US2263822A

US2263822A - Fuel burner control system

Info

Publication number: US2263822A
Application number: US253337A
Authority: US
Inventors: John M Wilson
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1939-01-28
Filing date: 1939-01-28
Publication date: 1941-11-25
Anticipated expiration: 1958-11-25

Description

Nov. 25, 1941. J. M. wlLsoN 2,263,822

FUEL BURNER CONTROL' SYYSTEM Filed Jan.v 28, 1939 2 Sheets-Sheet l l Figi 7 Nov. 25, .1941.' l J. M. wlL'sbN 2,263,822

FUEL BURNER cNTRoL' SYSTEM Filed Jan. 28, 1939 2 sheets-sheet 2 Patented Nov. 25, 1941 FUEL BURNER CONTROL SYSTEM John M. Wilson, Minneapolis, Minn., assignor to Minneapolis-Honeywell tor Minneapolis,

Minn., a corporation lof Company. Delaware Application January 28, 1.939, Serial No. 253,337

18Claims.

This invention relates to an automatic control system for a fuel burner having certain features of safety incorporated therein.

The main object of the invention is to provide a simple and relatively inexpensive control system in which no electromagnetic relays are used and which employsa relatively few number of parts, but yet gives protection against failure to obtain combustion, name failure after combustion has been established, and power failure.

In many prior art systems it has `been customary to have a safety switch, a combustion responsive switch, and at least one electromanetic relay for operating the motor circuit. It is an object of this invention to simplify this arrangement by having the thermal timer which actuates the safety switch also act as the motive power vfor closing the burner motor circuit, there.- by eliminating the electromagnetic relay.

A further object of the invention is to provide a thermal timer which is under the control of the room thermostat, or any other desired control instrumentality, and whichvhas a cold position which it holds the burner motor control s tch in open position, an intermediate position or range in which it holds the burner 'motor control switch closed, iand a hot position Y in which it opens the safety switch. A combustion responsive device is added to limit the movement of the thermal timer to its intermediate range if combustion has been established so that the burner motor switch will be held in running position.

A further object is to provide a thermal timer of the above type which locks the system in inoperative condition on flame failure or failure to obtain combustion, thereby requiring a manual resetting operation. These and other objects will become readily apparent as the following specification is read in the light of the accompanying drawings. in which Figure 1 is adiagrammatic view of my novel control system,

Figure 2 is a, diagrammatic view of the same system using a slightly different type of com-v bustion responsive device, and

Figure 3 is a view of the same system using a third type of combustion responsive device.

Referring now particularly to the drawings, a

room thermostat which is generally indicated at Il comprises a bimetallic coil I2 which actuates a switch arm i3 on variations in temperature in such a manner that on a decrease in switch arm will engage the stationary contact Il. Although a room thermostat has been disclosed it will be understood that this may be replaced by any type of controlling instrument desired, such as, for example. a boiler thermostat, a boiler pressure responsive device, or outdoor thermostat.

The safety switch which is generally indi- `cated at I6 comprises a pair of resilient contact blades Il and il which are anchored ,at

one end to the brackets il and 20, respectively. The free end of the contact blade Il is adapted to rest on the insulating tip 22 of the manual reset member 23. This reset member is slidably supported in a plate 24 and has a button 2l for manual operation. It is provided also with an insulated tip 26 which is adapted to cooperate with the free end of the contact blade il under certain ,conditions to be described hereinafter.

l an amount suiiicient for the block Il to drop oil! the end 33 of the timer, the blade il will drop down until its free end engages the tip 2l on a reset member 23 thus separating the contacts 34 and I5. An electric heating element l0 is provided for heating the bimetal timer 22.

