US3259121A

US3259121A - Broil-bake oven gas control system

Info

Publication number: US3259121A
Application number: US390341A
Authority: US
Inventors: Earl A Mcgee
Original assignee: Magic Chef Inc
Current assignee: Magic Chef Inc
Priority date: 1964-08-18
Filing date: 1964-08-18
Publication date: 1966-07-05
Anticipated expiration: 1983-07-05

Description

July 5, 1966 E. A. McGEE BROIL-BAKE OVEN GAS CONTROL SYSTEM 4 Sheets-Sheet 2 Filed Aug. 18, 1964 INVENTOR ATTORNEYS Earl 2. H0666 E 2 \nd E 23 July 5, 1966 E. A. McGEE 3,259,121

BROIL-BAKE OVEN GAS CONTROL SYSTEM Filed Aug. 18, 1964 4 Sheets-Sheet 5 INVENTOR Earl ll. M0666 ATTORNEYS July 5, 1966 E. A. McGEE 3,259,121

BROIL-BAKE OVEN GAS CONTROL SYSTEM Filed Aug. 18, 1964 4 Sheets-Sheet 4 2 -5. J J 75 75 T 5 ELIT I 76 /44 5 i I l @6144 43 A My 66 m 66,

INVENTOR EarZ A. M0566 ATTORNEYS United States Patent 3,259,121 BROIL-BAKE OVEN GAS CONTROL SYSTEM Earl A. McGee, Cleveland, T enn., assignor to Magic Chef, Inc., Cleveland, Tenn. Filed Aug. 18, 1964, Ser. No. 390,341 6 Claims. (Cl. 12639) This invention relates to the structure of an oven and a gas control system therefor wherein the oven is provided with both a baking burner and a broiling burner.

Ovens with both baking and broiling burners are not new but heretofore the broil burner has been required to be ignited by hand and noautomatic controls were provided therefor. According to the present invention such an oven is provided wherein the broil burner is in its usual location at the top of the oven enclosure and a bake burner is located near the bottom of the enclosure. However, the bake burner is located below the rear edge of the oven enclosure and a portion of the oven structure encloses the bake burner, then slopes upwardly and forwardly to about the front edge of the oven enclosure, thus providing a maximum of clearance space under the oven structure permitting its use over a work table, range top, or the like, while still providing for minimum over-all height of the structure and efficient heating of the enclosure for baking purposes.

The broil and bake burners are both provided with automatic ignition systems and are both under the control of a thermostat. In the gas line leading to the broil burner, a heat controlled valve is provided to prevent delivery of gas to the broil burner until such time as an ignition system is ready to ignite that gas and means are provided for preventing inadvertent energization of the ignition system when the broil burner is not to be used. A pressure responsive switch is a part of the automatic ignition system for the 'broil burner and means are provided to prevent undesired build-up of pressure at that switch at times when the oven is hot and a selector switch is positioned to turn the broil burner off. In addition, a safety ignition system is provided for the broil burner wherein the burner is automatically relighted in the event the selector valve is indavertently or momentarily turned off then on again or when a thermostat high temperature control turns the broil burner oif.

It is, therefore, a principal object of this invention to provide an oven having bake and broil burners and a gas control system therefor rendering the operation of both burners completely safe and automatic.

It is also an object of this invention to provide an oven of the type set forth and having automatic ignition, selectively operable, for each burner.

Another object is to provide an oven of the type set forth with means to prevent simultaneous operation of both burners.

Still another object is to provide an oven as described with means to prevent delivery of gas to either of the burners unless ignition is available at the selected burner.

A further object is to provide an oven of the above type wherein thermostatic control means are provided for the bake burner and a high limit thermal control is effective for the broil burner.

Still another object isto provide such an oven with an automatic pressure responsive ignition switch for the broiler burner and means associated therewith to prevent inadvertent operation 'of the pressure switch unless the broil burner has been selected to function.

Yet another object is to provide a standby safety pilot for the broil burner.

An additional object is to provide an oven structure adapted for use in a wall over a range or work table and provide maxi-mum space over the range and/ or work table while keeping the all-over height of the oven to a minimum.

