US2210852A

US2210852A - Interlock control system

Info

Publication number: US2210852A
Application number: US237371A
Authority: US
Inventors: William F Falkenberg
Original assignee: Individual
Current assignee: Individual
Priority date: 1938-10-27
Filing date: 1938-10-27
Publication date: 1940-08-06
Anticipated expiration: 1957-08-06

Description

w. F. FALKENBERG 2.210.852

INTERLOCK CONTROL SYSTEM Filed Oct. 27, 1938 3 Sheets-Sheet l Imfento r.

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w. FALKENBERG 2210,852

IHTERLOCK CONTROL SYSTEM Filed Oct. 27, 1938 3 Sheets-Sheet 2 Imfen. OT- W?! 277% i 'flttorr gy.

1940. w. F. FALKIENBZERG 2,210,852

' INTERLOCK CONTROL SYSTEM I Filed Oct.- '27, 193s s Sheets-Sheet s Patented Aug. 6, i940 UNITED STATES PATENT OFFICE INTERLOCK CONTROL SYSTEM William F. Falkenberg, Chicago, Ill.

Application October 27, 1938, Serial No. 237,371

16 Claims.

The invention relates to improvements in interlock control systems and more particularly to an improved burner and pump set interlock and con-,

' within the combustion chamber.

Certain known liquid fuel or gas fired burner systems employ pump set control means which consists of a switch actuated by air pressure within the burner blower, but as this air pressure is exteremly variable in volume and pressure due to the air required for fires of different sizes and because, in small installations, the pressure is insuflicient to .actuate the switch, such control means is extremely unreliable and wholly unsuited to some installations. Another objection to an air switch actuating means is the known fact'that frequently such devices fail because of mechanical defects and tampering.

In coal fired systems, coal is delivered automatically to a. hopper from which it is automatically discharged into the fire box. Should the discharge mechanism jam or otherwise become inoperative, wholly or partially, it is very desirable that fuel delivery to the hopper cease. Automatic stoppage of the delivery to the hopper can be effected with the improved interlock control system described herein.

The improved interlock control system illustrated herein, is designed to be used in conjunction with known types of room thermostats and stack switches used in automatic control systems and overcome the foregoingand other objectionable features of many known types of installations, although it will become obvious that the system can also be employed in manually controlled and ignited burner systems.

It is, therefore, an object of the invention to provide an interlock control system wherein operation of the fuel pump, valve or other fuel delivery mechanism is entirely controlled by operation of the burner blower motor.

Another object is to provide an interlocking control for the burner and pump-set in a liquid fuel burner system wherein fuel delivery occurs only while the burner blower motor is operating at its maximum rated speed.

Other objects are to provide an eflicient liquid fuel burner system wherein long suction fuel lines are eliminated and anti-siphon valves are not required, and by means of which pump vibrations in the heating system are entirely eliminated and the initial voltage drop oninitial start of the blower motor is minimized.

Another object is to provide an interlock safety control system of a type that may readily be installed in an individual or a multiple system of liquid or coal fired burners.

The foregoing and such other objects of the invention as will more fully appear her inafter as the description proceeds, will be more readily understood from a perusal of the following specification, reference being had to the accompanying drawings, in which:

Fig. 1 is a schematic view showing a liquid fuel. burner installation incorporating features of the invention.

Fig. 2 is a wiring diagram of one form of improved interlocking control system for liquid fuel burners having a single phase blower motor.

Fig. 3 is a wiring diagram showing a threephase blower motor in the circuit.

Fig. 4 is a schematic view showing diagrammatically the improved interlocking control incorporated in a bank of gas fired burners.

An exemplary arrangement of the various units making up one installation ofv the improved interlock control system is generally shown in Fig. 1, in which II is a heating plant having a combustion chamber I2 and stack l3. The liquid fuel burner, shown at M, has a blower l5 operated by blower motor IS. A suitable pilot I1 is provided having a connection i 8 with a gas source and a solenoid valve IS, the coil of which is so connected in the ignition circuit of the system that said valve opens when the plug 2| sparks.

Liquid fuel is delivered to the burner from a reservoir 22 through a fuel line 23 having a solenoid shut-off valve 24 and a pump 25 therein. A motor 26 is provided to operate the pump 25.

