US3744955A

US3744955A - Electric ignition mechanism

Info

Publication number: US3744955A
Application number: US00198591A
Authority: US
Inventors: H Tyler; J Clifford
Original assignee: Robertshaw Controls Co
Current assignee: Robertshaw Controls Co
Priority date: 1971-11-15
Filing date: 1971-11-15
Publication date: 1973-07-10
Anticipated expiration: 1990-07-10

Abstract

An electric ignition mechanism is disclosed for controlling a fuel burner system employing an electric ignitor having a positive temperature coefficient of resistance such that the current flow therethrough is dependent upon the temperature of the igniter. An electrical circuit, including a heat expansable wire in series with the ignitor and a coil in parallel with the ignitor, senses temperature responsive changes in the electrical characteristics of the ignitor. First and second switches are controlled by the wire and coil and are connected to control the operation of a fuel valve so that the system will be placed in a fail-safe condition in response to any failure of a component or electrical power. A spring is provided between the two switches to ensure the positive actuation of the second switch.

Description

mited States Patent 1 Tyler et al.

[11] 3,744,955 [451 July 10,1973

ELECTRIC IGNITION MECHANISM Inventors: Hugh J. Tyler, Santa Ana; Joseph Clifford, Corona, both of Calif.

Robertshaw Controls Company, Richmond, Va.

Filed: Nov. 15, 1971 Appl. No.: 198,591

Assignee:

References Cited UNITED STATES PATENTS /1972 Willson 431/66 12/1970 Willson 431/66 Primary Examiner-Edward G. Favors Attorney-Anthony A. O'Brien [57] ABSTRACT An electric ignition mechanism is disclosed for controlling a fuel burner system employing an electric ignitor having a positive temperature coefficient of resistance such that the current flow therethrough is dependent upon the temperature of the igniter. An electrical circuit, including a heat expansable wire in series with the ignitor and a coil in parallel with the ignitor, senses temperature responsive changes in the electrical char acteristics of the ignitor. First and second switches are controlled by the wire and coil and are connected to control the operation of a fuel valve so that the system will be placed in a fail-safe condition in response to any failure of a component or electrical power. A spring is provided between the two switches to ensure the positive actuation of the second switch.

Claims, 5 Drawing Figures ee 52 88 z\ \\\\s 44 \

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FIG?) ELECTRIC IGNITION MECHANISM BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention pertains to an electric ignition mechanism, more particularly to an ignition mechanism for an electric igniter having a positive temperature coefficient of resistance.

2. Description of Prior Art A recent trend in the field of igniting devices for fuel burners is to utilize electric igniters in place of the wellknown standing pilot flames. It has been found that burner operation can be monitored by this type of igniter itself if the igniter element is constructed of a material having a resistance which varies with temperature. If the resistance of the igniter varies as a function of the temperature of the igniter, the operation of the igniter and burner can be controlled by sensing igniter resistance.

Inherently dangerous conditions can arise in fuel burners resulting from improper operation of ignition systems. It is therefor necessary that such systems be capable of being shut down upon the failure of any component or interruption of electricity to the system. More specifically, it is important that the valve supplying fuel to the burner be open only when heat is called for and igniting temperatures exist to prevent a buildup of raw fuel. It is also desirable to have the system capable of recycling when igniting temperatures are not timely provided, once electricity is restored after an interruption or when the fuel valve has been closed due to other failures in the system such that another attempt is made to initiate ignition whereby burner operation is not restricted when the system is capable of proper operation.

While, as previously mentioned, the resistance of electric igniters has been used in electric ignition systems, such systems have been crude and have not provided the comprehensive fail-safe or recycling characteristics required. Furthermore, such ignition systems have not taken full advantage of the temperature variable resistance characteristics of the electric igniter element.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to construct a novel electric ignition mechanism utilizing the electrical characteristics of an electric igniter which vary with temperature.

Another object of the present invention is to utilize an electric igniter having a positive temperature coefficient of resistance as a sensor and control element as well as for ignition purposes.

A further object of the present invention is to provide an electric ignition mechanism having electrical sensing means connected to an igniter to sense the changing electrical characteristics and control the operation of a fuel valve in response to these changes.

Yet another object of the present invention is to provide an interlocked snap switch in a switching portion of an electric ignition mechanism which will assure positive and reliable fail-safe operation of the mechanism.

