US3692014A

US3692014A - Pilot and main fuel gas supply means for pressurized gas-fired space heater

Info

Publication number: US3692014A
Application number: US147015A
Authority: US
Inventors: James D Boucher; Jesse L Hopkins
Original assignee: General Electric Co
Current assignee: Trane US Inc
Priority date: 1971-05-26
Filing date: 1971-05-26
Publication date: 1972-09-19
Anticipated expiration: 1989-09-19

Abstract

This invention provides improved means for supplying pilot and main fuel gas to the fuel gas-air mixing area of a gas-fired space heater employing pressurized combustor means to produce a highly intense heat source flame. These improved means include control, igniter, and sensor means for assuring that fuel gas will be first supplied to the mixing area pilot portion and ignited therein before any fuel gas is supplied to the mixing area main portion. In particular accordance with the present invention, pilot gas ignition sensor means are provided which are located wholly outside of the mixing area and are thus spared from direct exposure to the intensely hot flame.

Description

United States Patent Boucher et al.

[54] PILOT AND MAIN FUEL GAS SUPPLY MEANS FOR PRESSURIZED GAS- FIRED SPACE HEATER [72] Inventors: James D, Boucher; Jesse L. Hopkins,

both of Tyler, Tex.

[73] Assignee: General Electric Company [22] Filed: May 26, 1971 [21] Appl. No.: 147,015

[52] US. Cl ..126/110 R, 431/43, 431/90, 431/285 [51] Int. Cl. ..F23g 9/08, F24h 3/08 [58] Field ofSearclLl26/91 A, 110 R, 110 B, 116 R; 431/42, 43, 47, 59, 90, 285

[ 51 Sept. 19, 1972 Weatherston ..431/285 Kegan et al ..431/285 X [57] ABSTRACT This invention provides improved means for supplying pilot and main fuel gas to the fuel gas-air mixing area of a gas-fired space heater employing pressurized combustor means to produce a highly intense heat source flame. These improved means include control, igniter, and sensor means for assuring that fuel gas will be first supplied to the mixing area pilot portion and ignited therein before any fuel gas is supplied to the mixing area main portion. In particular accordance with the present invention, pilot gas ignition sensor means are provided which are located wholly outside of the mixing area and are thus spared from direct exposure to the intensely hot flame.

9 Claims, 5 Drawing Figures PATENTEDSEP 19 I972 SHEH 2 OF 2 A 2! 2O 21 2 xi A a L.

F'IGS INVENTORS JAMES D. aouc E & JESSE. L. HOPKINS THEIR, ATTORNEY PILOT AND MAIN FUEL GAS SUPPLY MEANS FOR PRESSURIZED GAS-FIRED SPACE HEATER BACKGROUND OF THE INVENTION This invention relates to means for supplying heat to 5 a space such as a room, house, building or the like and, more particularly, to improved means for supplying pilot and main fuel gas to the fuel gas-air mixing area of a gas-fired space heater employing pressurized combustor means to produce a highly intense heat source flame.

U.S. Pat. Nos. 1,703,854 Fonseca, 1,842,339 Doen, and 3,213,922 Weber are noted merely as being of interest in that they describe conventional fuel gas supply means that are employed with conventional atmospheric burners for gas-fired heaters. These priorart fuel gas supply means are clearly distinguished from the present invention in that they are employed with a conventional atmospheric burner rather than pressurized combustor means and their pilot ignition sensors are located either closely adjacent to or in direct contact with the burner flame.

Copending, commonly-assigned, U.S. patent applications Ser. No. 47,433, filed June 18, 1970, by Roy W. Abbott (now Patent 3,670,713) and Ser. No. 66,800, filed Aug. 25, 1970 by James D. Boucher (now Patent 3,667,451}, are hereby incorporated herein by reference. These patent applications describe gas-fired space heaters which are superior to conventional 3 means in that they permit considerable weight and volume reduction in heat exchanger means material by employing a combination of highly efiicient heat exchanger means for transferring heat generated by combustion of a fuel gas-air mixture to an air stream circulated between the heat exchanger and the space to be heated, and pressurized high intensity combustor means for mixing fuel gas with pressurized combustion air to produce and ignite the fuel gas-air mixture.

