US2696874A - Oil burner safety control - Google Patents

Description

Dec. 14, 1954 p, SCHELL OIL BURNER SAFETY 4CONTROL Filed Feb. l0, 1951 6 m e 4 M 7 7 c. 2 M a M0, w 0a f. IIIII. ,70.

r .v 19 "5 a 6 7 E Aw v 5 5 w=my w v United States PatenttOfiice 2,696,874 -Patented Dec. 14, 1954 2,696,874 0.1L BURNER SAFETY coNrRoL lalllschell, Elkhart, Ind., assignor to` Penn Controls, Inc.,

` Goshen, Ind., a corporation of Indiana v.

'4 Application February 10, 1951, Serial No. 210,329 9 claims. (ci. rss- 28) "one or more 1'ecycles, and then if3 failure. persists, results :in shutfdown..

`One object of the invention is to provide an arrangement of relaysand'switch actuators for controlling the fuel valve, the burner "blower motor, and the ignition iofan oil burner or thev like insuchvmanner that'all 'the mentionedfeatures can be obtained with complete safety of operation inconnection with each feature.

Another object is tofprovide' a..combination ,of main and delay relays forclosing the blower motor :circuit when the roorn thermostat first calls for heat to provide an `initial purging/period duringV which the ignition 1son and lclose Athe circuit to the fuel valve which may be of the solenoid type by-the energization of the` delay relay so that combustion air and ignition are already supplied 'before the oil or other fuel is supplied to the burner.

Still another object is to provide themain relay and the delay relay` so interconnected electrically that upon shut-down as a result of combustion. failure subsequent to successful establishment of combustion, apost purge period occurs in which the motor continues to operate after the fuel valve is closed.

A further object is to provide the circuit with a safety switch for' effecting the normal shut-down following initial combustion failure and also shut-down after attempts to recycle following subsequent combustion failure, the safety switch requiring manual resetting so that the troublethat causedshut-down must be remedied before such manual resetting operation can be performed.

Still a furtherfob'ject is to providea combustion switch arrangement connected with the main and delay relays so as to cooperate withthem in providing the recycling after subsequent combustion failure with final shut-down after recycling and initial yshunt-down after initial combustion failure. f

An additiohalobject is to provide an arrangement of two v'relays in a control'circuit `-wherein one relay v'pulls in 'iirstand.v starts rthe 'burner operatior'randthe' se`co'nd` relay pulls" ii1'"1ater and drops out last. during the stopping operation, the 'desired sequencer of 'operations being effec'ted by a pair 'of motor switches connected in parallel and' a pairoffuel valve switches connected in series with one'switch of e'ac'h pair actuated by the first relay kconsists in the'l construction, arrangement and combinafully set`forth,` pointed out inmy claims and illustrated 'I' in the accompanying dravyil'uzs,` wherein: j

The figure is an electrodiagrammatic View of an oil burnerfembodyingmy present invention. V

'On th accompanying"drawi`rigs I have used the re erence' characterM'to' indicate *theblower' motor of a burner B which is shownl diagrammatically and-is the vuisiialtypeoiili-burner used in o il heated furnaces. The blewerdifhgimtliglghgwn;at28,1 ,fflt'f' ,Tart

v1A fuel valve is providedforlcontrolling the flowof -'fuel-'to\ th rner nozzle* inf. a'nvell known manner",l this ofitheslenbidtype andfits solenoid being lay DR fromr through 78 to 80.

minutes.

indicated at S. For igniting the combustible mixture fof atomized oil and'air, ignition mechanism is shown 'diagrammatically at Ign.

' 'A transformer T is shown having a primary P and a 'pairjof s econdarycoils vS1 and S2 of low voltage, for

instance l2 volts each, the 'primary being for the usual Yhouse current of llO or` 220 volts Supply wires 10 and `12 fo'r the primary P are shown'and these also supply current for the -`motorl M,'the ignition Ign and the solemotor switch MS1 and wires16 and 17 'lead from the 'motor switch'to the motorl M, another wire 18 connectingthe motor'to `the line wire'12. A .second motor "switch MS2.is 1in parallel'with the motor' switch MS1 Ithrough wires 20 and 30.

j" The'ignitionswitch Ign is viir'parallelwith the motor M' through 'wires' 3l),4 22," 24 andA 26, an ignition switch valve switches FVS1 and FVS2 in'series with each otherand in circuitwith the fuel valve solenoid S. 1This completes -the 110^^v`olt side-of thettransformer circuit.