This improved electric ignition mechanism has the advantage of utilizing an igniter for temperature sensing as well as ignition, providing positive and reliable operation, and having both fail-safe and recycling modes of operation.

The present invention is employed with a fuel burner system having a burner an electrically actuated valve for feeding fuel to the burner, and an electric igniter in igniting proximity to the burner and having a positive coefficient of resistance such that the electrical characteristics thereof are dependent upon igniter temperture. The invention is characterized by an electrical in put, sensing means connected in circuit with the igniter to be responsive to changing electrical characteristics thereof, and control means responsive to the sensing means and operative to control the flow of fuel through the valve and the energization of the igniter. The sensing means includes a thermally expansible wire heated by the passage of current therethrough while the control means includes lever means which move in response to expansion and contraction of the wire to control opening and closing of switch means to effect the desired and called for operation.

Further advantages of the present invention will be apparent from the following description taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an electric ignition mechanism embodying the present invention and showing the mechanism in a de-energized state;

FIG. 2 is a schematic diagram similar to FIG. 1 but showing the mechanism with the igniter at ignition temperature;

FIG. 3 is a schematic diagram similar to FIG. 2 but showing the mechanism after the igniter has dropped below ignition temperature;

FIG. 4 is schematic diagram showing the mechanism of FIG. 1 in a released condition; and

FIG. 5 is a schematic diagram similar to FIG. 1 with the mechanism shown at the start of a recycle operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present electric ignition mechanism is shown in FIGS. 1 through 5 in connection with thermostat l0 and electrically operated valve 12 which controls the flow of fuel from a source, now shown, through conduit 14 to burner 16. The valve 12 may be controlled by any suitable electric actuator such as a solenoid, which has not been illustrated since acceptable valves of this type are readily available. An electric igniter 18 is located in igniting proximity to the ports of burner 16. The igniter 18 is formed from material having a positive temperature coefiiciant of resistance such as, molybdenum disilicide, and will exhibit a change in the current passing therethrough as the temperature of the igniter changes. l-Iot wire 20 extends between

terminals

22 and 24 which are mounted in fixed spaced relationship in casing 26. Hot wire 20 has a thermal expansion characteristic such that it expands as it beats and contracts as it cools. At normal room temperature the hot wire 20 has a relatively contracted state, as shown in FIG. I. Casing 26 encloses the remainder of the mechanism except for pulling link 28 which extends through opening 30 in the casing with one end fastened to the hot wire 20 at a central location and the other end connected to lever 32 which has one end pivotally connected at 34 to frame member 36. Helical compression spring 38 is mounted between lever 32 and casing 26 to bias the lever 32 for rotation in the counterclockwise direction. An insulated roller 40 is carried on the free end of extension 42 of lever 32. Frame 46 is fixed within the housing and has valve blade 44 extending therefrom in cantilever fashion with contact 48 on the free end thereof. A cooperating contact 50 is mounted near the free end of snap blade 52 at a position to close with contact 48. The free end of the snap blade 52 is adapted to engage with the insulated roller 40. A U-shaped spring 54 is connected between the snap blade 52 and the free end of blade 60. The blade 60 carries a contact 56 near its free end positioned to engage with a fixed contact 58 mounted on frame 62 which is mounted in the casing 26 so as to be moved by screw64 in order to vary the spacing between

contacts

56 and 58. Both the snap blade 52 and the blade 60 are mounted to extend in cantilever fashion from the frame 102 which is fixed in casing 26 and connected to the terminal 24.

The free end of lever 32 extends through an opening 66 in the end 68 of spring 70. The spring 70 is mounted on armature 72 which in turn is pivoted about pivot 74. Spring 76 biases the armature 72 towards clockwise rotation about the pivot 74 with the force exerted by spring 76 being adjusted by means of screw 78. The armature 72 normally rests against the ends of a horseshoe magnet 80. The pivot 74 is located at the outer edge of the righthand leg of the magnet, as shown in the drawings, and coil 82 is wound about the lefthand leg and is connected to the ignitor.

A bi-metallic spring 84 is mounted between the casing 26 and the free end 68 of spring 70 and normally biases the spring in an upward direction. A feedback spring 86 is mounted across the armature 72, in a manner similar to spring 70, and carries on its free end a kickoff projection 88 which is positioned to engage snap blade 52.

