US2338786A - Fuel burner control system - Google Patents

Description

Jan. 11, 1944. H, T, SPARROW 2,338,786

FUEL BURNER CONTROL SYSTEM Filed June 16, 1941 2 Sheets-Sheet 1 62 J h 1 655 I 68 80 IIOO kw-" L Invmi'ofl Huber? '1. Sparrow- Bg fi ,e/ M, KML-L'.

Jan. 11, 1944. H. T. SPARROW 2,338,786

FUEL BURNER CONTRbL SYSTEM Filed June 16, 1941 2 Sheets-Sheet 2 coupon Patented Jan. 11, 1944 UNITED STATES PATENT- OFFICE FUEL BURNER CONTROL SYSTEM Kpplic'ationJune.16,1921, Serial No. 398,250

13 Claim.

This invention relates to fuel burner control systems of the type requiring a timer for shutting the system down in the event that combustion is not established within a predetermined period of time following a call for heat. These systems obviously require some device which will stop the running of this time period after combustion has been established.

One of the main objects of the present invention is to provide a thermal timer for accomplishing the desired result, this timer moving, when energized, in a direction to deenergize the fuel delivery device as long as combustion has not been established, but to reverse its direction of movement upon the establishment of combustion, the direction of movement of said thermal timer being controlled by a combustion responsive resistance as long as said timer is energized.

A further object is to utilize this thermaltimer for controlling the starting circuit and therefore time the scavenger period between successive energizations of the fuel delivery device. It is also contemplated that this timer may be used for controlling the running circuit so that the system will recycle in the event of a flame failure.

A still further object is to provide a thermal timer which is compensated both for variations in ambient temperature and also for variations in line voltage. The thermal elements forming the thermal timer are preferably in the form of double helices.

Although this invention will be described in connection with an oil burner control system, it will readily be apparent that it may be adapted forithe control of systems involving other types of fuel.

The above and other objects will become more apparent as the following specification is read in the light of the accompanying drawings, in which:

Figure 1 is a diagrammatic representation of an oil burner control system incorporating my invention, and

Figure 2 is a diagrammatic representation of a slightly modified system.

Referring now to Figure 1, the reference numeral l indicates generally a thermal timer including two heat motors II and I2. Each of these heat motors comprises a double helix bimetal thermostatic element such as isdisclosed in the patent to Parsons 2,121,259. Inasmuch as the strip of bimetal from which these heat m0- tors are made is of a relatively great length, they may be satisfactorily heated merely by passing current through them, thus eliminating the necessity of a separate electric heater.

The thermal elements ii and I2 are mounted at one end on the brackets 13 and I4, respectively, and at the upper ends carry insulating members I5 and I6, respectively, which in turn engage opposite ends of a lever I! which i pivotally mounted as shown at l8, When the two thermal elements H and 12 are at substantially the same temperature, they wili'tend to assume the same length with the result that the lever I! will occupy a horizontal position as shown in Figure 1. If the current flow through element ll should become greater than the current flow through element I2, element II will increase in temperature andcontract and rotate the lever il in a counter-clockwise direction, whereas if the element [2 should be the more highly heated the opposite is true and the lever I! will be rotated in a clockwise direction.

Ann 20 is insulatingly carried by the lever I l and when the lever is in its horizontal position. arm 20 is adapted to engage beneath the resilient blade 2| of a biased open safety switch indicated generally at 22. The upper blade 23 of the safety switch is in closed circuit position with respect to the blade 2l when blade 2| is held by the arm 20, as shown in the drawings. When the lever l I rotates counterclockwise a predetermined amount, the arm 20 moves from beneath the blade 2i which then moves downwardly due to its bias and opens the safety switch 22. The reference numeral 24 indicates a manual reset of the trip-free type for resetting the safety switch 22 to closed position once it has been opened. The ,SDecific construction of this type of switch is old in the art and forms no part of the present invention.