Mounted as shown at Il is a resilient blade carrying at its free end a contact 43 which 40 is adapted to cooperate with contact 44 carried -This switch is of the usual slip friction type' on the freeend of the bimetal timer 32. The contact I3 is so podtioned that it will be engaged by contact u as the timer nexes toward the left as shown in Figure l before the timer moves from beneath the insulated block Il. In other words, these contacts will engage before the switch contacts 34 and II are tripped open by the timer. Connected in the circuit with the contacts 4I and u is a 'combustion responsive switch indicated generally at I l which comprises a switch arm 41 and a stationary contact-M.

and responds to the heat of combustion in such a manner that on an'increase in temperature temperature beyond a predetermined point than dus m Iune establishment of comunicata switch arm 41 will engage its contact 46 and on a decrease in temperature due to failure of combustion the switch arm will separate from the contact 46.- It is understood that this switch depends for operation on a change in temperature and not on any predetermined temperature.

Pivoted at. 56 is an arm 5I whichcarries a mercury switch 52 and link 53 is connected at one-end to the arm I and at the other end Operation In considering the operation of this system let it be assumed that the room thermostat II is satisiied so that its contact arm I3 is separated from the stationary contact I4 and that this condition has existed for a sullicient period of time v for the blmetal timer 32 to have returned to itsA cold position as shown in Figure 1. Under these circumstances the mercury switch 52 will be in open position so that'no current will flow to the burner motor or the ignition means and therefore there will be no heat supplied to the room. Under these circumstances the temperature in the room will fall causing the blmetal element I2 to move the switch arm I3 into engagement with the contact I4 which will, establish the followingl circuit: from line wire 56, conductor 66, contact I4, switch arm I3, bimetal element I2, conductor 6I, bracket I3, contact blade I1, contacts 35 and 34, contact blade I6, bracket 26, conductors 62 and 63, electric heater 46, protective resistance 64, and conductor 65 which is connected to line wire 51. This circuit will energize the heater 46 which will raise the temperature of the blmetal timer 32 causing it to fles toward the left as indicated by the arrow in Figure l.

15, switch arm 41, contact 46, conductor 16, contact blade 42, contacts 43 and 44, blmetal timer 32 and bracket 3I.

The closure of this shunt circuit will deenergize the heater 46 and the blmetal timer will therefore start-to cool and as a result will flex toward the right separating the contacts 43 and 44. This in turn breaks the shunt circuit and reenergizes the heater 46 which will cause the bimetal timer to again ex toward the left to re'- make the shunt circuit. Thus the blmetal timer will`continu e to oscillate back and forth making and breaking the contacts 43 and 44 as long as the circuit to heater 46 remains closed through the room thermostat and as long as the combustion responsive switch 41, 46 remains closed in the shunt circuit for the heater, In this position f of the timer 32 the burner motor mercury switchV 52 will be maintained in closed position and the system will run normally. The resultant in- I means 55 is also carried by the room thermostat The blmetal timer will therefore continue to flex The timer through the v link 53 will rotate arm 5I causing .the mercury 66in the tube 52' to flow to the opposite end of 'the tube closing the following circuit: from line wire 56 through the above traced circuit including the conductors 60, room thermostat II, conductor 6I, safety switch blades I1 and I3 to conductor 62 then through conductor 61, mercury 66, conductor 66, conductor 66, motor'54, conductors 16 and 1I back to line wire 51. A