Other objects and advantages will become apparent to those skilled in the art as the description proceeds with reference to the accompanying drawings wherein:

FIG. 1 is a front view of a structure embodying the present invention and showing merely one form of the apparatus;

FIG. 2 is an enlarged fragmentary side view along the line 22 of FIG. 1, with parts broken away for clarity of illustration;

FIG. 3 is a schematic diagram of the gas control system for. the burners of the oven shown in FIG. 1;

FIGS. 3A and 3B are fragmentary diagrams of a portion of FIG. 3 showing the selector valve in different positions;

FIG. 4 is a vertical sectional view through a heat controlled valve employed with the present invention;

FIG. 5 is a vertical sectional view, the structure being schematically shown, through a selector valve suitable for the present invention;

FIGS. 6 and 7 are sectional views taken respectively on the lines 66 and 77 of FIG. 5, showing the relationship of the parts when the valve is in its off position;

FIGS. 8 and 9 are sectional views similar to FIGS. 6 and 7 but showing the parts when the valve is in its bake position; and

FIGS. 10 and 11 are sectional views similar to FIGS. 6 and 7 but showing the position of the parts when the valve is turned to its broil position.

Referring first to FIGS. 1 and 2, the structure shown comprises a more .or less conventional range 2 constituting the base of the apparatus and an oven 4 supported thereover and which constitutes the subject matter of the present invention. The oven 4, as shown, is supported directly on the range base 2 and extends forwardly thereover but spaced thereabove, thus leaving the entire burner top of the range 2 freely accessible for use. It is contemplated, however, that the oven 4 may be inset in a wall and that the burner top of the range 2 be replaced by a pull-out work table or the like and the range 2 eliminated. In either event, the space below the oven 4 and over the entire top of range 2 or the work table referred to is acessible to the housewife.

Referring now to FIG. 2, the oven 4 is provided with a broil burner 6 therein, located in the usual position near the top of the oven enclosure 8. The oven is provided with a bottom or floor partition 10 dividing the enclosure into a generally cubical cooking compartment 8 and a portion defining a burner compartment 12. The cooking compartment is provided with the usual adjust-able racks 14. A baking burner 16 is mounted in the lower portion or burner compartment 12 of the oven below the rear edge thereof and below the partition member 10. The burner compartment 12 as defined at least in part by a bottom rear enclosure wall portion 18 and an upwardly and forwardly sloping portion 20 extending from the portion 18 to the forward edge of the bottom of the cooking compartment 8. In this manner, the baking burner 16 is effective to provide the required heat for baking products in the compartment 8 but is located at the rear of the oven so as not to interfere materially with the open space 22 below the oven and above the range top or work table 24. Baking burners are conventionally located ,By the structure thus far described, however, adequate clearance space 22 is provided while maintaining the spacing between the oven enclosure and the surface 24 to a minimum and thus keeping to a minimum the over-all height of the entire apparatus.

Referring now to FIGS. 3, 3A and 3B, the gas control I V system includes a supply conduit 30 by which gas is supplied to the system and which will be referred to as a gas supply. The conduit 30 is connected to a distributor valve 32 from which a conduit 34 leads to a selector valve 36. Communication between the supply 30 and conduit 34 is, however, normally blocked by valve 32, which is opened only when a heater pilot burner, to be described, is lighted. The distributor valve 32 is under the control of a heat responsive mercury bulb 38 adjacent the bake burner 16 and in position to be heated by a heater pilot 42. The heater pilot 42 is supplied with gas through a conduit 44 under the control of a thermostatic control valve 46. Thus, when the heater pilot 42 is ignited, it heats the mercury bulb 38 which operates (in a well known manner) to cause the valve 32 to open and direct gas into the conduit 34. A conduit 48 is also connected to the distributor valve 32 but is not under the control thereof, being at all times in communication with the supply 30 so as to keep a constant supply of gas in a manifold 50. A conduit 52 extends from the manifold 50 to the thermostatic control valve 46 and is connected to the conduit 44 when the thermostat valve is turned to the on position. When the thermostatic control valve 46 is in its on position, a heat responsive bulb or temperature sensor 54 detects the temperature of the oven enclosure and will shut the valve 46 off when the oven temperature reaches the preset value, thus denying gas to the heater pilot 42 and permitting mercury bulb 38 to cool off which results in closing the distributor valve 32. A conduit 56 also extends from the manifold 50 to a constant pilot burner 58 adjacent the heater pilot 42. The constant pilot burner 58 is thus always lighted after once being ignited and is operable to ignite the heater pilot burner 42 whenever the thermostatic control valve 46 is opened to place the oven in operation. The structure thus far described is more or less conventional as a control system for baking burners and is described herein only to give a full and complete understanding of the novel features of the present invention.