t"Thrlowifvolt-age circuits supplied by the secondary 4coilsS1 and lS2 of theA transformer consist of feed wires 43, 44 and 45 for one side of the line and 46, 47 and 48 for the other side. interposed betweenthe wires 43vand 44 is a safety switch SS which switch is normally closed and is adapted to be' opened by a safety switch actuator SSA- which may comprise anV electrical heater. Theheat -ofwthe` heater-aifects abimetal element 69 for warping it upwardly-to operil the switch'- SS- after about 90 seconds vof energization of SSA. -A dog '71 drops under the bimetal element-to retain the safety switch SS in the open position, and manual reset is required as by thumb pres- 4sure appliedto` the end 73 of the dog 71.

circuit for-MR may be traced from S1 through 43, SS,

44, 50, RT, 52, 54, MR, 56, 58 and 46 to S2. At the same time some of the current shunts the coil of MR, passing from52 through 64, DSA1 andl66 to 58. The delay switchactuator DSA is aheatercoil fora bimetal which is 'adapted 'toclose a snap acting delay switch DS after a few seconds of energization of the heater DSA. :l

The delay` switch DS controls a circuit for a delay re- The main relay MR when energized4 closes the motor switch MS1 and the fuel valve switch FVS1.as indicated by dot-andsdash lines and arrowheads. The actuation of SS by SSA and DS by with dot-and-dash lines and arrowheads.

I also .provide'an' ignition switch actuator ISA whic is a heater acting on abimetal element 77 adapted to op'en acombustion failure-:- switch-' CFS and the, ignition switch IS1 as indicated by dot-and-,dash lines and arrow- The opening o'f'these switches occursA about 60 seconds afterlSAfis energized and, whenever itis deenergized the switches CFS and IS1 reclose vin about two The switch CFS is initially in circuit with DR and SSA andwith a, combustionv switch CS1, from the .wire'fso'through SZQCES, 7 4, 72, SSA, 68, CS1, 70 and l47 back to' 46.-

When the holding switch HS closes, the circult for Ythecoil of the delay relayDR is maintained from the coil S1 of the transformer by the current from 74 owingthrough 62, HS, and 60 to the center tapy of the transformer secondary. Subsequently the combustion swltch CS1 opens andV another combustion switch CS2, closes -under. theactio'n of a bimetal element 79 whichis responsive to combustion. The switches CS1 .an d CS2 are preferably notof thesnap-acting type. When combustion has been established, the movement of 79 upwardly opens the switch CSla-.aesiassathsswitqhaii Closure 0f CS2 establishes a circuit from 80 through 84, ISA, 8 6, VCS2, ardl48L!tof4L1otrnaintaimthedelay.` relay. DR energlged after-CFS opens aswill'hereiiiafter'appear. 1 4: 3

PRACTICAL OPERATION Normal starting When the room thermostat RT calls for heat, it energizes the main relay MR and the delay switch actuator DSA. Energization of MR closes MS1 to energize the burner motor M and the ignition lgn as IS1 is normally closed, and closes FVS1 to ready the circuit of the fuel valve FV for subsequent energization of the solenoid S when the delay relay DR closes.

In a few seconds DSA causes the bimetal element 75 to snap the delay switch DS closed so that DR is energized through CFS, SSA, and CS1. This closes the motor switch MS2 for subsequent post purge purposes, and closes FVS2 to energize the fuel valve so that fuel is supplied after an initial purge period effected by energization of the motor M and ignition Ign between the time that MR is energized and the time that DS closes. Energization of DR also closes the holding switch HS for holding DR energized even though CS1 opens subsequently due to combustion establishment.

Combustion establishment also closes CS2 to energize ISA and thereby open the ignition IS1 about 60 seconds after combustion has heated the bimetal 79. This is usually sufficiently long to insure that combustion will be maintained. The burner is now in normal running condition and the combustion failure switch CFS is opened by ISA so that the relay DR will be subsequently de-energized to open the motor switch MS2 and the fuel valve switch FVS2 upon combustion failure opening the switch CS2 as will be later described.

Normal shut-down When RT becomes satisfied, it opens the circuit through MR and DSA. De-energization of MR opens the motor switch MS1. The circuit, however, through the motor is still maintained through the second motor switch MS2 so that the burner motor continues to operate to provide a post purge period after normal shut-down ofthe burner to purge the-furnace of any fumes or explosive gases that might be contained therein after the fuel valve closes because of FVS1 being opened. The motor is then turned off in about 30 seconds when DS or CS2 opens, that is, which ever opens first.