Operation of the burner may be controlled automatically in the usual manner by means of a stack control unit 2'! and a room thermostat 28, or by an aquastat or steam pressure, or in conjunction with other limiting or controlling devices. As shown, the unit 2'! and thermostat 28 operate in conjunction with each other to control starting and stopping of the burner and ignition. However, to delay the fiow of fuel to the burner until the airflow within the burner reaches its required velocity and volume, and to prevent the delivery of fuel to said burner should the air pressure fail, novel interlock control means is provided in the circuit controlling operation of the pump motor 26. As indicated generally in Fig. 1, and in detail in Fig. 2, an interlock control unit 29 is arranged in the pump motor circuit which circuit is entirely independent of the control circuit of the system.

As will be more fully explained hereinafter, operation of the pump motor 26 is entirely dependent upon operation of the blower motor 16 at its maximum rated speed. Accordingly, should any interruption occur in the operation of the blower motor due to any cause such as for example, low voltage, jamming of blower, seizing of the bearings, etc., however slight, the flow of fuel to the burner is instantly stopped, then after a short delay the blower motor is stopped. After a cooling period the system then recycles, that is, the ignition and the blower motor are turned on and alter the period of acceleration of the blowe motor, the flow of fuel is resumed.

Another feature of the present system is means to prevent, momentarily, the flow of fuel to the burner while the blower motor is stopping after being shut down. This undesirable flow usually occurs because a counter voltage, generated in the motor, is impressed on the coil of the solenoid shut-off valve 24 during the interval while the motor is coming to a full stop, thus momentarily opening said valve and permitting the fuel line to discharge its contents into the burner. This is avoided in the instant arrangement by connecting the blower motor directly to the house line through a single-pole magnetic switch 5 which switch is operated by the stack control unit 2?. Hence any counter voltage impressed upon the open blower motor circuit is ineffective.

The specific structure, wiring and operation of a wholly automatic burner system embodying the improved interlock control will be more readily understood from a perusal of the following description of operation.

House lines 3l--32 are connected, through.

switch 33, with the primary 36 of transformer 35 mounted within the automatic stack control unit 2?. One terminal of the transformer secondary 3G is connected by lead 3i to one terminal 38 in the usual room thermostat 28. The other terminal of said secondary is connected by line 39 to a holding coil ll, asafety switch 12 being provided in the circuit. When the room thermostat 28 calls for heat it closes first the terminal 38 and then a contact 33 therein. This establishes a circuit through the holding coil ti which can be traced from the terminal 38 of the room thermostat 28 through the transformer secondary 3t, safety-switch contact :32, holding coil 3%, safetyswitch heater t l, lead $5 to contact as in the stack thermostat i'i, through one switch arm 68, which is in closed position with contact it, and lead 39 to the room thermostat contact 63. The energized holding coil '55 then closes the various holding contacts 5!, 52 and in the control unit 21, establishing a control circuit, an ignition circuit and a blower motor circuit.

The blower motor circuit, however, is completed only when the single pole switch 5 1, actuated by'holding coil 55, is in closed position. The coil 55 is energized through a circuit including one of the house lines 56 and returns through leads 5? and 58, holding contact 5! and lead 59. The blower motor 58 then receives its current through lead lil, switch 5 lead 62 and return leads 63-65 and 56.

The control circuit can be traced from the contact 38, in the room thermostat 28, through the transformer secondary 36, safety-switch contact 42, holding coil 4|, safety-switch heater 44, lead 45, contact 46, switch arm 48 and holding contact 52 to contact 65 oil the room thermostat 28. The safety-switch 44 starts to heat, the blower motor l6 operates and the ignition Ill-2| is energized through holding contact 5|, lead 58, contact 88 in a mercury switch 61 and leads 58-68.

With the mechanism in this position a heating coil II of an oil valve time switch 12 is heating preparatory to closing its contact 13 for energizing the coil 14 in the oil valve 24 through lead 15. At the expiration of the interval determined by the setting of the oil valve time switch 12 the contact 13 will close and the oil valve 24 will open. However, no oil can as yet be delivered to the burner because the oil pump 25 is not in operation.

Some control units 21 have the coil 14 of the oil valve 24 directly connected to the control circuit so that said oil valve opens when the control circuit is completed through holding contact 5|.

Operation of the oil pump 25 is delayed to prevent delivery of oil to the burner until the blower motor 16 has reached full running speed, thus preventing flooding of the burner or delivery of oil at any time prior to delivery of the proper amount of air to effect complete atomization. Delayed operation of the pump motor 28 is effected by providing two interlocked relay switch closing elements 78 and I1 (in unit 29) in a secondary circuit connected with and receiving a generated voltage from the starting winding 18 of the blower motor.