The pressurized combustor means described in these aforenoted patent applications include'a trough having side walls pierced by pressurized combustion air inlet apertures, and fuel gas supply conduit means that extend through the trough and have a surface pierced by apertures through which fuel gas can enter the trough and be turbulently mixed with the pressurized combustion air in a gas-air mixing area defined by the apertured trough side walls and conduit means surface. This turbulent mixture of fuel gas and pressurized combustion air is ignited within the mixing area to provide an extremely intense heat source flame whose combustion products are discharged to the inlet end of the heat exchanger through hollow interconnecting conduit means.

Typically, the heat intensity of this flame is at least 300,000 BTU of heat per hour per cubic foot of the interconnecting conduit means, a far greater intensity level than was encountered with the atmospheric burners that have heretofore been most frequently employed in conventional gas-fired space heaters.

Copending, commonly-assigned U.S. Patent application Ser. No. 93,722, filed Nov. 30, 1970, by Theodore C. Brandt et al (now Patent 3,677,892) describes ini detail means which are particularly adapted for supplying pilot and main fuel gas to the fuel gas-air mixing area of a gas-fired space heater of the aforedescribed type. These Brandt et al. fuel gas supplying means include control, igniter, and sensor means 'fofassuring that fuel gas will be first supplied to the mixing area pilot portion and be ignited therein before any fuel gas is supplied to the mixing area main portion. While these Brandt et al means have performed fairly satisfactorily, the sensor which they employ to sense pilot gas ignition is located within the mixing area and are thus subjected to direct exposure to the intensely hot flame. Because of this, the sensor is expensive in con- .it aisfitbstsslassitrsqi i y- The present invention is particularly concerned with providing improved means for supplying pilot and main fuel gas to the fuel gas-air mixing area of such gas-fired space heaters that employ pressurized combustor means to product a highly intense heat source flame wherein the construction cost, and replacement frequency of the pilot ignition sensor means can be greatly reduced.

SUMMARY OF THE INVENTION This invention provides improved means for supplying pilot and main fuel gas to the fuel gas-air mixing area of a gas-fired space heater of the aforedescribed type that employs pressurized combustor means to produce a highly intense heat source flame. These improved fuel gas supplying means include control, igniter, and sensor means for assuring that fuel gas will be first supplied to the mixing area pilot portion and ignited therein before any fuel gas is supplied to the mixing area main portion. In particular accordance with the present invention, pilot gas ignition sensor means are provided which are located wholly outside of the mixing area and are thus spared from direct exposure to the intensely hot flame. Because of this, the construction cost and replacement frequency of the sensor means can be greatly'reduced.

. BRIEF DES CRIPTION OF THE DRAWINGS The present invention is illustrated in the accompanying drawings, wherein:

FIG. 1 is a somewhat schematic, partly-sectioned, front elevational view of a space air-conditioning unit utilizing gas-fired space heater means incorporating a presently preferred form of the improved pilot and main gas supply means of the present invention;

FIG. 2 is an enlarged fragmentary sectional view taken along line 2-2 of FIG. 1;

Referring now to the drawings and, more particularly to FIG. 1 thereof, there is illustrated, somewhat schematically, an air-conditioning unit 10 utilizing gas-fired space heater means 11 incorporating a presently preferred form of improved fuel 'gas supply means, generallydesignated and shown in detail in FIGS.