Subsequent combustion failure This is combustion failure after combustion has been successfully established, and is the result of some fault such as water in the fuel extinguishing the llame. The combustion chamber or stack, upon cooling, permits the bimetal element 79 to also cool for reclosing the combustion switch CS1 and reopening the combustion switch CS2. The opening of CS2 breaks the circuit for the coil DR as the only circuit at the time is through ISA and CS2. This immediately stops the ow of oil because of FVS2 opening but the motor M is kept energized for a post-purge period because even though switch MS2 opens, switch M81 remains closed. Even a reclosure of the combustion switch CS1 causes nothing further to happen until the ignition switch actuator ISA has cooled suiciently to reclose the ignition switch IS1 thus providing ignition through M51 still closed at 30.

Cooling of ISA takes about two minutes and will cause re-establishment of the ignition circuit through IS1 and re-establishment of the delay relay circuit through CFS, SSA and CS1. Energization of DR then recloses the fuel valve v FVS2 to give the burner a chance to try to start again. If the fault causing the recycling can correct itself as is often the case, normal combustion will again be established.

After one or more recycling attempts, however, if combustion is not re-established, a shut-down will occur because when CFS recloses, the safety switch actuator SSA is re-energized and if CFS does not open because of CS2 closing to energize ISA, the safety switch actuator SSA will open the safety switch SS thus de-energizing both relays MR and DR. Then, of course, the trouble must be remedied and the safety switch manually reset at 73 for placing the burner back in operation.

Initial combustion failure shut-down without recycling. Thislis accomplished byv CS failing to open so that SSA heats sufliciently (in about 90 seconds) to trip the safety switch SS off as in shut down after subsequent combustion failure.

The arrangement disclosed using two relays, one of them delayed in relation to the other, each relay operating a motor switch, the motor switches being in parallel, and each relay operating a fuel valve switch, the valve switches being in series, provides the desired features of initial and post-purging, recycling after subsequent combustion failure and delayed ignition tripping in addition to eventual shut-down after subsequent combustion failure if the fault that caused the failure does not become remedied.

The disclosure also provides the motor M started and operated a short period of time before the oil valve is opened to secure initial purge. Post purge is provided by M required to operate a short period of time following opening of RT at which time the oil is turned off. Delayed ignition tripping is provided by ISA requiring some time to trip ignition after the combustion thermostat indicates that combustion has been established, and is therefore dependent on same. The recycling operation gives the burner a successive chance to start after flame failure following normal burner operation.

The ignition switch actuator ISA has a resistance high enough to hold the relay DR in once its circuit has been established, yet prevent it from being pulled in when there is a momentary opening of the delay switch DS. Normally this would not occur but if the room thermostat is tampered with, it may be left open long enough to deenergize the delay switch actuator DSA a sufficient length of time that the bimetal 7S opens the switch DS. Then if the room thermostat is closed so as to result in DS closing again before CS2 opens and CS1 closes, the current from S1 through 43, SS, 44, 45, DS, 78, DR, 80, 84, ISA, 86, CS2, 48, 47 and 46 to S2 will not be sufficient to pull DR in. This is particularly necessary to prevent danger from absence of ignition because of IS1 being open when DS closes after the momentary interruption therethrough referred to.

Some changes may be made in the construction and arrangements of the parts of my oil burner control without departing from the real spirit and purpose of my invention, and it is my intention to cover by my claims any modified forms of structure or use of mechanical equivalents which may be reasonably included within their scope.

I claim as my invention:

l. In an oil burner control mechanism to control a burner motor, its ignition means and fuel supply means in response to a call for heat by a room thermostat; said burner control mechanism comprising a primary circuit, a transformer connected therewith and having a secondary provided with an intermediate tap, an initial room thermostat circuit connected across said secondary and adapted to be controlled by the room thermostat, a motor switch, burner motor and ignition circuits controlled thereby, a relay coil operable upon energization by closure of the room thermostat to close said motor switch and thereby initiate operation of the burner motor and its ignition means, a delay switch actuator included in said room thermostat circuit, a delay switch actuated thereby, a delay relay having an actuating coil in circuit with said delay switch and having a switch operable to control the fuel supply means, combustion switch contacts and a safety switch actuator in the delay relay coil circuit, said combustion switch contacts being closed in the absence and open in the presence of combustion, a safety switch included in said room thermostat circuit, said safety switch being normally closed but opened by said safety switch actuator upon energization thereof for a predetermined period of time, holding contacts closed by energization of said delay relay coil and connected between said delay relay coil and said intermediate tap, a normally open combustion switch which is closed in the presence of combustion, said last switch being in circuit with said delay relay coil across said secondary, an ignition switch actuator also in said last named circuit, and an ignition switch for said ignition means opened by energization of said ignition switch actuator, said last combustion switch shunting said combustion switch contacts and said safety switch.