The broil burner 6, located in the upper portion of the oven enclosure, is supplied with gas through a conduit 60 communicating with the outlet of a heat controlled valve 62. The inlet of the heat controlled valve 62 is connected to a conduit 64, which, in turn, is connected to an outlet of the selector valve 36. The selector valve 36 is provided with a second outlet communicating with conduit 66 extending to and supplying gas to the bake burner 16. The heat controlled valve 62 is normally closed but is caused to open by the operation of a mercury bulb 68, which detects the control temperature for valve 62. A pressure switch 70 is provided with a diaphragm (not shown) responsive to the differential pressure between

conduits

60 and 64 and is normally open, but when a differential pressure exists on the opposite sides of the diaphragm, a switch is caused to close which energizes a glow coil 72 adjacent a heater pilot 74 supplied with gas through a conduit 76 controlled by the selector valve 36. Gas for the heater pilot 74 is directed to the selector valve 36 from manifold 50 and through conduit 78. A further conduit 80 extends from the manifold 50 to a valve 82, to be more fully described hereinafter, and which controls the how of gas from conduit 80 into conduit 84 which supplies a standby pilot burner 86, also to be described more fully hereafter.

The selector valve 36 will be described in more detail later but is of such construction that when in the ott position shown in FIG. 3 no gas is fed to the heater pilot conduit 76 and both

conduits

66 and 64 are isolated from the gas inlet conduit 34 so that gas is not directed to either of the

burners

6 or 16. However, the

conduits

64 and 66 are at this time placed in communication with each other (as indicated by dotted lines 65 in FIG. 3) for a purpose to be presently described.

When the selector valve is turned to the bake position, as shown schematically in FIG. 3A, it functions to direct gas from the supply conduit 34 into conduit 66 leading to the bake burner 16 and the baking burner is thus placed in operation. However, since conduit 64, leading to the heat controlled valve 62, would normally be closed at both ends, the gas trapped therein would expand due to the heat of operation of the baking burner and thus actuate the pressure switch 70 and energize glow coil 74 even though it is not desired to place the broil burner in operation, and the glow coil would remain energized during the entire baking cycle. In the position shown in FIG. 3A, the selector valve 36 also establishes communication between

conduits

64 and 76 so that the gas then in conduit 64 is vented to the atmosphere of the oven through the heater pilot 74 and then only atmospheric pressure is impressed on the two sides of the pressure switch, since the other side is exposed to the atmosphere through the broil burner 6. As shown in FIG. 3,

conduits

64 and 66 are in communication, thus atmospheric pressure is present in conduit 64 and the pressure switch cannot be inadvertently closed even though the oven may be at a high temperature at that time.

When the selector switch 36 is turned to its broil position, as shown in FIG. 3B, gas is directed from the supply conduit 34 into the conduit 64 and to the heat controlled valve 62. At the same time gas is admitted from conduit 78 into conduit 76 leading to the heater pilot 74. Since the pressure switch 70 now has gas pressure in the upstream side and atmospheric pressure on the broil burner side, the switch is closed and energizes glow coil 72, which ignites the gas issuing from heater pilot 74. This causes mercury bulb 68 to become heated and to open the valve 62, thus admitting gas to the broil burner which is then ignited by the heater pilot. At the same time, a mechanical connection between the mechanism in valve 62 and valve 82 causes the latter to open and supply gas to standby pilot conduit 84 so that standby pilot burner 86 is ignited, from heater pilot 74, and remains ignited and burning even at some times when valve 62 may be closed, as will be described, and when valve 62 is open but not being supplied with gas. Thus the inadvertent or momentary turning of the selector valve from broil to the off position will result in extinction of the heater pilot 74 but as long as bulb 68 remains hot, valve 62 will remain open for a while and glow coil 72 cannot be energized. At this time the pressure on opposite sides of the diaphragm of pressure switch 70' is equal and glow coil 72 cannot be energized even if the selector valve is turned immediately back to the broil position. Under these conditions the standby pilot 86 will effect ignition of the heater burner 74 to reinitiate operation of the broil burner.

As stated previously the entire oven compartment is under the control of the thermostatic valve 46, which in turn is controlled by the temperature sensor 54. While it is not neceessary or desirable to maintain a specific temperature in the oven while broiling, yet it is desirable to limit the maximum temperature of the compartment, for safety reasons. Thus, while broiling, the temperature sensor 54 will detect the temperature and when it reaches the maximum safe limit, it will at least temporarily turn off the thermostatic valve 46, thus extinguishing the heater pilot 42 and permitting the mercury bulb 38 to cool. This results in closing distributor valve 32 and denying gas to conduit 34. It is to be remembered that the selector valve 36 remains in its broil position and thus gas continues to flow from the manifold 50 to the upper heater pilot 74. This keeps valve 62 open (even though no gas is flowing therethrough) and provides an active pilot light to reignite the broil burner when the oven compartment cools sufficiently for the thermostatic valve 46 to open again and reheat mercury bulb 38.

