supply valve in said low fire mode thereof.

2. The fuel-fired air heating furnace of claim 1 wherein:

said minimum flow rate of fuel is approximately forty percent of said maximum flow rate of fuel.

3. The fuel-fired air heating furnace of claim 1 wherein:

under a first furnace operating condition both of said low and high fire switches are open, with said draft inducer fan being driven at said low speed thereof and said heat exchanger being at a first temperature,

under a second furnace operating condition said low fire switch closes and said high fire switch remains open, with said draft inducer fan being driven at said low speed thereof and said heat exchanger being at a second temperature higher than said first temperature,

under a third furnace operating condition said low and high fire switches are closed, with said draft inducer fan being driven at said high speed thereof and said heat exchanger being at said first temperature thereof, and

under a fourth furnace operating condition said low and high fire switches are closed, with said draft inducer fan being driven at said high speed thereof and said heat exchanger being at said second temperature thereof.

4. The fuel-fired air heating furnace of claim 3 wherein:

said low and high fire switches are pressure-electric switches coupled to said draft inducer fan to sense a pressure therein, and

said pressure in said draft inducer fan is (1) less than about -0.5" W.C. when said furnace is in said first operating condition, (2) within the range of from about -0.7" W.C. to about -0.9" W.C. when said furnace is in said second operating condition, (3) within the range of from about -1.2" W.C. to about -1.4" W.C. when said furnace is in said third operating condition, and (4) within the range of from about -1.5" W.C. to about -1.75" W.C. when said furnace is in said fourth operating condition.

5. The fuel-fired air heating furnace of claim 1 wherein said fuel supply valve is a gas fuel supply valve.

6. A fuel-fired air heating furnace comprising:

a heat exchanger through which hot combustion gas may be flowed;

a two-speed draft inducer fan operatively connected to said heat exchanger and having selectable high and low speeds;

an air blower operative to flow air to be heated across said heat exchanger;

a burner operative to receive fuel from a source thereof, burn the received fuel, and flow resulting hot combustion gas into the interior of said heat exchanger;

a fuel supply valve coupled to said burner and having a main portion and a servo portion, said fuel supply valve being selectively operable in either (1) a low fire mode in which said fuel supply valve is operative to deliver a fixed minimum flow rate of fuel from said source thereof to said burner, or (2) a high fire mode in which said fuel supply valve is operative to deliver, from said fuel source to said burner, a fuel flow which is modulatable from said minimum flow rate of fuel to a predetermined maximum flow rate of fuel; and

control means for regulating the operation of said fuel supply valve, said burner and said draft inducer fan, said control means including;

an ignition switch having an input side, and an output side coupled by first and second electrical leads to said main fuel supply valve portion,

a microprocessor coupled to (1) said draft inducer fan drive motor by high and low speed electrical signal leads, (2) said servo portion of said fuel supply valve by a variable output third electrical lead, and (3) said input side of said ignition switch by a fourth electrical lead,

a normally open high fire pressure-electric switch operably interposed in said third electrical lead, said high fire pressure-electric switch being coupled to said microprocessor by a first electrical switch open/switch closed sensing line, and being further coupled to the interior of said draft inducer fan by a first pressure sensing line, and

a normally open low fire pressure-electric switch operably interposed in one of said first and second electrical leads, said low fire pressure-electric switch being coupled to said microprocessor by a second electrical switch open/switch closed sensing line, and being further coupled to the interior of said draft inducer fan by a second pressure sensing line,

said control means being operative to start-up said furnace by sequentially initiating operation of said draft inducer fan motor at said high speed thereof, and then lighting said burner during high speed operation of said draft inducer fan if and only if both of said high fire and low fire pressure-electric switches are closed, and then control the heat output of said furnace, in response to the magnitude of a heating demand signal received from a conditioned space served by said furnace, by selectively (1) operating said fuel supply valve in said low fire mode with said draft inducer fan drive motor being operated at said low speed thereof, or (2) operating said fuel supply valve in said high fire mode, with said draft inducer fan drive motor being operated at said high speed thereof, and modulating the fuel flow through said fuel supply valve between said minimum flow rate and said maximum flow rate.

7. The fuel-fired air heating furnace of claim 6 wherein:

said fixed minimum flow rate of fuel is approximately forty percent of said maximum flow rate of fuel.

8. The fuel-fired air heating furnace of claim 6 wherein:

said burner is a gas burner, and

said fuel supply valve is a gas supply valve.

9. The fuel-fired air heating furnace of claim 6 wherein:

under a first furnace operating condition both of said low and high fire switches are open, with said draft inducer fan motor being driven at said low speed thereof and said heat exchanger being at a first temperature,

under a second furnace operating condition said low fire switch closes and said high fire switch remains open, with said draft inducer fan motor being driven at said low speed thereof and said heat exchanger being at a second temperature higher than said first temperature,

under a third furnace operating condition said low and high fire switches are closed, with said draft inducer fan motor being driven at said high speed thereof and said heat exchanger being at said first temperature thereof, and

under a fourth furnace operating condition said low and high fire switches are closed, with said draft inducer fan motor being driven at said high speed thereof and said heat exchanger being at said second temperature thereof.

10. The fuel-fired air heating furnace of claim 9 wherein:

said pressure in said draft inducer fan is (1) less than about -0.5" W.C. when said furnace is in said first operating condition, (2) within the range of from about -0.7" W.C. to about -0.9" W.C. when said furnace is in said second operating condition, (3) within the range of from about -1.2" W.C. to about -1.4" W.C. when said furnace is in said third operating condition, and (4) within the range of from about -1.5" W.C. to about -1.75" W.C. when said furnace is in said fourth operating condition.