2. In an oil burner control mechanism to control a burner motor, its ignition means and fuel supply means in response to a call for heat by a room thermostat; said burner control mechanism comprising aprimary circuit, a transformer connected therewith and having a secondary provided with an intermediate tap, an initial room rthermestat, circuit ,connais-'d .activs #Sd -sodary and "including a" room therrr'lostat;l a` lntor switeh,`f bur`n`er frnotor land Aignition circuits controlled'. thereby, af relay hcoil operable upon energization by closure of vthe room thermostat to ,close said motor switch andthereby initiate operation ofthe burner motor andits ignition means, an Hignition kswitch for the ignitionfrneans, a .delay` switch actuatorincluded inlsaid room thermostat circuit,4 a delay yswitch actuated thereby, a delay relay having an actuat-` ing coilin circuit `with said delay fswitch and having a switch operable-to controlnthe Afuel supply means, a delayed safety switch actuator and` a combustion failure switch and cor'nljqustion switcht contacts in ,the delay relay circuit, said combustion switchcon'tac'ts b'eing'closed vin `the absence and open inthe presenceofcombustion, a lsafety switch connected in saidfroorn thermostat circuit and` normallyclosed, .said safety switch' being'y operatively s'lsociyatcdwithand'opened byfsaid sa'fty switch actuator 'up'o'ri energization; thereof for-'a predeterminedperiod offtime, vholding contacts vclosedfby energization of said "delay relayl'coil and connected .between said combustion failre 1"switch and. said; intermediate. tap, an ignition vactuator'of` delayed type roperatively associatedwith. said ignition switch; and said combustion'.l failure switch,y a 'normally open 4conlbustionlswitchL which is cl'osedin the l 'presence 'of combustion, said `last 'switch-and said ignition .'switch .actuator being in circuit with said delay relay coil 'acrossf said secondary and inf shunt relation to said safety switch actuator, /combustion'faiiure switch and, combus- 'tion switchcontacts; v l i 3f, lena burner control to`control a burner motor, fuel "supply means lincluding aefuelvalveand ignition lmecha- 'nismfortheburner in response to acallfor heat by a room'thermostatg said 4burner controlcomprising a motor circuit having a pair, of switchesthereinl connectedin parallel with eachotheria circuit adapted to connect the ignition mechanism inshunting relation to, the motor and having a normally closed ignitionv switch therein, a fuel valve circuit having .a pair of Lswitches therein connected in series with each other, a transformer having a pair of ,secondary coils connected in'series with eachy other, a t main, relay coil,` arcircuit'across said secondaryz coils `adapted v`t o;be controlled by the room thermostat and including said main relay coilya'delay switch actuator, a. circuit connecting 'said actuator in shunting relation to saidjmain'. lrelaycoil; a delay switchfactuatedfby said delay switch 'actuatorga delayrelaycoil, a circuit therefor through said delay switch, a normally closed cornbustion switch and a normally closed combustion failure switch in circuit with said delay relay coil, each of said fuel valve switches being controlled by one of said relay coils, each of said motor switches also being controlled by one of said relay coils, a holding switch for said delay relay coil connected between said secondary coils and said delay relay coil through said combustion failure switch, a normally open combustion switch and an ignition switch actuator also in circuit with said delay relay coil, said ignition switch actuator being operable to open said ignition switch and said combustion failure switch a time period after closure of said normally open combustion switch, said normally open combustion switch and said ignition switch actuator being in shunt relation to said normally closed combustion switch and said combustion failure switch.