circuit including conductors

12 and 13 connects the ignition means in parallel with the motor 64 from the conductor 66 to the conductor 1I. "I'his circuit therefore energizes the burner motor and the ignition means which should produce combustion within the furnace. If combustion is established the combustion responsive device 46 will be heated causing the switch arm 41 to engage the stationary contact 46. Closure of this switch will not have any effect at the present time because the circuit to contact 46 is opened at the contacts 43 and 44. However the bimetal timer 32 is still moving toward the left under the influence of the '46 from conductor 63 to the bracket 3I to which the heater 46 is connected. This circuit may be traced as follows: from conductor 6 3, conductor to the left pressing back the contact 43 and the resilient blade 42 until the insulated block 36 drops oil' the end of the timer 32 permitting the contact blade I6 to move downwardly and break the circuit at 34, 35. As these two contacts are in series withthe heater, burner motor, and ignition means they will all be immediately deenerg'ized, The blmetal timer 'will then start to cool but the block 36 will prevent it from flexing vtoward the right and reclosing the switch 34, 35. In order to accomplish this it is necessary to manually position the restarting member 23 which will rst raise the spring blade I1 and then raise the blade I6 lifting it free of the blmetal element 32. vIf the timer 32 has cooled sufilciently it will move toward the right and open the burner motor switch 52 and the blade I6 will be reset in its contact lclosed position. It will be noted. that as the blade I 6 is being reset that the blade I1 *is moved out of the path of movement of the blade I6 so that the contacts N and '35 4can only be engaged if the timer 32 has moved to the right sufllciently to hold the blade I6 in its contact closed position. In other words, this safety switch is trip free. After the system has been manually reset in this manner it is conditioned to restart in the normal manner.

On power failure the blmetal timer 32 would cool and return toits original position opening escasas the switch l2 so that upon resumption of power the system would recycle normally.

InfFigure2thereisdisclosedasystemwhich operates on the same principle as the system shown in Figure 1 except that a slightly different arrangement of the combustion responsive device is disclosed. The room thermostat, safety switch, thermal timer, manual reset member. mo-

tor, motor switch, burner motor and ignition means are identical with-those in Figure l and will not-again be described. The heater |43 for lthe timer |32 is also shunted by the combustion responsive device but in this case the combustion responsive device moves the contact I 43 bodily instead of operating a switch in series therewith. 'Ihis is accomplished by providing a combustion responsive thermostat ill which longitudinally moves an operating rod IBI which. in turn, is connected through a slip friction |82 with a lever |53 which is shown to be pivoted at |54. I'he lever |33 carries at its free end the contact |43 and two stops |25 and |86 limit the movement of the lever |83.

When the combustion responsive thermostat |3l`is cold the lever |33A is in the position shown in Figure 2 and in this position the contact |43 out of the path of-movement of the contact |44 carried by the timer |32. On a call for heat by the room thermostat ill a circuit is closed from the line wire |56 through conductor |60, room thermostat Ill, conductor iii, contact blade H1,

In case combustion is not established by energiding the motor |54 and ignition means |55 the combustion responsive thermostat ill will not reBDOnd and the lever |33 will remain against the stop |35. With the lever in this position the contact |43 cannot be engaged bythe contact |44 and the bimetal timer |32 will trip the safety switch ||6 to open position as described in con- 'nection with Figure l. Also if combustion should The system disclosed in Figure 3. is different from the systems dlsclosed'in Figures 1 and 2 in that the combustion responsive device provides a mechanical stop for preventing the thermal timer from tripping the safety switch after combustion has been established. This system comprises a room thermostat 2|| for operating a. switch arm 2|2 to engage its stationary contact 2|3 on a decrease in temperature. The safety switch 2|4 comprises a pair of resilient contact blades 2|5 and 2|5 which carry cooperating contacts 2|1 and 2|! in the same manner as in Figures 1 and 2. The bimetal timer 220 carries at its upper end an insulated tip 22| which normally holds the contact blade 2li in closed position. The timer 220 is connected by means of link 223 with a pivoted arm 224 for actuating mercury switch 225 which controls the delivery of electrical energy t0 the burner moto;` 226 and the ignition means 221. The timer is provided with an electric heater 230' and a protective resistance 23| is located in series therewith.