2-5, provided in accordance with the present invention. The air-conditioning unit includes a cabinet 13 that is fluid-connected by supply and return conduit means 14 and 15 to a space 16 (shown in block), such as a room, house, building or the like, so as to condition air contained within the space 16 as by supplying heat to or removing heat from an air stream that is circulated between the space 16 and the cabinet 13 by space air blower means 17 mounted within the cabinet 13. The air-conditioning unit 10 mounts the gas-fired space heater means 11 within the cabinet 13 to supply heat to the space air stream in combination with cooling means, including an evaporator section 18 also contained within the cabinet 13, to remove heat from the space air stream.

The gas-fired heater means 11 are generally similar to those described in the aforenoted patent applications. They employ highly efficient heat exchanger means 20 comprising a plurality of metal, generally vertically arranged, finned heat exchanger tubes 21 for transferring heat generated by combustion of a gas-air mixture to the air stream circulated by the blower 17 between the heat exchanger means 20 and the space 16. The heat exchanger means 20 is combined with high intensity combustor means 22 which fuel gas with pressurized air to provide and ignite the gas-air mixture and produce a downwardly extending highly intense heat source flame 23 (FIG. 2). The combustor means 22 is interconnected to the upper ends of the heat exchanger tubes 21 by hollow metal interconnecting conduit means 24 such that the products of combustion from the heat source flame 23 are forced downwardly through the heat exchanger 20 and outwardly to the atmosphere through a vent 25 by blower means 27 which supply the pressurized combustion air. The vent 25 is connected to an exhaust plenum 26 provided at the bottom or outlet ends of the heat exchanger tubes 21.

The combustor 22 includes an inverted generally horizontally arranged trough 29 having a pair of opposed, spaced apart, downwardly extending

long side walls

30 and 31 that are provided with a plurality of apertures which define primary combustion air inlet apertures 32 and secondary combustion air inlet apertures 33. The upper surfaces of the inverted trough 29 are surrounded by the combustion air supply plenum 34 that is fluid-connected to the combustion air supply blower 27 via a combustion air blower discharge conduit 35 and a hollow box-like combustion air inlet control chamber 36. The control chamber 36 is similar to that described in detail in copending, commonly-assigned U.S. patent application No. 69,043, filed Sept. 2, 1970, by Richard C. Barnett et al. and includes a similar damper 37 pivotally mounted within the control chamber 36 by hinge means 38 adjacent the connection of the conduit 35 to the chamber 36 and an adjustable, norma1ly-open, electric control switch 39 which has its actuator 39a interposed between the damper 37 and the combustion air supply plenum 34 and is actuatable to closure by engagement by the lower portion of the damper 37.

In accordance with the present invention, the improved means 12 are provided for supplying pilot and main fuel gas to the combustor means 22. These improved fuel gas supplying means 12 are generally similar to, but are an improvement of, those described in detail in the aforenoted Brandt et al. patent application. The improved means 12 include a hollow box-like fuel gas supply conduit 40 that extends longitudinally through the inverted trough 29 with its long axis arranged generally horizontally. Its lower surface 41 is pierced by a plurality of fuel gas inlet apertures that have their axes arranged generally vertically and are located above the trough primary combustion air inlet apertures 32. The apertured

trough side walls

30 and 31 and gas conduit surface 41 define a gas-air mixing area 42 wherein fuel gas entering via the gas conduit apertures can be mixed with pressurized combustion air admitted via the

trough wall apertures

32 and 33 and be ignited by igniter means, such as a spark plug 43, to produce the heat source flame 23 which extends along substantially the entire length of the open lower or outlet end of the inverted trough 29.

The intensity of the flame 23 is generally similar to that produced by the pressurized combustor means described in the aforenoted patent applications and is typically at least 300,000 BTU of heat per hour per cubic foot of the interconnecting conduit means 24. This is a far higher flame heat intensity level than has been obtained with the atmospheric combustion means heretofore most frequently employed in conventional gas-fired space heaters.