In Fig. 2 of the drawings, a single phase 220 volt motor I6 is shown. Referring specifically to the wiring commonly employed when a singlephase motor is used, said motor receives its energy through power lines 52 and 6364. The current passes through lead 62, safety fuse 19, lead 811 to running winding 82-83, Current also flows through the starting winding 18, centrifugal switch 84 and lead 85 to a point midway between the running windings 82-83. The adding of the starting winding 18 is necessary to start the blower motor.

Then the single phase blower motor operates at its maximum velocity to insure the required delivery of air, the centrifugal switch 84 is actuated to break the contact through the lead 85 and make contact at 86 whereby the starting winding is disconnected from the source of power and the motor continues to operate on the running windings 82-83. However, voltage is generated in the starting winding 18 by operation of the motor which varies with the speed of said motor. This voltage is impressed upon the holding coil 96 of relay switch Tl, through the switch 84, contact 86, lead 81, terminal 68, to terminal 93 through movable contact 92, which is closed during the starting period, thence through lead 95 to coil 96 of relay switch Ti.

When the voltage generated in winding i8 is suiilcient to close relay switch Ill and thereby connect terminals 9l-98 by moving contact 91, a lock-in circuit can be traced from the starting winding 18 through switch 84, contact 86, lead 87, contact 88, lead 89, contact 9!, moving contact 91, contact 98 and lead 95 to holding coil 96 in relay switch 17. The other switch elements lfll, carried in the relay switch Tl, constitute a two-pole switch for closing the house line 3l-32 direct to the pump motor 26 thus starting operation of the pump to deliver fuel throu h the open oil valve 24 into the burner l4 for atomization by the maximum volume of air now being 1 delivered.

When combustion takes place, the other switch arm 48 in the stack thermostat 41 closes a circuit through contact I02 to shunt the safety switch heater 44. A circuit is now closed from the terminal 38 of the room thermostat 28, through the transformer secondary 36, safety-switch contact 42, holding coil 4|, holding contact 53, contact I02 in switch element 48, holding contact 52, to contact in the room thermostat. On continued stack temperature rise the contact 46 of the stack thermostat 41 is broken without effecting the circuit and the contact at 65 in the mercury switch 61 breaks, cutting off the ignition and breaking the circuit through said switch 16. At the same time a switch contact I03 in the mercury switch 6'! closed shunting the oil valve switch contact 13 allowing it to cool. The burner is now in normal operation with the 'generated voltage from the starting winding I8 of the blower motor l6 flowing to centrifugal switch arm 84, contact 88, leads 81 and 89 to terminals 9| and 98 of the switch element 11 to maintain the coil 96 therein energized for holding the multiple switch in closed position and maintaining the pump motor in operation. I

When the room thermostat 28 calls for less heat the contact 38 therein opens deenergizing the holding coil 4| and opening the holding contacts 5|, 52 and 53 to shut down the burner. In the case of flame failure the reduction instack temperature will open the contact I02 almost immediately, deenergizing the holding coil 4| and shutting down the burner.' After a predetermined length of time depending-uponthe setting of the mechanism, the mercury switch 61 will operate to again close contact 66 and then contact 46 will close allowing the control to re-start if the room thermostat 28 is still calling for heat. If the fuel fails to ignite, the stack temperature will not rise and at the end of the safety-switch timing period the switch 42 will then shut down the control. stopping the burner.

Should operation of the blower motor l6 cease momentarily or even slow down due, for example, to momentary interruption of current, broken blower, dragging or seized hearings or any of the multiple of things that will decrease its maximum operating speed, the generated voltage impressed by the starting winding 18 will drop rapidly thus allowing the multiple relay switch 11 to open the switch and break the circuit to the coil 96. Opening of this switch 11 also breaks the line circuit to the pump motor 26 thus stopping the flow of fuel. The broken line circuit to the pump mo-' tor cannot be re-made, irrespective of the opera tion of the various mechanism herein described for controlling the starting and operation of the various units, until generated voltage in the starting winding 78 is impressed on the coils 94-96. Obviously this cannot occur until the control mechanism in unit 21 has recycled to energize the ignition circuit and made'contact be

tween terminals

88 and 93 in the relay switch 16.