The right-hand end of the lever H, as seen in Figure 1, carries a resilient blade 21 which, when the lever I1 is in horizontal position. is in engagement with the stationary contact 28 and also in a strong part of the field of the permanent magnet. Lever llcarries a second contact arm 30 which is adapted to cooperate with the stationary contact 3!. When the lever ll moves in a clockwise direction, contact arm 30 will engage the stationary contact 3! before the permanent magnet 29 will permit the resilient blade 21 to snap to open circuit position. Therefore the switches 21, 28 and 30, 3| overlap as the lever ll moves in a clockwise direction. On movement of the lever il in the opposite direction, contacts 30 and 3| will separate before the blade 21 moves close enough to the magnet 29 to cause it to snap it into engagement with the stationary contact 28. Therefore, the switches do not overlap when the lever l1 moves in a counter-clockwise direction. This type of switching action is shown in my Patent No. 2,223,796.

When the two thermal elements H and I2 are energized by an electric current flowing through them, the relative amount of current flowing through each one of them is controlled by means of two resistances 35 and 36. The resistance 35 is connected in parallel with the thermal element H by means of the conductors 31, 38 and 95 and the resistance element 36 is connected in parallel with the thermal element l2 by means of conductors 94, 38 and 39. Both of the resistances 35 and 36 may be mounted directly in the combustion chamber of a fur nace indicated at 40. The element 36 is located so as to respond directly to the radiant heat of combustion, whereas the element 35,1esponds only to the ambient temperature in the combustion chamber and is shielded iromdirect heat radiation from the flame by some such means as the shield 4|. The result of this arrangement isthat in the absence of combustion the two resistance elements 35 and 36 will be at substantially the same temperature, but when com bustiori is established the temperature of the element 36 will be considerably higher than that of element 35 due to its absorption. of radiant heat energy from the flame. The resistance of these two elements when they are at the same temperature is unequal, element 36 having a lower value than 35 so that in the absence of combustion the thermal element H will d aw a larger current and hence will be mmfev highly energized than the thermal element l2."-' Upon the establishment of combustion, the resistance-"value of the element 36 will increase above that-cf ele#-- ment 35, at which time the thermal element it? will draw the higher current and therefore be more highly energized than the thermal element ii.

In as much as these two thermal elements it and it oppose each-other it will be clear that changes in ambient temperature will have no effect upon the lever it. Moreover, as current flows through both elements simultaneously,

variations in line voltage will aifect them both alike and will not have any tendency to move lever 81.

This system is provided with the usual oil burner motor 45 and ignition device 46 for producing combustion within the furnace 40. They are under the control of the electromagnetic relay which has a winding 41; This relay controls a plurality of switch arms 48, 49, 50, and 5| which when the relay winding 41 is energized. are moved to engage the stationary contacts 52,

and 55, respectively. The main control is effected by the usual roam thermostat, which nttiiisinstance is shown comprising a birn'e' .coil-56'ifiited at one end and carrying at its 'i'r'eeT'eiid gs pair of switch bladestil and 58, which when the temperature decreases to a predetermined value engage the stationary contacts 53 and 69, respectively. The stationary contacts are so arranged that the blade 5? will first engage contact 59 and then at a slightly lower temperature the blade 58 will engage contact 66. The power for operating the system is derived from two line wires 52 and 63.

-A step-down transformer 64 is provided with a primary winding 65 connected at one end to the line wire 62 by means of conductor 66 and con- In Figure 1 the parts are shown in the position which they will occupy when the room thermostat is satisfied. At this time no current will flow through the two thermal elements H and I2 inasmuch as the circuit thereto is open at the switch blade 50. These elements have reached substantially equal temperatures and therefore the lever I1 is in horizontal position at ill which time resilient'blade 2-1 engages the starting contact 28. The oil burner 45 and the ignition device 46 are deenergized and therefore the temperature in the room in which the thermostat 56 is located will begin to fall. The switch blade 51 will first engage the stationary contact 59 but this will establish no circuit at this time due to the fact that the switch arm 5| is out of engagement with the contact 55. On a further decrease in room temperature, the switch blade 53 will engage stationary contact 60 and establish the following circuit: from one side 10 of the secondary winding 88 through conductor 1|, relay winding 41, conductor 12, safety switch 22, conductor 13, contact 59, switch blades '51 and 58, contact 60, conductors 14 and 15, contact 28, switch blade 21, lever l1, and conductor 16 back to the other side 11 of the secondary winding 68. This circuit energizes the relay winding 41 which causes the switch blades 48, 49, 50,-and 5| to engage their respective stationary contacts. Engagement of the switch blade 48 with the stationary contact 52 establishes a circuit from line wire 62 through conductor 86, contact 52, switch blade 48, conductors 8! and 82, burner motor 45, and conductors 83 and 84 back to line wire 63. The ignition device 46 is connected in parallel with the burner motor 45 by means of the conductors 85 and B6.