4. In a burner control to control a burner motor, its ignition mechanism and a fuel valve in response to a room thermostat; said control comprising a transformer, an initial room thermostat circuit adapted to be controlled by a room thermostat, a main relay having a coil, a delay switch, a delay switch actuator, a delay relay having a coil in circuit with and adapted to be energized upon closing of said delay switch, a burner motor circuit having a pair of parallel connected motor switches therein, a fuel valve circuit having a pair of series connected fuel valve switches therein, an ignition circuit for the ignition mechanism having an ignition switch therein, said ignition circuit being connected in shunt with the motor and thereby controlled by said motor switches, said main and delay relay. coils being each operable to close one of said motor switches and one of said fuel valve switches upon energization 'of said relay coils, combustion responsive delayed acting means for opening said ignition switch, a switch operated by said delayed action means and in c lrcult with said delay relay coil, said combustion responsive delayed v:acting: means-uponresponding: te *combustion 'failure "tleei'ergizin'gr 4sa'idfd'ejlay 'relay coil'j to"'perir1it 'opening of th'e'! motor 'switch and' the fuel valve y'switch controlled 'thereby tostop thefiow ofr'uel butp'ermit the motor to 5 'operate-becauseof-the other motorfswlitch'being closed,

ffsaid `ignition"'switch upon'l'reclosing'due to Ithe deenerl'g'izati'o'njofV said delayed/acting' means reestablishing ignition, and the `'circuit of said delay vrelay coil being reestablis'hedlby said combustion responsive delayed' act- 10 ing in'eans to recycle th'e burnerfif' thejfa'ult thatlc'aused fl'ar'e Pfailure i's'gby'then remedied.'

v5. "A'bu'rner lc'ontr'ol fo'r cortrollinga burner having a 'niotorlfor? operating said b ufrner,a fuel valve for the Ifuel i `to Hsaid burner and'jignition'mechanism forthe burner in l5 espoii's'efto" a'r'oo'in thermostat; 'said control comprising g operating circuit f'ori'the burner "inclu'dingla pair of -mot'or' 'switclie l'iii-parallel' with-eachother', an operating circuit for the fuel valve` including a'pair of fuel valve fsw'itches in' series 'with eachother and an'loperating'circuit lH ffor"theignitionfinechanisin including an ignition switch,

controlling circuits for theforego'ngl elements comprising 'a' '-niain'relay'coil and' a delay relay coil, .a room thermof 'stat circuit adapted to 'b'e'controll'ed by the room thermos 'tat'frleriergizingsaid mainv relay coil and said delay v relay.4 coil', delay 'means' interposed"betweensaid'circuit and said'delay relaycoilt energize said delay relay coil -a time" per'iod` afterene'rgization of said main relay coil, yeach'of ,'said'A relay coils' being 'op'er'ativel'y associated )witl' one 'f's'aid motor lswitches and one of said fuel valve switches'ftoclose the 'same upon'energization of ',tli'e'frelay coils, 'a normally closed "combustion failure switch and a normally cl'o'se'd combustion switch in series c'ir'c'uit 'with' said-',delay rela'y coil, `an ignition switch actuator- 'anda normally open combustion switch also in 'se'riefs'eircuitwith'said'delay relay coil and shunting said y='combustio'nffailureswitch 'and said normally closed com- "bustio'fn 's'witchQ'said 'ignition switch' actuator being'operatively 'associated with saidc'ombustion failure switch and vwitli'said-'ignition-"switch toopen them upon predeter- 40 mined energization of' saidjgnition switch actuator under ,control of said normally 'open combustion switch, and 'a' circuit-including 'a'holding switch connected through "-said' delayfrelay coil and said co'mbustion'failure switch, f'said'holding .switch being closed bylenergization of 4said 'delayf'rel'a'y coil." l, t.

f' 6. A burner control'to control a burner having a motor #l orf'operatin'g the burner,a fuel valve for'the fuel `to the burner and ignition mechanism for the burner under control of a room thermostat; said control comprising an operating circuit for the burner including a pair of motor switches in parallel with each other, an operating circuit for the fuel valve including a pair of fuel valve switches in series with each other and an operating circuit for the ignition mechanism including an ignition switch, controlling circuits for the foregoing elements having a safety switch, a main relay coil and adelay switch actuator therein, said circuits being adapted to be energized by the room thermostat, a delay relay coil, a delay switch therefor closed by said delay switch actuator to energize said delay relay coil a time period after energization of said main relay coil, each of said relay coils being operatively associated with one of said motor switches and one of said fuel valve switches to close the same upon energization of the relay coils, a normally closed combustion failure switch, a safety switch actuator for said safety switch, a normally closed combustion switch in series circuit with said delay relay coil, an ignition switch actuator and a normally open combustion switch also in series circuit with said delay relay coil and shunting said combustion 70 failure switch and said normally closed combustion switch, said ignition switch actuator being operatively associated with said combustion failure switch and with said ignition switch to open them upon predetermined energization of said ignition switch actuator under control of said normally open combustion switch, and a holding switch connected through said delay relay coil and said combustion failure switch and closed by energization of said delay relay coil.