The mechanism is connected to a source of power, not shown, through transformer 90 which provides power to the mechanism through lead 92, terminal 94, leads 96, 98, terminal 100, terminal 102, leads 104, 106 and 108.

Leads

110 and 112 connect the ignitor 18 to the coil 82.

The operational sequence of the mechanism is shown in FIGS. 1 through 5. When the mechanism is in a ready condition, as shown in FIG. 1, the thermostat is open indicating that no heat is called for. In this condition valve 12 is de-energized and closed so that no fuel will flow from the source to the burner, thus preventing an unwanted buildup of raw fuel when the igniter is not in an igniting condition.

In the ready condition the hot wire is relatively taut, as shown, so that the pulling link 28 is in its lower most position holding the rotating lever 32 down, after a clockwise rotation about pivot 34, and against the force of helical spring 38. The insulated roller 40 is engaged with'and holds the valve blade-44 down keeping the contact 48 away from contact 50 on the snap blade 52. The insulated roller also holds down the end of the snap blade 52 causing the C-shaped spring 54 to hold contact 56 in engagement with the stationary contact 58. The end of lever 32 extending through opening 66 in end 68 of spring 70 holds the spring against the bias of spring 76 and likewise holds armature 72 in contact with the ends of magnet 80.

When the thermostat 10 is closed, calling for heat as shown in FIG. 2, the circuit including the ignitor 18 and the hot wire 20 will be energized. This circuit comprises transformer 90, lead 106, thermostat 10, lead 104, frame 62,

closed contacts

56 and 58, blade 60, frame 102, terminal 24, hot wire 10, terminal 22, frame 100, lead 98, igniter 18, lead 96, terminal 94 and lead 92. The hot wire 20 will expand as its heats, thus allowing-the start of an upward movement of link 28 caused by the action of the spring 38 against lever 32. However, the

contacts

48 and 50 will be remain open for a substantial upward movement of link 28. Spring will start to relieve the force it applies the armature 72 when lever 32 no longer holds down the lower end of opening 66, but a sufficient force will be available to keep the armature 72 firmly in engagement with the ends of electromagnet 80.

When the temperature of the igniter 18 reaches its proper ignition temperature, as shown in FIG. 2, voltage is supplied to coil 82 to energize the electromagnet and insure that armature 72 will be held against the force of the bias spring 76. No retaining force is applied by the lever 32 to the end of the spring 70 at this time and the insulated roller 40 of the lever 32 moves out of contact with the valve blade 44 thus allowing

contacts

48 and 50 to close thereby energizing the valve 12 which will open and allow fuel to flow to the burner where it will be ignited by'the igniter 18.

The temperature responsive bimetal spring 84 normally supplies a small bias against the end 68 of spring 70. As the ambient temperature of the mechanism rises, the resistance of coil 82 becomes higher which tends 'to allow the armature 72 to drop out prematurely. Bimetal spring 84 is arranged to decrease the force it applies to the spring end 68 as the temperature rises so that the tendency of the armature 72 todrop out prematurely will be counteracted and the mechanism therefore will be temperature compensated.

In the event that an external effect cools the igniter 18 while the line voltage remains high, as shown in FIG. 3, the temperature of the igniter may drop below ignition temperature without a sufficient drop in voltage to the coil 82 to drop out the system because of the high current flow. This condition is made fail-safe by using the normally high current to further expand hot wire 20 and allow the link 28 to rise further so that spring 38 will press lever 32 upwards against the feedback spring 86. This causes the mechanism to move to the release position, shown in FIG. 4, thereby shutting off the fuel flow by de-energizing valve 12.

When the armature 72 is released by the electromagnet 80 for any one of many reasons, this indicates that igniter 18 is no longer at ignition temperature or that the thermostat 10 has opened to no longer call for heat. The armature 72, when released, rotates in a clockwise direction about pivot 74 under the influence of spring 76. The kickoff projection 88 onfeedback spring 86 will be drawn up against the snap blade 52 causing contact 50 to move away from contact 48 to de-energize and close the valve 12 to stop the flow of fuel to the burner. The snap blade 52 also moves up to a position which will cause spring 54 to snap contact 56 away from contact 58. The igniter 18 and the wire 20 will then be deenergized and, as the hot wire 20 cools and contracts, the mechanism will return to the ready condition shown in FIG. 1, assuming that the thermostat 10 is not closed to again call for heat or that recycle has started if the disturbance is only temporary.