Engagement of the switch arm 5| with contact 55 sets up a holding circuit for, the relay winding 41 which is independent of the switch blade 56 and contact 36. This circuit may be traced from the end 10 of the secondary'68, conductor H, relay winding 41, conductor 12, safety switch 22, conductor 13, contact 55, switch blade 51, thermostat 56, conductor 81, contact 55, switch arm 5|, conductors 88, 89, and 15, contact 29, switch blade 21, lever l1, and conductor 16 back to the end 11 to secondary winding 68. There- .tore at this time the switch blade 58 may sepa rate from the contact 60 and have no effect on the relay winding 41 which will remain energized until the first-to-close switch blade 51 separates from the contact 59.

Closure of the switch blade 50 with the stationary contact 54 sets up a circuit for the thermal elements I and 2, as well 'as the resistances 35 and 36. This circuit may be traced from the end 11 of the transformer secondary 68 through conductor 16, lever l1, switch blade 21, contact 28, conductors 15, 90, 89, and 92, contact 54, switch arin 50, conductor 93, thermal element I2, conductors 94 and 95, thermal element H, and conductor 96 back to the tap 91 on the secondary winding 68. The tap 91 is used in this case because it is not desired to have the full secondary voltage applied across the thermal elements II and H. The above circuit causes a current flow through the thermal elements II and I2 which will result in their becoming heated due to the resistance of the bimetal of which they are composed. As set forth above, the relative values of current flowing through these elements is determined by the values of resistance of the two resistance elements 35 and 36 which are connected in parallel with the thermal elements H and I2, respectively. Inasmuch as the temperature of the two resistances 35 and 36 will be the same on an original call for heat when there is no combustion present in the furnace, the resistance value of the element 36 will be lower than that of the element 35 causing the thermal element I l to be more highly energized than the thermal element l2. This will cause a contraction of the thermal element It and a slow movement of the lever 11 in a counter-clockwise direction.

If combustion is not established within a predetermined period of time, the continued movement of lever I! in a counter-clockwise direction will cause the arm 20 to move from beneath the resilient blade 2I'and the safety switch will therefore open due to its bias. This will deenergize the relay winding 41 and all of the switch arms 48, 43, 50, and 5! will drop out, thereby de- 46, and breaking the circuit through the thermal elements Hand 12. It will be noted that the safety switch 22 cannot be reset automatically but must be manually reset by the member 24.

In the event that combustion is established before the safety switch has been opened, the temperature of the resistance element 36 will increase to a value higher than that of the element 35, and at this time the thermal element l2 will become more highly energized than the thermal element l I. This will cause the lever H to start rotating in a clockwise direction. It will move past its original horizontal or starting position and will continue until it moves the switch blade 30 into engagement with the contact 3| and later will snap the resilient switch blade 21 outof engagement with contact 28. The relay winding 41 will now be held in by means of a circuit extending from the end of the secondary winding 68 energizing the burner motor 45, ignition device I through conductor H, relay winding 41, conductor 12, safety switch 22, conductor 13, contact 59, switch blade 51, thermostat 58, conductor 81, contact 55, switch arm 5|, conductors 88, 89 and I00, contact 53,'switch and", conductor Illl, contact 3|, switch blade 30, lever H, and conductor T6 back to the end H of the secondary'winding 68. This is the running condition of the system.