As best shown in FIGS. 2, 3 and 5, the present invention preferably provides integrated means for supplying both pilot and main fuel gas to the combustor fuel gasair mixing area 42 by installing partition means 44 within the gas supply conduit 40, which is thus divided by the partition means 44 into a first or pilot portion 40p and a second or main portion 40m. As illustrated, the partition means 44 is mounted adjacent one of the corners of the conduit 40 and the pilot portion 40p is fluid-connected to a first or pilot group of gas inlet apertures 41p which pierce the gas supply conduit lower wall 41, while the main portion 40m is fluid-connected to a second or main group of the gas inlet apertures 41m that also pierce the gas supply conduit lower wall 41. The pilot 40pand the main 40m portions of the gas supply conduit 40 are fluid-segregated from one another by the partition means 44 and, preferably, the pilot group of gas inlet apertures 41p comprises approximately l0 per cent of the total number of all such fuel gas inlet apertures, while the main group of gas inlet apertures 41m comprises the remaining approximately per cent thereof.

In the illustrated embodiment of the invention, both the pilot 40p and main portions 40m of the fuel gas supply conduit 40 are fluid-connected to a common source 45 of fuel gas, such as natural gas, propane, or the like. The pilot portion 40 pis fluid-connected to the fuel gas source 45 via first or pilot piping means 45p including a normally-closed electrically-powered first or pilot control valve 46;). The main portion 40m is fluidconnected to the fuel gas source 45 via second or main piping means 45m including a normally-closed, electrically-powered second or main control valve 46m.

As best shown in FIG. 5, the normally-closed electrically-powered pilot control valve 46p has its solenoid 46ps electrically connected by conductor means 47 across power source lines L and L in electrical series with the spark plug 43 and the normally-open damperactuatable air control switch 39, and by conductor means 48 in electrical parallel with other electrical circuitry which includes the solenoid 47ms of the normally-closed main control valve 46m that is connected in electrical series with pilot gas ignition sensor means 49.

In particular accordance with the present invention, the pilot gas ignition sensor means 49 are located wholly outside of the combustor fuel gas-air mixing area 42 and are thus spared from direct exposure to the intensely hot flame 23. In the illustrated embodiment, the sensor means 49 comprises a normally-open, inexpensive, thermally-sensitive switch 50 including a bimetallic switch actuator 51 that is adapted to move to a position closing or bridging the normally-open contacts 50a and 50b of the switch 50. The switch actuator 51 is mounted in a heat-transferring relationship to a metal disc 52 that is, in turn, welded to the exterior of a metal by-pass conduit or tube 53 which extends between and fluid-connects the portion of the fuel gasair mixing area 42 adjacent the pilot group of gas inlet apertures 41p to the exhaust plenum 26 such that fluid can pass therebetween without passing through the heat exchanger means 20. While various materials may be employed to construct the by-pass conduit 53, good results have been obtained using a tube having its upper portion made of l-Iastelloy X and its lower portion made of 409 stainless steel.

The bimetallic actuator 51 is adapted to move to a position bridging the switch contacts 50a and 50b (FIG. 5) and electrically energizing the solenoid 47ms to open the main gas valve 47m only if it has sensed emission of a predetermined amount of heat from the bypass conduit 53. This should occur only after ignition of fuel gas admitted via the pilot group of gas inlet apertures 41 p and the subsequent passage of the products of combustion therefrom downwardly through the by-pass conduit 53.

As further shown, the sensor means 49 are preferably housed within a generally rectangular box 54 that is provided with a readily removable access cover 55. The purpose of this box 54 is to shield the heat-sensitive sensor means switch 50 from the air stream that is circulated between the air conditioner cabinet 13 and the space 16 by the blower means 17.

With the aforedescribed arrangement of the present invention, the gas-fired heater means 11 can be activated in the following manner. First, the combustion air supply blower 27 is started and the combustion air control switch actuator 39a is adjustably positioned as described in detail in the aforenoted Barnett et al. application, such that it is engaged by the damper 37 and pivoted to its closed position (FIG. 5) only just when the flow of combustion air from its supply blower 27 to the combustor gas-air mixing area 42 is such that there will be optimum mixing therein of the combustion air and the fuel gas.