In this modication the thermal timer 220 carries a pin 232. The combustion responsive thermostat 235 carries a pin 236 on which is slip frictionally mounted a sleeve 231'having a projection 238 which is adapted to cooperate with a pair of

stops

239 and 240 which limit its movement. In the Acold position ofthe combustion responsive thermostat 235 the sleeve 231 is withdrawn into engagement with the stop 240 at which time the bimetal timer 220 may flex to the left under -the influence of the heater 230 an amount sufllcient to trip the safety switch 2| to open position. The circuit to the heater may be traced as follows: from line wire 245, conductor 246, contact 2|3, switch arm 2|2, thermostat 2li, conductor 241, contact blade 2|5, contacts 2|1 and 2| 8, contact blade 2|6,

conductors

248 and 249, electric heater 230, protective resistance 23| and conductor 250 back to the line wire 25|. This will cause the bimetal timer 220 to f'lex toward the left and close the switch 225 thereby energizing the burner motor and -the ignition means as in Figures 1 and 2. If combustion is-not established the bimetal timer will continue to flex to the left until the contact blade 2| 6 drops off the end of tip 22| opening the safety switch contacts 2|1. and 2|8 which will deenergize the burner motor and the ignition means as well as the heater 230. The safety switch may be reset to closed position by means of the manual reset member` 255 in the same manner as in the nrst two modifications.

If, after the bimetal timer 220 has energized the burner motor and ignition means, combustion is established the combustion responsive thermostat 235 will expand moving the sleeve 231.. into engagement with the stop 233 and in this position as the bimetal timer 220 continues to flex toward the left the rod 232 will engage the sleeve 231 whereby the timer will be prevented from further movement so that it cannot trip the safety switch 2|4 to open position. This is the running condition of this system and the heater 23h will remain continuously energized instead of being alternately energized and deenergized. If a flame failure should occurv under these conditions the combustion responsive thermostat 235 would contract and remove the sleeve 231 from engagement with the pin 232 and the timer 225 would then be permitted to flex toward the left sufficiently to trip the safety switch 2|4 to open position. 0n power failure the heater 230 would be deenergised and the timer 220 would return to -sponsive switch is open. e

. its cold position so that the system would recycle on resumption of power in the same manner as Figures 1 and 2.

'Thus it will be seen that VI have provided a relatively simple control system for an oil burner 5.

- that a combustion responsive device is provided for preventing the timer from tripping the safety switch as long as combustion is present. This system eliminates the use of electromagnetic relays and also provides protection against failure to obtain combustion on starting, flame failure after combustion has once been established, 13 and power failure.

It will be appreciated that these systems have been shown for the purpose of illustration only and that the scope of the invention is to be limited only by the appended claims.

I claim as my invention:

l. In a fuel burning system, in combination, an electrical timing device which moves in a first direction when energized and returns in the opposite direction when deenergized, a main switch for energizing said electrical timing device, means in control of the supplying of fuel, means controlled by said electrical timing device to operate said last named means to supply fuel when said timing device moves to a first position and to stop the supply of fuel when the timer `moves to a second. position spaced from said first position upon movement of said timer in said rst direction, and circuit connections including a contact operated by the timer and a contact controlled b y combustion conditions to deenergize said electrical timing device when the same reaches an intermediate position between said first and second positions and to again energize said electrical timing device upon retraction of the same from i0 said intermediate position.

2. In a fuel burning system, in combination, an electrical timing device which moves in a first direction when energized and returns in the opposite direction when deenergized, a main switch for energizing said electrical timing device, means in control of the supplying of fuel, means controlled by said electrical timing device to operate said last named means to supply fuel when. said timing device moves to a first position and to stop the supply of fuel when the timer moves to a second position spaced from saidfirst position upon movement of said timer in said first direction, and circuit connections including a closed-whenhot combustion responsive switch, a stationary contact, and a movable contact controlled by said timing^ device to deenergizeY said electrical timing device when the same reaches an intermediate position between its first and second positions and to again energize said electrical timing de.- ivice upon retraction of the same from such intermediate position, said stationary contact being incapable of limiting the movement of said elec'i trical timing device when said combustion re- 3. In a fuel burning system, in combination, an

electrical timing device'which moves in a first direction when energized andretumsin-the Vopposite direction when deenergized, a main switch for energizing said electrical timing device, means in control of the supplying of fuel, means controlled by said electrical timing device to operate said last named means to supply fuel when said timing device moves to a first position and to stop the supply of fuel when the timer moves to a sec- .7-5

ond position spaced from said first position upoii movement of said timer in said first direction, circuit connections comprisingk a switch having first and second contacts, said 'first contact being controlled by said electrical timing device, and combustion responsive means for moving said second contact to a hot position upon the establishment of combustion to be engaged by said first contact as said electrical timing device reaches an intermediateposition between its first and second positions to complete the circuit connections and deenergize said electrical timing means and to again energize the same as it retracts from said intermediate position, said second contact in its cold position being out of the path of movement of said rst contact.