FIG. 5 illustrates the mechanism at the start of a recycle operation. The hot wire 20 has cooled and contracted to a point where the lever 32 begins to move the end 68 of spring 70 downward compressing spring 38 and moving armature 72 into engagement with both ends of the electromagnet 80. The insulated roller 40 on extension 42 pulls the valve blade 44 down ahead of the snap blade 52 ensuring that the

contacts

48 and 50 do not make; this prevents the unintendend energization of the valve which would allow flow of fuel to the burner. As the hot wire cools further it enables extension 42 and roller 40 to move the snap blade 52 downward enough for spring 54 to snap the movable contact 56 up to make with contact 58. When

contacts

56 and 58 close they will reenergize igniter 18 and start a relighting cycle, assuming the thermostat is closed.

The sequence of operation of first closing the armature 72 firmly against the electromagnet 80 and then snapping contact 56 up to close with the contact 58 to turn on the system provides a positive interlock without depending on armature 72 to operate magnetically through a small magnetic gap. If the igniter 18 comes up to ignition temperature, conditions shown in FIG. 2 will prevail and the system will energize valve 12 and a normal heating cycle will be provided.

The present invention has been described in connection with a specific embodiment which must be considered as merely illustrative since many variations and modifications are possible without departing from the spirit of the invention.

What is claimed is:

1. An electrical ignition mechanism for fuel burner systems having a burner, electrically operated valve means controlling flow of fuel to said burner and an electric igniter in igniting proximity to said burner, said igniter having a positive coefficient of resistance such that the electrical characteristics thereof are dependent upon the temperature of the igniter, said mechanism comprising an electrical input means connected to a source of electricity and controlled by thermostatic switching means;

sensing means including a thermally expansible wire connected in series with said igniter, said wire sensing changes in the electrical characteristics of said igniter by being heated by the passage of current therethrough;

control means connected to said sensing means and adapted to selectively energize and deenergize said valve means from said input means in response to the temperature of said electric igniter,

said control means including lever means connected with and movable in response to expansion and contraction of said wire, first and second switch means connected in parallel to control energization of said igniter and said valve means, respectively, from said input means and to be actuated sequentially by said lever means, and means responsive to movement of said lever beyond a given point to positively actuate said second switch means with a snap movement whereby energization of said valve means is responsive to the sensed temperature of said electric igniter.

2. An electrical ignition mechainsm according to claim 1 wherein said snap movement means interconnects movable contacts of said first and second switch means in such manner that a given movement of the first switch positively actuates the second switch.

3. An electric ignition mechanism according to claim 1 wherein said sensing means further comprises coil means connected in parallel with said igniter responsive to the voltage developed across said igniter, said igniter exhibiting a high resistance at igniting temperature, and

said control means further comprises armature means responsive to energization of said coil means and adapted to actuate said switch means.

4. An electric ignition mechanism according to claim 3 further comprising first and second spring means acting in opposition on said armature means to normally hold said armature means in a first position,

said lever means engaging said first spring means and controlling the force exerted thereby on said armature so that as said force is reduced said armature is biased by said second spring means to a second position.

5. An electric ignition mechanism according to claim 4 further comprising means to adjust the force exerted by said second spring means.

6. An electric ignition mechanism according to claim 1 further comprising spring means biasing said lever means whereby said wire is always under tension.

7. An electrical ignition mechanism for fuel burner systems having a burner, electrically operated valve means controlling flow of fuel to said burner and an electric igniter in igniting proximity to said burner, said igniter having a positive coefficient of resistance such that the electrical characteristics thereof are dependent upon the temperature of the igniter with a high resistance being exhibited at ignition temperature, said mechanism comprising an electrical input means connected to a source of electricity and controlled by thermostatic switching means;

sensing means including a thermally expansible wire connected in series with said igniter to sense changes in the electrical characteristics of said igniter by being heated by the passage of current therethrough, and coil means connected in parallel with said 9gniter responsive to the voltage developed across said igniter;

control means connected to said sensing means and adapted to selectively energize and deenergize said valve means in response to the temperature of said electric igniter, said control means including lever means connected with and movable in response to expansion and contraction of said wire, first and second switch means connected to control energization of said igniter and said valve means, respectively, from said input means and to be actuated sequentially by said lever means, armature means responsive to energization of said coil means and adapted to actuate said switch means, first and second spring means acting in opposition on said armature means to normally hold said armature means in a first position, temperature compensating means acting against asid first spring means to vary the force exerted thereby in accordance with ambient temperature, said lever means engaging said first spring means and controlling the force exerted thereby on said armature so that as said force is reduced said armature is biased by said second spring means to a second position, and means responsive to movement of said lever beyond a given point to positively actuate said second switch means with a snap movement whereby actuation of said valve means is responsive to the sensed temperature of said electric igniter.