When the temperature in the room increases, the thermostat 56 will cause the switch blade 51 to separate from the stationary contact thereby deenergizing the relay winding 41 and dropping out the switch arms 48, 49, 50 and 5|. This will result in the immediate deenergization of the oil burner motor 45 and the ignition device 46 and will also break the circuit through the termal elements H and I2. They will therefore return to the same temperature and move the lever I! back to its horizontal position, as shown in Figure 1. It will be noted that the relay winding cannot be reenergized until the lever ll recloses the switch blade 21 and stationary contact 28. scavenger period.

A power failure while the system is in running condition will deenergize the relay winding 4'! and, as set forth above, this winding cannot be reenergized until the lever H has returned to It is this action which times the horizontal position. This will prevent the burner motor from being brought back on before the combustion chamber has been purged of unburned vapors.

In the event of a flame failure during the running of the system, the temperature of the element 36 will rapidly decrease and become equal to that of the element 35, at which time the thermal element H will become more highly energized than the thermal element l2. This will cause the lever H to rotate in a counter-clockrecycle by reenergizing its original starting circuit if the switch blade 58 is engaged with contact 60.

In the modification shown in Figure 2, the

thermal timer i0 is exactly the same as in the modification shown in Figure 1, with the exception that the lever l'l directly carries a movable contact H0 which engages stationary contact III. This is the only movable contact carried by the lever H and hence there is no overlap or non-overlap action. The safety switch 22 is exactly the same as is the transformer secondary 68 with its tap 31. Also the thermal element 56 remains the same and the relay winding 41 with its four switch blades 48, 49, and 5|. Also, the same burner motor 45 and ignition device 48 may be used.

This modification differs from the modiflcation of Figure 1 in that one of theresistance elements I I2 which is connected in parallel with the thermal element H has a fixed resistance value and is positioned-so as not to be subject to thetemperature of combustion. The resistance elemerit H3 connected in parallel with the thermal element l2 has a positive temperature co-efilcient of resistance and is very sensitive and may be mounted in the stack H4 which carries the products of combustion from the furnace. When there is no combustion present in the furnace, the resistance element H2 has a higher resistance value than the element H3 and these resistances may be so chosenthat when they are near the same temperature the wattage consumed in thermal element H is approximately 50% greater than that consumed in the element l2. This always results in the rotation of the lever I! in a counter-clockwise direction. Upon the establishment of combustion the temperature of the element H3 will increase and at a temperature of about 400 may increase to such a value that the wattage consumed in the thermal element It will now be approximately 50% greater than that consumed in the thermal element H with the result that the lever I! will now rotate in a clockwise direction.

Operation thermostat, a circuit will be established from one end 10 of the secondary winding 68 through conductor 1|, relay winding 41, conductor 12, safety switch 22, conductor 13. contact 59,'switch blade 51, switch blade 58, contact 60, conductors 14 and 15, contact I i I, movable contact l in, lever- I 1, and conductor 18 back to the other side 11 of the secondary'winding. The above circuit will energize the relay winding 41 and result in the switch arms 48, 49, 50, and 5i moving into engagement with their respective stationary contacts 52, 53, 54, and-55. Engagement of switch arm 48 with contact 52 will result in the energization of the burner motor 45 by means of a circuit extending from line wire 62 through conductor H6, contact 52, switch arm 48, conductors I I5 and I I1, burner motor 45, and conductors H8 and H9 back to line wire 63. The ignition device 46 is connected in parallel with the burner motor 45 by means of conductors I20 and l2l. Engagement of switch arm 5| with contact 55 will close a holding circuit for relay winding 41 which is independent of switch blade 58 and contact 60 in the same manner as described in con nection with Figure 1. Engagement of switch blade 50 with contact 54 will close the circuit through the thermal elements Ii and I2 in the same manner as that traced in connection with Figure 1. Engagement of contact arm 49 with contact 53 will set up the following holding circuit for the relay winding 41: from the end 10 of the secondary winding 88, conductor 1!, relay winding 41, conductor 12, safety switch 22, conductor 13, contact 59, switch blade 51, thermostat 56, conductor 81, contact 55, switch arm 5i, conductors 88, 89 and I00, contact 53, switch blade 49, conductor I22 directly to lever l1 and conductor 16 back to the other side 11 of the secondary winding 68. It will be noted that this holding circuit is set up immediately the relay winding 41 is energized and maintains this energizlation independently of the contacts H0 and H The thermal timer i0 functions in exactly the same manner as itdoes in connection with Figure 1 to open the safety switch in the event that combustion is not established within a predetermined time. If combustion is established, however, the resistance value of the element H3 increases to such an extent that the thermal timer reverses its movement and moves past its starting position, separating the contacts H0 and HI. In the event of a normal shut-down by the thermostat 56 or a shut-down due to power failure, the relay winding 41 will be deenergized and cannot be reenergized again until the thermal timer i0 recloses its contacts H0 and ill. The thermal timer therefore times the scavenger period following a normal shut-down or a shutdown from power failure.