4. A fuel supply control system comprising in combination, a timer, an electrical means, a circuit for energizing said electrical means, a condition responsive element in control of-said circuit, said electrical means upon energization thereof delivering energy to said timer to produce a timed movement thereof from an original position, said timer upon deenergization of said electrical means having a timed movement back to its original position, fuel supplying means, a circuit therefor, a first switch in said circuit, said timer on movement from its original position closing said first switch to render said fuel supplying means operative, a second normally closed switch in said circuit. said timer upon continued movement from its original position opening said second switch, and means including means res ponsive to combustion for controlling said electrical means to deliver just suicientpower to said timer whereby it holds said first switch closed and does not open said second switch.

5. A fuel supply control system comprising in combination, a timer,` an electrical means, a circuit for energizing said electrical means, a condition responsive element in control of said cir-A cuit, said electrical means upon energization thereof delivering energyto said timer to produce a timed movement thereof from an original position, said timer upon deenergization of said electrical means having a timed movement back to its original position, fuel supplying means, a circuit therefor, a first switch in said circuit, said timer on movement from its original position closing said first switch to render said fuel supplying means operative to supply fuel, a second normally closed switch in said motor circuit and also in the circuit to said electrical means, said timer upon continued movement from its original position opening said second switch, means including means responsive to combustion for controlling said electrical means to deliver Just sufficient power to said timer whereby it holds said firstswitch closed and does not open said second switch, means to lock said second switch in open position after it has been opened by said timer due to lack of combustion, and means for manually reclosing said second switch.

6. A fuel supply control system comprising in combination, a timer, an electrical means, a cir cuit for energizing said electrical means, aondition responsive element in control of said circuit, said electrical means upon energization" thereof delivering energy to said timer tofpro-V duce a timed movement thereof from an origina-l;E position, said timer upon deenergization'of suivit.;i electrical means having a timed movement baci fuel supplying means, circuit therefor, a iirslt switch in said circuit, -a

to its original positio circuit for deenergizing said electricahmm comprising a combustion responsive switch which is closed whenever combustion is present and a stationary contact, said timer on movement from its original position rst closing said first switch to render said fuel supplying means operative and then engaging said stationary contact completing the deenergizlng circuit for said electrical means providing said combustion switch is closed, which causes a reversal in movement of said timer and a reopening of said last mentioned circuit to again reverse movement of said timer, said timer continuing to oscillate back and forth, a second normally closed switch in said` fuel supplying means circuit and the circuit for said electrical means, said timer continuing its movement away from its original position if said cornbustion switch is open to open said second switch, and means to prevent said timer from reclosing said second switch after it has once opened it 7. A fuel supply control system comprising in combination, a timer, an electrical means, a cir- -cuit for energizing said electrical means, a condition responsive element in control of said circuit, said electrical means upon energization thereof delivering energy to said timer to produce a Vtimed movement thereof from an original position, said timer upon deenergization of said electrical means having a timed movement back to its original position, fuel supplying means, a circuit therefore, a first switch in said circuit, said timer on movement from its original position closing said first switch to render said fuel supplying means operative, a second normally closed switch in said fuel supplying means circuit and also in the circuit to said electrical fmeans, said timer upon continued movement its originalposition opening said second }-'switch, means for deenergizing said electrical means comprising a contact, and a combustion responsive device for moving said contact from a cold position in which it is beyond the path of movement of said timer and a hot position in which it is' engaged by said timer before said timer opens said second switch, engagement of said timer and said contact causing deenergization of said electrical means causing said timer to reverse its movement which in turn causes it to disengage said contact and reenergize said electrical means to'again cause said timer to move ix the original direction, said timer conl therefor, a iirst switch in said circuit,a normally closed safety switch in said circuit, a thermal timeroperatively, associated with said first switch, a heater for said timer, a -circuit for said heater, a condition responsive switch in control of said heater circuit, said thermal timer under the control of said heater having a cold position in which said first switch is in open position, an intermediate position in which said first switch is in closed position. and a hot position in which said timer trips said safety switch to open position. a shunt circuit for said heater, a switch in said shunt circuit, and a combustion responsive element, one contact of said last named switch being moved bysaid thermal timer, the other contact being movedby said combustion responsive element in such a manner thatw'hen said combustion responsive element is in its cold position the contact moved thereby is out of the path of movement of the contact moved by the thermal timer, and when' the combustion responsive element is in its hot position the contact moved thereby is in such a position that it engages the contact moved by the thermal timer when the thermal timer is in its intermediate position thus completing the shunt circuit aroimd said heater, whereby said heater is alternately energized and deenergized as said timer alternately breaks and makes the two contacts.