As previously mentioned it also is desirable that the line circuit to the blower motor 16 be independent of the control cystem to prevent'a back flow of induced current into said system which results in momentary opening of the fuel valve 24 while the blower motoris slowing down after being shut off. This type of circuit is desirable to prevent the flow of fuel into the combustion chamber after the motor speed has dropped below ment 11.

that sufficient to effect atomization of fuel and to this end the cut-off switch 54 having contact I04 and N15 is provided in one line of the blower motor circuit which is actuated upon exciting the holding coil 55. The coil 55 is energized through lead 51 upon closing of the holding switch 5|. It will thus be seen that the blower motor circuit is independent of the control circuits and any voltage genorated in the motor windings while stopping, will not be impressed upon or effect operation of any of the control elements.

When the blower motor l6 stops, the centrifugal switch 84 therein returns to its initial position thus again placing the starting winding in circuit with the motor running windings. Should the centrifugal switch 84 stick or look in position against the contact 86, the fuel pump 25 cannot be operated after the blower motor has come to a Should current be supplied to the blower motor while said switch is stuck, any induced voltage impressed on the starting winding is insumcient to actuate the interlock pump control elewhen excited by current generated during full speed operation of the blower motor.

Fig. 3 shows the wiring for a three-phase motor Mid. and the same numerals previously used identify the same elements thereof. Note should be made however, that no starting winding is present in a three-phase motor hence a winding must be incorporated therein whereby the voltage generated in coil 18a by operation of the motor. is impressed on the holding coil in the relay switch 7 7. The centrifugal switch 84 cuts the coil 18a out of the interlock control circuit when the motor slows down, even momentarily.

It is, therefore, obvious that any interruption in the full operating rated speed of the blower motor IE or 16d will instantly. stop the delivery of fuel to the burner, and that delivery cannot be resumed until the'entire control system has recycled to place the pilot ignition system in operation and operate the blower motor at its maximum rated speed.

Accordingly, the system hereinabove described is foolproof and more positive than methods now used which often depend upon the extremely variable air pressure from fan or blower of the burner motor. Also small sized burners have been found to have insufficient pressure to operate known types of air control switches and said switches are readily tampered with. In the improved entirely automatic electrical interlock control system described herein, operation of the various control units is not effected by vibration which is minimized by placing the fuel pump motor at a point remote from the burner'mechanism nor is it effected by variations in the air volume or pressure or switch failures. The improved control,

.being tamper proof, cannot be locked in any position by the operator inasmuch as the windings of the blower motor and the windings of the holding coils in switches 16 and 11 controlling opera- .tion of the fuel pump motor 26 are rated and Said element 11 can be operated only although it will be readily appreciated that other liquid or even solid fuel may be used. As shown, three blower motors Ifia, 16b and lGc are provided at A, B and C although any additional required number may be added upon adding the necessary interlocking control units 29.

Referring particularly to the system illustrated, the blower motor 16a is operating to create the required burner air pressure and the centrifugal switch therein has closed to impress the generated voltage on the holding coil 96a to close multiple magnetic switch Ila, the ignition system having previously energized coil 94a to close magnetic switch 16a. Closing of switch Ila completes the circuit to the solenoid valve 26a which opens to deliver fuel to the burner unit at A. The closing of switch Ila also closes the line circuit to blower motor l6b in unit B so that said motor is now placed in operation.

As soon as this motor (lGb) reaches its maximum rated speed, the centrifugal switch 84b therein, makes contact to close the circuit, for the impressed voltage therein, to holding coil 96b of multiple magnetic switch 11b. As the holding coil 84b of switch 16b already is energized by the ignition circuit, there is no further delay in opening the solenoid fuel valve 26b and in starting the next succeeding blower motor IBc. As soon as this motor l6c reaches its maximum rated speed the voltage generated therein energizes holding coil 960 to close the circuit to gas valve 260. Obviously this successive starting and delivery of fuel can be repeated to operate a bank of any number of burner units. Should any one of the blower motors fail, the supply of fuel to its burner is immediately shut off and the burner units following in succession are likewise shut down so that, upon re-starting, each burner unit will come into operation after the next previous burner unit is functioning.

When this bank burner system is used with liquid fuel fired burners, such as fuel oil, the solenoid valves 26a, 26b and 26c are replaced with pump motors 26 (Fig. 2).

It is obvious also that the improved interlock control system for delayed feeding of fuel can be efficiently incorporated in heating systems not using the automatic control units 21-28 or the automatic ignition means l92l, and that said interlock control system may be advantageously incorporated in any system requiring delayed action of the fuel delivery means to guard against premature delivery of fuel or delivery after the fire has been extinguished because of failure of associated mechanisms.