Movement of the actuator 39a to its closed position by the damper 37 will then simultaneously electrically connect the solenoid 4612s of the pilot control valve 46p and the spark plug 43 across the electric power source lines L and L thus opening the normally-closed pilot control valve 46p and admitting fuel gas to the mixing area 42 from the fuel gas source 45 via the pilot piping means 45p and the pilot group of fuel gas inlet apertures 41p, This fuel gas is next turbulently mixed with the pressurized combustion air that is admitted to the mixing area via the adjacent primary combustion air inlet apertures 32 and this turbulent pilot" mixture is ignited by the nowenergized spark plug 43 and then further turbulently mixed with the pressurized combustion air that is admitted via the secondary combustion air inlet apertures 33. This produces a pilot" portion of the total heat source flame 23 covering only about 10 per cent of the downwardly opening trough mouth adjacent the spark plug 43.

At least a part of the products of combustion from the ignition of this pilot portion of the fame 23 will now pass downwardly through the bypass conduit 53 and, will cause a sufficient amount of heat to be emitted from the bypass conduit 53 to actuate the bimetallic switch actuator 51 to close the normally-open contacts 500 and 50b of the pilot ignition sensor means switch 50 and thereby electrically connect the solenoid 46ms across the electrical power source lines I. and L This will, of course, operate the normally-closed main control valve 46m to be opened to allow admission of fuel gas to the fuel gas-air mixing area 42 from the fuel gas source 45 via the main piping means 45m and the main group of fuel gas inlet apertures 41m of the fuel gas supply conduit 40. The gas thus admitted will then be ignited by the pilot portion of the heat source flame 23 which has already been ignited by the spark plug 43 as previously described.

As best shown in FIGS. 1-3, the remaining structure comprising the air conditioning unit 10 is generally similar to that described in the aforenoted Boucher patent application.

It should be apparent to those skilled in the art that while there has been described what, at present, is considered to be presently preferred embodiments of this invention in accordance with the Patent Statutes. changes may be made to the disclosed apparatus without actually departing from the true spirit and scope of this invention. It is, therefore, intended that the appended claims shall cover such modifications and applications that may not depart from the true spirit and scope of the present invention.

What is claimed is:

1. In a gas-fired heater for supplying heat to a space such as a room, house, building and the like, and ineluding heat exchanger means for transferring heat generated by combustion of a fuel gas-air mixture to an air stream circulated between the heat exchanger means and the space to be heated, in combination with combustor means for mixing fuel gas with pressurized combustion air and igniting that mixture to produce an intense heat source flame whose combustion products are discharged into the heat exchanger means inlet and forced through the heat exchanger means and outwardly to the atmosphere through a vent connected to an exhaust plenum at the outlet of the heat exchanger means and wherein said combustor means includes a trough having side walls pierced by pressurized combustion air inlet apertures and hollow fuel gas supply conduit means that extend through said trough and have a surface pierced by apertures through which fuel gas can enter said trough and be turbulently mixed with said pressurized combustion air in a gas-air mixing area defined by said apertured trough side walls and conduit means surface, improved means for supplying fuel gas to said mixing area and igniting it therein, said improved means comprising:

a. first means for fluid-connecting a source of fuel gas to said mixing area, said first means including hollow conduit means having a surface located within said mixing area and pierced by a first group of apertures through which fuel gas can enter said mixing area;

. second means for fluid-connecting a source of fuel gas to said mixing area, said second means comprising hollow conduit means having a surface located in said mixing area and pierced by a second group apertures through which fuel gas can enter said mixing area;

c. igniter means mounted in said mixing area; and

. control means for controlling admission of fuel gas to said first and second means such that fuel gas will be first supplied to said mixing area via said first group of fuel gas inlet apertures and ignited by said igniter means before any fuel gas is supplied to said mixing area via said second group of fuel gas inlet apertures, said control means including:

1. a first controlvalve for fluid-connecting said first group of fuel gas inlet apertures to their fuel gas source,

2. a normally-closed electrically powered second control valve for fluid-connecting said second group of fuel gas inlet apertures to their fuel gas source,

3. sensor means mounted wholly outside of said mixing area for sensing ignition of fuel gas entering said mixing area through said first group of inlet apertures, and

4. means for electrically interconnecting said sensor means with said second control valve such that said normally-closed second control valve will not be operated to open unless said ignition has been sensed by said sensor means.

2. The invention of claim i, wherein said sensor means comprises a thermally-sensitive electric switch.

3. The invention of claim 2, further including by-pass conduit means for fluid-connecting a portion of said mixing area adjacent said first group of inlet apertures to said exhaust plenum such that fluid can pass between said portion of said mixing area and said exhaust plenum without passing through said heat exchanger means.

4. The invention of claim 3, wherein:

a. said thermally-sensitive switch includes a thermally-sensitive switch actuator,

b. said switch actuator is mounted adjacent said bypass conduit means to receive heat emitted therefrom, and

c. said switch actuator is operable in response to receipt of a predetermined amount of heat to actuate said thermally sensitive switch to cause said normally-closed second control valve to open.

5. The invention of claim 4, wherein said switch actuator comprises a bimetallic member.

6. In a gas-fired space heater combining heat exchanger means for transferring heat generated by combustion of a fuel gas-air mixture to an air stream circulated between the heat exchanger means and the space to be'heated, with combustor means for mixing fuel gas with pressurized combustion air in a gas-air mixing area and igniting that mixture to produce a heat source flame whose combustion products are discharged into the heat exchanger means inlet and forced through the heat exchanger means and outwardly to the atmosphere through a vent connected to an exhaust plenum at the heat exchanger means outlet and further including first and second means for fluidconnecting a source of fuel gas to said mixing area, igniter means mounted in said mixing area, and control means for controlling admission of fuel gas to said first and second means such that fuel gas will be first supplied to said mixing area via said first means and ignited by said igniter means before any fuel gas is supplied to said mixing area via said second means and wherein said control means include a first control valve for fluid-connecting said first means to its fuel gas source, a normally-closed electrically powered second control valve for fluid-connecting said second means to its fuel gas source, sensor means for sensing ignition of fuel gas entering said mixing area through said first means, and means for electrically interconnecting said sensor means with said second valve such that said second valve will not be operated to open unless said ignition has been sensed by said sensor means, an improvement comprising:

a. by-pass conduit means for fluid-connecting a portion of said mixing area adjacent said first means to said exhaust plenum such that fluid can pass between said mixing area portion and said exhaust plenum without passing through said heat exchanger means, and

b. said sensor means being mounted wholly outside of said mixing area and adjacent to said bypass conduit means.

7. The invention of claim 6, wherein said sensor means comprises a thermally-sensitive electric switch.

8. The invention of claim 7, wherein:

c. said switch actuator is operable in response to receipt of a predetermined amount of heat to ac tuate said thermally sensitive switch to cause said normally-closed second control valve to open.