10. Ina fuel .burning system, in combination, an electrical timing device which moves in a r'st direction when energized and returns in the opposite direction when deenergized, means in control of the supplying of fuel, a safety switch in control of said last named means, said safety switch comprising a pair of switch elements biased to open circuit position, said timer in deenergized position engaging beneath one of said elements to mechanically hold said safety switch closed, a main switch for energizing said timing device to cause it to move from its deenergized tinuing to oscillate back and forth to hold said first switch closed.

8. A fuel supply control system comprising in combination, fuel supplying means. a circuit therefor, a nrst switch and a normally closed safety switchinsaid circuit,athermaltimer'cp eratively associated with said first switch, a heater for said timer, a circuit for said heater, a condition responsive switch `in control of said heater circuit, said thermal timer under the control of said heater having a cold position'in which said first switch is in open position, an intermediate position in which said nrst switch is in closed position, and a hot position in which said timer trips said safety switch to open podtion,

a shunt circuit for said heater including a closedwhen-hot combustion responsive switch, and a second switch in said shunt circuit closed by said timer when it is in its intermediate position whereby said heater is alternately energized and deenergized as said timer alternately opens and closes said second switch while holding said first switch in closed position.

9. A fuel supply control -systemcomprising in v combination, fuel supplying means, a circuit position to a. second position in which itrenders said fuel supplying control means operative, continued energization causing said timer to moveing a relatively xed' end and a relatively free end, an electrical fuel feeding device, a control switch therefor, a safety switch in control of said fuel feeding device, said safety switch comprising a pair of switch elements biased to open circuit position, one of said elements being mechanically engaged by the free end of`said thermal timer and held in closed circuit position thereby, a main switch for energizing said thermal timer to cause it to move from its first cold position to a second position in which it closes the control switch for said electrical feeding device, continued energization of said thermal timer normally causing it to move to a third position in which it releases said one element thereby permitting said safety switch to open and deenergize said fuel feeding device, and means responsive to the establishment. of combustion for preventing said thermal timer from moving from its second to its third position.

12. In a fuel burning system, in combination,

a timer in the form of a thermal element having a relatively fixed end and a relatively free end, an electrical fuel feeding device, a safety switch control of said fuel feeding device, said safety iirst cold to a second position in which it closesk said control switch thereby establishing a second circuit including said mainswitch, safety switch, control switch, and electrical fuel feeding device, continued energization of said heater causing-said thermal timer, after a predetermined period of time, to assume a third positionin which it releases said one element and permits said safety switch to open, and means responsive `to the establishment of combustion for preventing said thermal timer from assuming its third position.