In the case of a flame failure, theresistance value of the element H3 will decrease due to a rapid decrease in temperature in the stack and the thermal element II will become the more highly energized, causing a rotation of the lever H in a counter-clockwise direction. The relay winding 41 will remain energized in this case until the lever H has rotated far enough to move the arm 20 from beneath the switch blade 2| 0f the safety switch 22 and permit this switch to open. This results in the deenergization of the relay winding 41 and hence takes all power off of the system. The system remains effectively locked out until the safety switch is normally reset and hence there is no recycle in this case as there is in Figure 1 which has the contacts 30 and 3| for deenergizing the relay winding 41 immediately following a flame failure.

It will be appreciated that the specific arrangement of the two elements 35 and 36 may be used in connection with th non-recycling system shown in Figure 2, and likewise the fixed resistance H2 and the stack mounted resistance H3 of Figure 2 may be used equally well with the recycling system shown in Figure 1.

It will be seen therefore that I have designed a system in which a single thermal timer controlled by combustion operates a safety switch as well as a starting switch and a running switch for controlling a system which recycles on flame failure and that the running switch may be eliminated if it is desired to provide a system which does not recycle on flame failure.

There will undoubtedly be man changes and modifications of the above two systems which will occur to those who are skilled in the art, and I therefore desire to be limited by the scope of the appended claims and not by the specific embodiments disclosed.

I claim as my invention:

1. A fuel burner control system comprising in combination, electrical means which, when energized, delivers fuel to a combustion chamber, a normally deenergized timer having a starting position, a main switch for energizing said electrical means and at substantially the same time energizing said timer for movement in a direction to deenergize said electrical means after a predetermined period of time, means responsive to combustion for controlling the energization of said timer, providing it has not already deenergized said electrical means, to reverse its direction of movement and move to a position on the other side of its starting position, and means operative after the establishment of combustion for preventing the reenergization of said electrical means, once it has been deenergized, until said timer returns to its starting position.

2. A fuel burner control system comprising in combination, electrical means which, when energized, delivers fuel to a combustion chamber, a normally deenergized timer having a starting position, a safety switch, a. main switch, an energizing circuit for said electrical means including said main switch and said safety switch, means operative upon closure of said main switch to cause energization of said timer, and combustion responsive means controlling the energization of said timer to control the direction of movement of said timer during the time said timer is energized to cause movement of said timer in a direction to open said safety switch in the absence of combustion and in the opposite direction in the presence of combustion, said timer returnin to its starting position upon deenergization.

3. A fuel burner control system comprising in combination, electrical means which, when energized, delivers fuel to a combustion chamber, a normally deenergized timer having a startin position, a safety switch, a starting switch, a main switch, a. starting circuit for said electrical means comprising said safety switch, starting switch, and main switch in series, a holding circuit for said electrical means comprising said safety switch and main switch but independent of said starting switch, said timer closing said starting switch when in starting position, said main a aarse switch, when closed, causing the energization of said timer, combustion responsive means controlling the direction or movement of said timer during the time it is energized, said combustion responsive means causing said timer to move in a direction to open said safety switch in the absence of combustion and in the reverse direction to a position on the other side or said starting position in the presence of combustion, said timer opening said starting switch as it moves to the said other side of its starting position.