9. The invention of claim 8, wherein said switch actuator comprises a bimetallic member.

Claims

1. In a gas-fired heater for supplying heat to a space such as a room, house, building and the like, and including heat exchanger means for transferring heat generated by combustion of a fuel gas-air mixture to an air stream circulated between the heat exchanger means and the space to be heated, in combination with combustor means for mixing fuel gas with pressurized combustion air and igniting that mixture to produce an intense heat source flame whose combustion products are discharged into the heat exchanger means inlet and forced through the heat exchanger means and outwardly to the atmosphere through a vent connected to an exhaust plenum at the outlet of the heat exchanger means and wherein said combustor means includes a trough having side walls pierced by pressurized combustion air inlet apertures and hollow fuel gas supply conduit means that extend through said trough and have a surface pierced by apertures through which fuel gas can enter said trough and be turbulently mixed with said pressurized combustion air in a gas-air mixing area defined by said apertured trough side walls and conduit means surface, improved means for supplying fuel gas to said mixing area and igniting it therein, said improved means comprising: a. first means for fluid-connecting a source of fuel gas to said mixing area, said first means including hollow conduit means having a surface located within said mixing area and pierced by a first group of apertures through which fuel gas can enter said mixing area; b. second means for fluid-connecting a source of fuel gas to said mixing area, said second means comprising hollow conduit means having a surface located in said mixing area and pierced by a second group apertures through which fuel gas can enter said mixing area; c. igniter means mounted in said mixing area; and d. control means for controlling admission of fuel gas to said first and second means such that fuel gas will be first supplied to said mixing area via said first group of fuel gas inlet apertures and ignited by said igniter means before any fuel gas is supplied to said mixing area via said second group of fuel gas inlet apertures, said control means including: 1. a first control valve for fluid-connecting said first group of fuel gas inlet apertures to their fuel gas source, 2. a normally-closed electrically powered second control valve for fluid-connecting said second group of fuel gas inlet apertures to their fuel gas source, 3. sensor means mounted wholly outside of said mixing area for sensing ignition of fuel gas entering said mixing area through said first group of inlet apertures, and 4. means for electrically interconnecting said sensor means with said second control valve such that said normally-closed second control valve will not be operated to open unless said ignition has been sensed by said sensor means.

2. The invention of claim 1, wherein said sensor means comprises a thermally-sensitive electric switch.

4. The invention of claim 3, wherein: a. said thermally-sensitive switch includes a thermally-sensitive switch actuator, b. said switch actuator is mounted adjacent said by-pass conduit means to receive heat emitted therefrom, and c. said switch actuator is operable in response to receipt of a predeteRmined amount of heat to actuate said thermally sensitive switch to cause said normally-closed second control valve to open.

6. In a gas-fired space heater combining heat exchanger means for transferring heat generated by combustion of a fuel gas-air mixture to an air stream circulated between the heat exchanger means and the space to be heated, with combustor means for mixing fuel gas with pressurized combustion air in a gas-air mixing area and igniting that mixture to produce a heat source flame whose combustion products are discharged into the heat exchanger means inlet and forced through the heat exchanger means and outwardly to the atmosphere through a vent connected to an exhaust plenum at the heat exchanger means outlet and further including first and second means for fluid-connecting a source of fuel gas to said mixing area, igniter means mounted in said mixing area, and control means for controlling admission of fuel gas to said first and second means such that fuel gas will be first supplied to said mixing area via said first means and ignited by said igniter means before any fuel gas is supplied to said mixing area via said second means and wherein said control means include a first control valve for fluid-connecting said first means to its fuel gas source, a normally-closed electrically powered second control valve for fluid-connecting said second means to its fuel gas source, sensor means for sensing ignition of fuel gas entering said mixing area through said first means, and means for electrically interconnecting said sensor means with said second valve such that said second valve will not be operated to open unless said ignition has been sensed by said sensor means, an improvement comprising: a. by-pass conduit means for fluid-connecting a portion of said mixing area adjacent said first means to said exhaust plenum such that fluid can pass between said mixing area portion and said exhaust plenum without passing through said heat exchanger means, and b. said sensor means being mounted wholly outside of said mixing area and adjacent to said by-pass conduit means.

8. The invention of claim 7, wherein: a. said thermally-sensitive switch includes a thermally-sensitive switch actuator, b. said switch actuator is mounted adjacent said by-pass conduit means to receive heat emitted therefrom, and c. said switch actuator is operable in response to receipt of a predetermined amount of heat to actuate said thermally sensitive switch to cause said normally-closed second control valve to open.