13. In a fuel burning system, in combination, a timer in the form of a thermal element having a relatively iixed end and a relatively free end, anv electrical fuel feeding device, a control switch therefor, a safety switch in control of said fuel feeding device, said safety switch comprising a pair of switch elements biased to open circuit position, one of said elements being mechanically engaged by the free end of said thermal element and held in closed circuit position thereby, a main switch for energizing said thermal timer to cause it to move from its iirst cold position to a second position in which it closes said control lswitch for said electrical fuel feeding device, continued energization of said thermal timer normally causing it to move to a third position in which it releases said oneelement thereby permitting said safety switch to open and deenergize said fuel feeding device, means responsive to the establishment of combustion for preventing said thermal timer from moving from itssecond to its third positionand means preventing said safety switch from automatically recl'osing.

.14. In a fuel burning system, in combination, a timer in the form of a thermal element having a relatively fixed end and a relatively free end, an electrical fuel feeding device, a safety switch in control'ofsaidi'el feeding device, said safety switch comprising a pair of vswitch elements biased to open circuit position, one of said elements being mechanically engaged andl held in closed circuit position by said thermal timer, a main switch, a heater for said thermal timer, a control switch, a first circuit including said main switch, safety switch, and heater for said thermal timer, said thermal timer moving as a? result of the energization of said heater from its first cold to a second position in which itcloses said control switch thereby establishing a second circuit including said main switch, safety switch, control switch, and electrical fuel feeding device, continued energization of said heater causing said thermal timer, after a predetermined period of time, to assume a third position in which it releases said -one i element and permits said safety switch to open, .means responsive to the establishment of combustion-for preventing said thermal .timer fromv assuming its third position,

aaeasaa once it has been opened, and means for resetting said safety switch to closed position.

a relatively ilxed end and a relatively free end,

an electrical fuel feeding device, a control switch therefor, a safety switch in control of said fuel feeding device, said safety switch comprising-a pair of switch elements biased to open circuit position, one of said elements being mechanically engaged by the free end of said thermal timer and held in closed circuit position thereby, a main switch for energizing said thermal timer to cause it to move'from its rst cold position to a second position in which it closes the control switch for said electrical feeding device, continued energization of said thermal timer normally causing it to move to a third positionV in which it releases said one element thereby permitting said safety switch to open and deenergize said fuel feedingV device, and means moved in yresponse to the establishment of combustion to mechanically engage said thermal timer to prevent it from moving into its said third position. 16. In a fuel burning system, in combination, an electrical fuel feeding device, a control switch therefor, a safety switch, an electrical timing device which moves in a first direction when energized and returns in the opposite direction when deenergized, electrical means for energizing said timer, a main switch, a first circuit including said main switch, safety switch and electrical means, said timer moving, as a result of energization by said electrical means, from a first position to a second position in which it closes said control switch thereby establishing a second cira .timing device which moves -in a first direction when energized and returns in the opposite direction when deenergized, electrical means for energizing said timer, a main switch, a first circuit including said main' switch, safety switch and velectrical means, said timer moving, as a result moans said safety inopen position v75 of energization by said electrical means, from a first position to a second position in which it closes said control switch thereby establishing a second circuit including said main switch, safety switch, control switch and electrical fuel feeding device, continuation of said rst circuit causing said timer, after a predetermined period of timesu to move into a third position in which it opens said safety switch, and means moved in response to the establishment of combustion to mechanically engage said thermal timer te prevent it from moving into its said third position.

18. In a fuel burning system, in combination, an electrical timing device which moves in a first direction when energized and returns in the opposite direction when deenergized, means in control vof the supplying of fuel.- a safety switch in control of said last named means, said safety switch comprising a pair of switch elements biased to open circuit position, said timer in deenergired position engaging beneath one of said elements to mechanically hold said safety switch closed, a main switch tor energizing said timing device to cause it to move from its deenergized position to a second position in which it renders said fuel supplying control means operative, continued energization causing said timer to move to a third position in which it releases said one- `element to permit said safety switch to open,

5 suflicient to cause it to move into its third position.

JOHN M. WILSON.