4. A fuel burner control system comprising in combination, electrical means which, when energized, delivers fuel to a combustion chamber, a normally deenergized timerhaving a starting position, a safety switch, a starting switch, a main switch, a starting circuit for said electrical means said relay including said main switch and safety switch. a switch closed by said relay for energizing said timer, and combustion responsive means in control of said timer during the time 'that it is energized, said combustion responsive means causing said timer to move in a direction to open said safety switch in the absence of combustion, but to reverse its movement and move to a position on the other side of its first position upon the establishment of combustion.

7. A fuel burner control system comprising in combination, a fuel delivery device, an electric relay which, when energized, renders said device operative to deliver fuel, a timer which occupies comprising said safety switch, starting switch,

and main switch in series, a holding circuit for said electrical means comprising said safety switch and main switch'but independent ofsaid starting switch, saidtimer closing said starting switch when in starting position, said main switch, when closed, causing the energization of said timer, combustion responsive means controlling the direction of movement of said timer during the time it is energized, said combustion responsive means causing said timer to move in a a starting position when' deenergized, a safety switch, a. main switch, a starting switch which 4 is closed" when said timer is in its starting position, an energizing circuit for said relay including said main switch, safety switch, and starting I switch in series, a holding switch, a holding circuit for said relay including said holding switch, main switch, and safety switch for holding in I said relay independently of said startingswitch,

direction to open said safety switch in the ab- I sence of combustionand in the reverse direction to a positlon'on the other side of said starting position in the presence ofcombustion, aholding switch in said holding circuit, said timer first closing said holding switch and then opening said starting switchas it moves past said starting position as 'a'jesult of the establishment of combustion, and first opening said holding switchand then closing said starting switch as it moves back toward its starting position, whereby said control system will recycle in the event of a fail--' been established.

5. A fuel burner control's'ystem comprising in combination, electrical means which, when ener- I gized, delivers fuel" to a combustion chamber, a.

normally deenergized timer having a starting position, a safety switch, a starting switch, a

main switch, a starting circuit for said electrical means comprising said safety switch, start-- ing switch, and main switch in series, a holding. circuit for said electrical means comprising said a,

me of combustion after combustion has once 1 i energization of said electrical means and said a switch closed by said relay for energizing said timer, and combustion responsive means in control of said timer during the time that it is energized, said combustion responsive means causing said timer to move in a direction to open said safety switch in the absence of combustion, but to reverse its movement and move to a position on theother side of its first position upon the establishment of combustion, said timer opening "said starting switch as it moves past its starting position asa resultv of the establishment of combustion.

8. A fuel burner control system comprising in combination, "a balanced thermal relay compris- I ing a' pair of heat motors, electrical means coni trolling the delivery of fuel 'to a combustion chamber, a main switch, closure of said main switch causing the substantially simultaneous Q thermal relay, and means comprising a combusi tive energization, said resistance; in the absence 'of combustion, causing one of said heat -motors.-1-"' to be more highlyv energizedthan the-other re--- sulting in a deenergization of said electrical, means by saidthermal relay after ,a predetera mined time interval, said resistance, in the presence of combustion, rendering said other heat safety switch and main switch but independent of said starting switch, said timer closing said starting switch when in starting position, said main switch, when closed, causing the energization of said timer, combustion responsive means controlling the direction of movement of said timer during the time it is energized, said combustion responsive meanscausing said timer to move in a direction to open said safety switch in the absence of combustion and in the reverse direction to a position on the other side of said starting position in the presence of combustion, said timer opening said starting switch as it moves to the said other side of its starting position, said holding circuit being independent of said timer except that said timer will move in its original direction upon a failure of combustion and open said safety tion responsive resistance connected in "circuit with said heat motors for controlling their relamotor more highly energized, thus preventing del energization of said electrical means if it has not already been deenergized.- 1

9. A fuel burner control system comprising in combination, a balanced thermal relay comprising a pair of heat motors, which-include electric heaters, electrical means controlling the delivery of fuel to a combustion chamber, a first resistance connected in parallel with one of said heaters, a second combustion responsive resistance connected in parallel with the other heater, said two parallel arrangements being connected in series, a main switch, closure of said main switch causing the energization of said electrical means and the establishment of a circuit through said electric heaters and resistances, said resistances being so chosen that in the absence of combustion one of said heat motors is more highly energizedthan the other causing said thermal relay to deenergize said electrical means after a predetermined time interval, said second resistance changing in value in' response to the establishment of combustion to cause said other heat motor to become more highly energized whereby said thermal relay moves in the opposit direction.