FIG. 4 shows one form of structure of the heat controlled valve 62. This valve comprises a body portion 90 having an inlet connected to the conduit 64 and an outlet to which conduit 60 is connected. The inlet communicates with a chamber 92 and the outlet communicates with a chamber 94. The two chambers are separated by a partition 96 having an opening 98, therein, the periphery of which defines a valve seat for a movable valve member 100. The member 100 is guided by a pin 102 for movement toward and from itsvalve seat and is urged by spring 105 to closed position wherein communication between the

conduits

60 and 64 is cut off. The pressure switch 70 is of conventional form and need not be further described except to point out that it is provided with a movable diaphragm, one side of which is in communication with the inlet chamber 92 through an on'fice 106 therein and the other side of the diaphragm is in communication with the outlet chamber 94 through an orifice 108 therein. The heat sensing mercury bulb device 68 is also of known and conventional construction and is provided with a central member 110 extending through a fitting 112 into the outlet compartment 94 of valve 62. Within the compartment 94 is a first lever 114 mounted for pivotal rocking movement about a fulcrum 116. The end of lever 114 opposite the fulcrum 116 bears upon a second lever 118 which is capable of rocking about a fulcrum 120. The other end of lever 118 extends to a position below the valve member 100 and a compression spring 122 between the

levers

114 and 118 urges lever 114 to swing in such direction so that an intermediate portion of the lever 114 bears against a ball 124 and so that the lever 118 is urged to rotate in a clockwise direction about its fulcrum 120 so that its right hand end moves away from the valve 100. When the mercury bulb device 68 is heated, the movable element thereof v110 causes the ball 124 to move downwardly and thus enforce counterclockwise rotation of the lever 114 which in turn causes counter-clockwise rotation of the lever 118 to move the right hand end of the latter upwardly and enforce opening of the valve 100. Thus, when the mercury bulb is heated the valve is opened to admit gas from conduit 64 to conduit 60 and to the broil burner 6.

As will be apparent, when valve 100 is closed, and gas pressure is admitted to conduit 64, that gas pressure will be communicated to one side of the diaphragm of the pressure switch through orifice 106. Since the outlet chamber 94 is at atmospheric pressure, the diaphragm will be deflected to close the switch 70 and energize glow coil 72 to ignite the heater burner 74. That burner heats mercury bulb 68, which results in opening valve 100 in the manner described. At that time gas pressure in the inlet and outlet chambers is equal and the orifices 106 and 108 direct equal pressure to opposite sides of the switch diaphragm and the same returns to its central position to open switch 70. The pressure switch and valve 62, as thus far described, are conventional and known to the art.

The valve 82 controlling flow of gas to the standby pilot 86 is mounted on the body 90 of the valve 62 and may be constructed as shown wherein a movable valve element 130 may close communication between

conduits

80 and 84. A spring 132 urges the valve 130 to open position but a stem 134 on the valve element 130 projects slidably int-o the outlet chamber 94 of valve 62 to a position Where it bears against a portion of the lever 118. The spring 122 is sufficiently stronger than the spring 132 to ensure that the valve element 130 is held in its closed position when the mercury bulb 68 is cold. However, when the lever 118 is caused to rock in a counterclockwise direction to open valve 100 (-in response to heating of the mercury bulb 68), the valve element 130 is released to the spring 132, and any gas pressure that exists in conduit 80, to thus move the valve 130 to the open position and direct gas therethrough into conduit 84 and standby pilot 86.

It is to be noted that when the mercury bulb 68 is cold, the right hand end of lever 118 is spaced downwardly from the lower end of valve member 100 and thus there is some lost motion therebetween. As the lever 118 starts to rock upwardly, the valve 130 can open before the valve is caused to open. Likewise, when gas to the broil burner is cut off, such as by operation of the selector valve 36, the mercury bulb 68 cools and lets lever 118 swing in such direction as to permit valve 100 to close under the influence of spring 104. The valve 100 will close and cut off communication with the broil burner before the valve element is forced to its closed position and the mercury bulb must cool down to a lower temperature before the valve 130 is closed. Thus, the standby pilot remains ignited and burning for an appreciable period of time after the heat responsive valve 100 is closed so that an immediate ignition source is available.