8. An electric ignition mechanism according to claim 7 wherein said temperature compensating means comprises a bimetallic element.

9. An electrical ignition mechanism for fuel burner systems having a burner, electrically operated valve means controlling flow of fuel to said burner and an electric igniter in igniting proximity to said burner, said igniter having a positive coefficient of resistance such that the electrical characteristics thereof are dependent upon the temperature of the igniter, said mechanism comprising an electrical input means connected to a source of electricity and controlled by thermostatic switching means;

sensing means including a thermally expansible wire connected in series with said igniter to sense changes in the electrical characteristics of said igniter by being heated by the passage of current therethrough;

control means connected to said sensing means and adapted to selectively energize and deenergize said valve means in response to the temperature of said electric igniter, said control means including lever means connected with and movable in response to expansion and contraction of said wire, first and second switch means connected to control energization of said igniter and said valve means, respectively, from said input means and to be actuated sequentially by said lever means, and means responsive to movement of said lever beyond a given point to positively actuate said second switch means with a snap movement;

said mechanism having a ready condition in which said thermostatic switching means is open, said wire is contracted and at ambient temperature, said first switch means is closed to enable said igniter and said second switch means is open to disable said valve,

a first operating condition in which said thermostatic switching means is closed, said wire is hot and partially expanded so that said lever closes said first and second switches to energize both said igniter and said valve,

a second operating condition in which said thermostatic switching means is closed, said wire is hot and further expanded so that said lever opens both said switches to deenergize said valve and said igniter, and

a reset condition in which as said wire cools and contracts said lever closes said first switch means while maintaining said second switch means open so that said valve will not be reenergized before said igniter.

10. An electrical ignition system for fuel burners comprising electrical input means connected to a source of elec tricity and controlled by a thermostat;

an electric igniter having a positive coefficient of resistance such that the electrical characteristics thereof are dependent upon the temperature of the igniter, said igniter being in igniting proximity to the burner;

electrically operated valve means controlling flow of fuel to said burner;

first switch means connected to control energization of said igniter means from said input means;

second switch means connected in parallel to said first switch means and to control energization of said valve means from said input means;

control means including a thermally expansible wire electrically connected in series with said igniter and linkage means mechanically connected to said wire and to said switch means, said wire responding to changes in passage of current through the igniter according to the temperature thereof and accordingly expanding and contracting, said linkage means responding to movement of said wire to sequentially actuate said switch means; and

means responsive to movement of said linkage means beyond a given point to positively actuate said second switch means with a snap movement whereby said valve means is only open when said igniter is at ignition temperatures.

Claims

1. An electrical ignition mechanism for fuel burner systems having a burner, electrically operated valve means controlling flow of fuel to said burner and an electric igniter in igniting proximity to said burner, said igniter having a positive coefficient of resistance such that the electrical characteristics thereof are dependent upon the temperature of the igniter, said mechanism comprising an electrical input means connected to a source of electricity and controlled by thermostatic switching means; sensing means including a thermally expansible wire connected in series with said igniter, said wire sensing changes in the electrical characteristics of said igniter by being heated by the passage of current therethrough; control means connected to said sensing means and adapted to selectively energize and deenergize said valve means from said input means in response to the temperature of said electric igniter, said control means including lever means connected with and movable in response to expansion and contraction of said wire, first and second switch means connected in parallel to control energization of said igniter and said valve means, respectively, from said input means and to be actuated sequentially by said lever means, and means responsive to movement of said lever beyond a given point to positively actuate said second switch means with a snap movement whereby energization of said valve means is responsive to the sensed temperature of said electric igniter.

3. An electric ignition mechanism according to claim 1 wherein said sensing means further comprises coil means connected in parallel with said igniter responsive to the voltage developed across said igniter, said igniter exhibiting a high resistance at igniting temperature, and said control means further comprises armature means responsive to energization of said coil means and adapted to actuate said switch means.