7. In a control for controlling a burner having a blower 80 motor, a fuel valve and ignition means under control of a room thermostat; said control comprising a pair of motor switches connected in parallel with each other for controlling the motor and the ignition means, a normally P closed ignition switch connected in series with said motor 8* switches for additionally controlling the ignition means,

a fuel valve circuit, a pair of fuel valve switches in series therein for controlling the fuel valve, a main relay coil and a delay relay coil each operable to close one of said motor switches and one of said fuel valve switches, a room thermostat circuit having said main relay coil therein, a delay relay coil circuit having said delay relay coil therein, delayed acting means energized with said main relay, a switch operatively associated with said delay acting means and connected to energize said delay relay coil a time period after energization of said main relay coil, a pair of combustion switches, one normally closed and the other normally open, a combustion failure switch and a safety switch actuator connected in series circuit with said delay relay coil and said one combustion switch, an ignition switch actuator connected in series circuit with said delay relay coil and said other combustion switch and in shunting relation to said combustion failure switch actuator and said one combustion switch, said ignition switch actuator being operatively associated with and arranged to open said ignition switch after a delayed period of time and also to open said combustion failure switch at substantially the same time, and a safety switch connected to open said room thermostat circuit and thereby shut down the operation of the burner, said safety switch being actuated by said safety switch actuator and operable to locked-out position requiring manual resetting.

8. In a control for controlling a burner having a blower motor, a fuel valve and ignition means under control of a room thermostat; said control comprising a pair of motor switches connected in parallel with each other, a circuit for the ignition means in series with said switches, a normally closed ignition switch connected in series in said last circuit, a fuel valve circuit, a pair of fuel valve switches in series therein for controlling the fuel valve, a main relay coil and a delay relay coil each operable to close one of said motor switches and one of said fuel valve switches, a room thermostat circuit having said main relay coil therein, a delay relay coil circuit having said delay relay coil therein, delayed acting means energized with said main relay, a switch operatively associated with said delay acting means and connected to energize said delay relay coil a time period after energization of said main relay coil, a pair of combustion switches, one normally closed and the other normally open, a combustion failure switch and a safety switch actuator connected in series circuit with said delay relay coil and said one combustion switch, an ignition switch actuator connected in series circuit with said delay relay coil and said other combustion switch and in shunting relation to said cornbustion failure switch actuator and said one combustion switch, said ignition switch actuator being operatively associated with and arranged to open said ignition switch and said combustion failure switch after a delayed period of time following closure of said second combustion switch, and a safety switch in said room thermostat circuit connected to open said room thermostat circuit and thereby shut down the operation of the burner under actuation by said safety switch actuator and operable to locked-out position requiring manual resetting.

9. In a control to control a burner having a blower motor, a fuel valve and ignition means under control of a room thermostat; said control comprising circuits adapted to control said motor, fuel valve and ignition means, a pair of motor switches connected in said motor circuit and in parallel with each other, said ignition circuit being in series with said motor switches and adapted to be connected in shunt relation to the blower motor, a pair of fuel valve switches in series in said fuel valve circuit for controlling said fuel valve, a main relay coil and a delay relay coil each operable to close one of said motor switches and one of said fuel valve switches, a room thermostat circuit for said main relay coil, delayed acting means interposed between the room thermostat circuit and said delay relay coil to energize the latter a time periodafter energization of said main relay coil, a pair of combustion switches, one normally closed and the other normally open, a combustion failure switch connected in series circuit with said delay relay coil and said one combustion switch, and a combustion failure switch actuator connected in series circuit with said delay relay coil and said other combustion switch, said actuator being operable to open said combustion failure switch following closure of said other combustion switch.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,732,174 Sweatt Oct. l5, i929 2,188,565 Bower Ian. 20, 1940 2,388,124 Crews Oct. 30, 1945 2,440,700 Rosche May 4, 1948 2,486,612 Pratt Nov. l, 1949 2,616,491 Nycum Nov. 4, 1952

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