10. A fuel burner control system comprising in combination, a balanced thermal relay comprising a pair of heat motors, which include electric heaters, electrical means controlling the delivery of fuel to a combustion chamber, a first resistance connected in parallel with one of said heaters, a second resistance connected in parallel with the other of said heaters, said two parallel arrangements being connected in series, both of said resistances being subject to the heat of combustion but only one of them being subject to the,

radiant heat of combustion, the resistance value of said one resistance being lower than the other when they are both at the same temperature and higher than the other in the presence of combustion, a main switch, closure of said main switch causing the energization of said electrical means and the establishment of a circuit through said electric heaters and resistances, one of said heat motors being more highly energized than the other on starting to cause the thermal relay to move in a direction to deenergize said electrical means after the elapse of a predetermined time, said one resistance increasing in value upon the establishment of combustion to cause the other heat motor to be more highly energized whereby said thermal relay reverses its direction of movement.

11. A fuel burner control system comprising in combination, abalanced thermal relay comprising a pair of heat motors, which include electric heaters, electrical means controlling the delivery of fuel to a combustion chamber, a first resistance connected in parallel with one of said heaters, a second resistance connected in parallel with the other of said heaters, said two parallel arrangements being connected in series, said first resistance being substantially fixed and unaflfected by the temperature of combustion, said second resistance being positioned to respond to the temperature of combustion, a main switch, closure of said main switch causing the energization of said electrical means and the establishment of a circuit through said heaters and resistances, the resistance values of said resistances being so chosen that when they are near the same temperature one of said heaters is in the neighborhood of 50% more highly energized than the other causing said thermal relay to move in a direction to deenergize said electrical means, but upon the establishment of combustion the resistance value of said second resistance increases until said other heater is in the neighborhood of 50% more highly energized than said one, whereupon said thermal relay reverses the direction of its movement.

12. A fuel burner control system comprising in combination, elertrical means controlling the delivery of fuel to a combustion chamber, a main controlling switch. a balanced thermal timer comprising a pair or electrothermal elements, said thermal timer assuming a starting position when deenergized, a starting switch associated with said timer and closed thereby when said timer is in its starting position, a second switch associated with said timer, means for causing energization of said electrothermal elements and said fuel controlling means upon closure of both said main controlling switch and said starting switch, said timer being operative upon continued energization of said electrothermal elements to terminate energization of said fuel controlling means, combustion responsive means "controlling said thermal timer and operative upon the establishment of combustion prior to the termination of the energization of the fuel controlling means to cause said timer to close said second switch and open said starting switch, and means operative upon closure of said second switch by said timer to maintain energization of said fuel controlling means despite opening of said starting switch.

13. A fuel burner control system comprising in combination, electrical means controlling the delivery of fuel to a combustion chamber, a main controlling switch, a balanced thermal timer comprising a pair of electrothermal elements, said thermal timer assuming a starting position when deenergizer, a starting switch associated with said timer and closed thereby when said timer is in its starting position, a second switch associated with said timer, means for causing energization of said electrothermal elements and said fuel controlling means upon closure of both said main controlling switch and said starting switch, said timer'beingoperative upon continued energization of said electrothermal elements to terminate energization of said fuel controlling means, combustion responsive means controlling said thermal timer and operative upon the establishment of combustion prior to the termination of the energization oi! the fuel controlling means to cause said timer to close said second switch and open said starting switch, and means operative upon closure of said second switch by said timer to maintain energization of said fuel controlling means despite opening of said starting switch, said thermal timer being operative upon the cessation of combustion to first open said second switch to interrupt energization of said fuel controlling means and after an interval of time to reclose said starting switch.

HUBERT T. SPARROW.

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