4. An electric ignition mechanism according to claim 3 further comprising first and second spring means acting in opposition on said armature means to normally hold said armature means in a first position, said lever means engaging said first spring means and controlling the force exerted thereby on said armature so that as said force is reduced said armature is biased by said second spring means to a second position.

7. An electrical ignition mechanism for fuel burner systems having a burner, electrically operated valve means controlling flow of fuel to said burner and an electric igniter in igniting proximity to said burner, said igniter having a positive coefficient of resistance such that the electrical characteristics thereof are dependent upon the temperature of the igniter with a high resistance being exhibited at ignition temperature, said mechanism comprising an electrical input means connected to a source of electricity and controlled by thermostatic switching means; sensing means including a thermally expansible wire connected in series with said igniter to sense changes in the electrical characteristics of said igniter by being heated by the passage of current therethrough, and coil means connected in paRallel with said igniter responsive to the voltage developed across said igniter; control means connected to said sensing means and adapted to selectively energize and deenergize said valve means in response to the temperature of said electric igniter, said control means including lever means connected with and movable in response to expansion and contraction of said wire, first and second switch means connected to control energization of said igniter and said valve means, respectively, from said input means and to be actuated sequentially by said lever means, armature means responsive to energization of said coil means and adapted to actuate said switch means, first and second spring means acting in opposition on said armature means to normally hold said armature means in a first position, temperature compensating means acting against said first spring means to vary the force exerted thereby in accordance with ambient temperature, said lever means engaging said first spring means and controlling the force exerted thereby on said armature so that as said force is reduced said armature is biased by said second spring means to a second position, and means responsive to movement of said lever beyond a given point to positively actuate said second switch means with a snap movement whereby actuation of said valve means is responsive to the sensed temperature of said electric igniter.

9. An electrical ignition mechanism for fuel burner systems having a burner, electrically operated valve means controlling flow of fuel to said burner and an electric igniter in igniting proximity to said burner, said igniter having a positive coefficient of resistance such that the electrical characteristics thereof are dependent upon the temperature of the igniter, said mechanism comprising an electrical input means connected to a source of electricity and controlled by thermostatic switching means; sensing means including a thermally expansible wire connected in series with said igniter to sense changes in the electrical characteristics of said igniter by being heated by the passage of current therethrough; control means connected to said sensing means and adapted to selectively energize and deenergize said valve means in response to the temperature of said electric igniter, said control means including lever means connected with and movable in response to expansion and contraction of said wire, first and second switch means connected to control energization of said igniter and said valve means, respectively, from said input means and to be actuated sequentially by said lever means, and means responsive to movement of said lever beyond a given point to positively actuate said second switch means with a snap movement; said mechanism having a ready condition in which said thermostatic switching means is open, said wire is contracted and at ambient temperature, said first switch means is closed to enable said igniter and said second switch means is open to disable said valve, a first operating condition in which said thermostatic switching means is closed, said wire is hot and partially expanded so that said lever closes said first and second switches to energize both said igniter and said valve, a second operating condition in which said thermostatic switching means is closed, said wire is hot and further expanded so that said lever opens both said switches to deenergize said valve and said igniter, and a reset condition in which as said wire cools and contracts said lever closes said first switch means while maintaining said second switch means open so that said valve will not be reenergized before said igniter.

10. An electrical ignition system for fuel burners comprising electrical input means connected to a source of electricity and controlled by a thermostat; an electric igniter having a positive coefficient of resistance such that the electrical characteristics thereof are dependent upon the temperature of the igniter, said igniter being in igniting proximity to the burner; electrically operated valve means controlling flow of fuel to said burner; first switch means connected to control energization of said igniter means from said input means; second switch means connected in parallel to said first switch means and to control energization of said valve means from said input means; control means including a thermally expansible wire electrically connected in series with said igniter and linkage means mechanically connected to said wire and to said switch means, said wire responding to changes in passage of current through the igniter according to the temperature thereof and accordingly expanding and contracting, said linkage means responding to movement of said wire to sequentially actuate said switch means; and means responsive to movement of said linkage means beyond a given point to positively actuate said second switch means with a snap movement whereby said valve means is only open when said igniter is at ignition temperatures.