The distributor valve 32, controlled by mercury bulb 38, may be of the same or similar construction as the valve 62, just described. However, the pressure switch and orifices 106 and 108 would be omitted, as would be any connection to another valve, such as 8-2. A manual valve 150 may be provided in conduit 48 to cut gas on from manifold 50 and thus completely disable the entire oven.

FIGS. 5 through 11 are schematic illustrations of the structure of the selector valve 36 and in the following description of the passageways within the valve 36 are identified by the same reference numerals as the conduit to which they are connected but with a added. Referring first to FIG. 5, the gas inlet 34 is directed to a tapered chamber in" which a rotatable tapered plug valve 142 is mounted. The plug 142 is provided with a cross slot 144 adjacent its large end and an axial bore 76 extending into its small end. The conduit 76 leading to the heater pilot 74 communicates with the small end of the chamber 140 whereas the conduit 78 communicates with that chamber intermediate its ends. A radial bore 146 (see FIG. 7) extends from the axial bore 76' outwardly of the rotatable plug 142 in the plane containing the inlet conduit 78. In the plane containing the cross slot 144, the body is provided with outlet passages 64 and 66' in the same axial plane as the inner end of the inlet 34 and respectively with the broil and bake

conduits

64 and 66.

FIGS. 6 and 7 are sectional views taken on the planes 6-6 and 7-7 of FIG. 5, respectively, when the selector valve is in its off position. Thus, it can be seen from FIG. 6 that gas in the inlet 34' cannot reach either outlets 64' or 66 while the latter are placed in communication with each other by the cross slot 144. Also, as seen in FIG. 7, the pilot gas inlet 78 is blocked. When the selector valve is turned to the bake position shown in FIGS. 8 and 9, the gas inlet 34 is placed in communication with outlet66 which directs gas to the bake burner conduit 66. At the same time the heater pilot conduit 76 of the broil burner communicates through bore 76' (FIG. 9) and passageway 146 with the broil burner outlet 64', thus placing conduit 64 in communication with conduit 76 whereby to admit atmospheric pressure to the upstream side of the pressure switch 70 (refer to the description of FIG. 4).

When the selector valve is turned to the broil position shown in FIGS. 10 and 11, gas is directed from the inlet 34, through cross slot 144 to outlet 64' and thence to conduit 64. At the same time gas is directed from the manifold 50 through conduit 78 and 'bore 146 to the passageway 76 and thence to conduit 76 to supply gas to the heater pilot 74 near the broil burner.

It is to be noted that in the event of electric power failure the broil burner can be manually started by first actuating the selector valve 36 to its broil position, which supplies gas only to heater pilot 74 (the valve 62 being closed at that time) and the pilot can be ignited manually. After that the operation is fully automatic, as already described.

While a single specific embodiment of the invention has been shown and described herein it is to be understood that the same is merely illustrative of the principles involved and that other embodiments may be resorted to within the scope of the appended claims.

I claim:

1. In a gas oven having a bake burner and a broil burner therein; a selector valve having an inlet arranged to receive gas from a supply thereof, said selector valve having at least two outlets for gas; a first conduit leading from one of said outlets to one of said burners; a second conduit leading from the other of said outlets to the other of said burners; a heat controlled valve normally closing said first conduit; a pressure responsive igniter means arranged to be responsive to the differential pressures, in said first conduit, on opposite sides of said heat controlled valve; said selector valve being selectively operable to connect either of said outlets to said inlet and to isolate both outlets from said inlet, said selector valve being further arranged to establish communication between said first and second conduits when said outlets are both isolated from said inlet whereby to direct atmospheric pressure from said burners to opposite sides of said heat controlled valve.

2. The arrangement defined in claim 1 including a heater pilot burned adjacent said first burner; a third conduit extending from said selector valve to said heater pilot burner, said selector valve being arranged to direct gas into said third conduit when actuated to connect said first conduit to said inlet; said igniter means including an electrical gas igniting member adjacent said heater pilot burner and a pressure responsive switch for energizing said igniting member; and heat responsive means adjacent said heater pilot burner arranged, when heated, to open said heat controlled valve.

3. The arrangement defined in claim 1 including a heater pilot burner adjacent said one burner; a third conduit extending from said selector valve to said heater pilot burner, said selector valve being arranged to establish communication between said first and third conduits when actuated to connect said second conduit to said inlet.

4. In a gas oven having at least a broil burner therein; a selector valve having an inlet arranged to receive gas from a supply thereof, said selector valve having at least one outlet for gas; a first conduit leading from said outlet to said burner; a heat controlled valve normally closing said first conduit; means responsive to actuation of said selector valve, in connecting said first conduit to said inlet, for opening said heat controlled valve; a standby pilot burner adjacent said one burner; a standby valve adjacent said heat controlled valve; further conduits for directing gas from said supply to said standby valve and from said standby valve to said standby pilot burner: and means interconnecting said standby valve and said heat controlled valve so that said standby valve is opened in response to opening of said heat controlled valve and is closed in response to closing of said heat controlled valve, said means for opening said heat controlled valve including a heat responsive element and a movable actuating member movable in response to said heat responsive element; said standby valve having a movable element engaging said actuating member, said heat controlled valve including a valve member operable by said actuating member, said actuating member and said valve member being arranged with lost motion therebetween such that said valve member closes before said actuating member closes said standby valve.

5. In a gas oven having at least a bake burner therein; means defining an oven enclosure having a generally cubical portion and a further portion therebelow; means defining a fixed table-like surface; means supporting said enclosure spaced above said table-like surface; said further portion of said enclosure being at the rear thereof and above the rear edge of said table-like surface, said further portion being bounded in part by a bottom wall sloping downwardly and rearwardly from adjacent the front of said enclosure to substantially said rear edge; said bake burner being positioned in said further portion, adjacent the rear thereof and substantially over said rear edge whereby to leave said generally cubical portion unobstructed by said burner while providing substantial clearance space between said enclosure and said table-like surface.

6. In a gas oven having a broil burner and another burner therein; a selector valve having an inlet arranged to receive gas from a supply thereof, said selector valve having at least two outlets for gas; a first conduit leading from one of said outlets to said broil burner; a second conduit leading from the other of said outlets to the other of said burners; a heat controlled valve normally closing said first conduit; a pressure responsive igniter means arranged to be responsive to the differential pressures, in said first conduit, on opposite sides of said heat controlled valve; said selector valve being selectively operable to connect said outlets to said inlet and to isolate both outlets from said inlet, said selector valve being further arranged to establish communication between said first and second conduits when said outlets are both isolated from said inlet whereby to direct atmospheric pressure from said burners to opposite sides of said heat controlled valve.

References Cited by the Examiner UNITED STATES PATENTS 2,639,765 5/1953 Hollman 158-133 2,791,212 5/1957 Hollman et al 12639 3,033,463 5/1962 Doner et al 23615 X 3,167,250 1/1965 Wantz et al 23668 3,186,398 6/1965 Poracki 12637 3,194,227 7/1965 Beach et al 12639 CHARLES J. MYHRE, Primary Examiner.

Claims

1. IN A GAS OVEN HAVING A BAKE BURNER AND A BROIL BURNER THEREIN: A SELECTOR VALVE HAVING AN INLET ARRANGED TO RECEIVE GAS FROM A SUPPLY THEREOF, SAID SELECTOR VALVE HAVING AT LEAST TWO OUTLETS FOR GAS; A FIRST CONDUIT LEADING FROM ONE OF SAID OUTLETS TO ONE OF SAID BURNERS; A SECOND CONDUIT LEADING FROM THE OTHER OF SAID OUTLETS TO THE OTHER OF SAID BURNERS; A HEAT CONTROLLED VALVE NORMALLY CLOSING SAID FIRST CONDUIT; A PRESSURE RESPONSIVE IGNITER MEANS ARRANGED TO BE RESPONSIVE TO THE DIFFERENTIAL PRESSURES, IN SAID FIRST CONDUIT, ON OPPOSITE SIDES OF SAID HEAT CONTROLLED VALVE; SAID SELECTOR VALVE BEING SELECTIVELY OPERABLE TO CONNECT EITHER OF SAID OUTLETS TO SAID INLET AND TO ISOLATE BOTH OUTLETS FROM SAID INLET, SAID SELECTOR VALVE BEING FURTHER ARRANGED TO ESTABLISH COMMUNCATION BETWEEN SAID FIRST AND SECOND CONDUITS WHEN SAID OUTLETS ARE BOTH ISOLATED FROM SAID INLET WHEREBY TO DIRECT ATMOSPHERIC PRESSURE FROM SAID BURNERS TO OPPOSITE SIDES OF SAID HEAT CONTROLLED VALVE.