Although exemplary arrangements incorporating the improved fuel control units and the blower motor cut-out are illustrated herein and described in detail, it is to be understood that the disclosures are illustrative and not restrictive, and that certain variations can be made in the system as a whole and in the detail structure of various parts without departing from the spirit of the invention or the scope of the appended claims.

I claim:

1. An interlock control system for delayed feeding of fuel to a burner including a motor operable to operate a blower to deliver air to said burner, a second motor to operate a pump to deliver fuel to said burner, a circuit for the pump motor, an open circuit having a holding coil therein, a switch operable to impress said open circuit with voltage generated in the blower motor while said motor is running, means to initially close said open circuit to energize said holding coil, and means effective while said holding coil is energized to maintain its circuit and the pump motor circuit closed.

2. In a control system for liquid fuel burners including a blower motor, a fuel pump motor and a holding coil, temperature control mechanism controlling operation of said blower motor and energization of said holding coil, and means jointly controlled by operation of said blower motor at its maximum rated speed and by mementary energization of said holding coil to render said fuel pump motor operable.

3. A burner control including a blower motor and an ignition circuit, a holding coil connected to be energized concurrently with said ignition circuit, means to control operation of the blower motor and to energize said ignition circuit, a fuel supply line, a pump in said supply line, a motor for said pump, a circuit for said pump motor, and means responsive to joint operation of the blower motor and energization of said ignition circuit to close the-circuit to said pump motor to operate the pump.

4. The combination with a heating device including a blower motor and a fuel line, electrical means to start said blower motor and open the fuel line, ignition means actuated by said electrical means, a pump in saidv fuel line, a motor to operate said pump, an open circuit for said pump motor, and electrical means energized by voltage generated during full speed operation of the blower motor operable while the ignition 'means is functioning to close said fuel pump motor circuit.

5. The combination with a heating device including a blower motor and a fuel line, electrical means to start said blower motor and open the fuel line, ignition means actuated by said electrical means, a pump in said fuel line, a motor to operate said pump, an open circuit for said pump motor, electrical means energized by voltage generated during full speed operation of the blower motor operable while the ignition means is functioning to close said fuel pump motor circuit, and means to maintain said fuel pump motor circuit closed after the ignition means ceases to function.

6. The combination with a heating device including a blower motor and a fuel line, temperature control means to start said blower motor and open the fuel line, ignition means actuated by said control means, a pump in said fuel line, a motor to operate said fuel pump, an open circuit for said pump motor, electrical means operable when the blower motor reaches its rated speed and while the ignition system is functioning to close said fuel pump motor circuit, an automatic cut-out for said ignition means operable upon combustion of fuel, and means in said electrical means to prevent closing of said open pump motor circuit should the blower motor regain its rated speed after. having slowed down without operation of said ignition system.

7. A system to control operation of the fuel delivery means of a mechanism including a blower motor and a temperature controlled ignition circuit electrical means energized by voltage generated during operation of said blower motor at its rated maximum speed to maintain said fuel delivery means in operation, and check control means in parallel with said ignition circuit to prevent starting of said fuel delivery means unless the ignition circuit is complete.

8. In a control system for automatically fired l6 burners including a blower motor and fuel delivery means and an ignition system, temperature control mechanism controlling operation of said blower motor andenergization of said ignition system, and electrical means energized by voltage generated during operation of said blower motor at its maximum rated speed and dependent upon energization of said ignition system operable to initiate operation of said fuel delivery means.

9. In a control system for automatically fired burners including a blower motor and fuel delivery means and an ignition system, temperature control mechanism controlling operation of said blower motor' and energization of said ignition system, means in said mechanism to deenergize said ignition system, and electrical means energized by voltage generated during operation of said blower motor at its maximum rated speed and actuated upon energization of the ignition system to initiate operation of said fuel delivery means, said electrical means including means to maintain operation of said fuel delivery means after the ignition system is deenergized and while said blower motor continues to operate at its maximum rated speed.

10. The combination with a heating device including a blower motor and a fuel line, means to start said blower motor and open said fuel line, ignition means including a circuit operable upon starting said blower motor, a fuel pump in said fuel line, a motor therefor, an open circuit for said pump motor, a switch having a holding coil in said open circuit, an open circuit connecting said holding coil with the starting winding of the blower motor, means to impress a generated voltage in said last named circuit when the blower motor is operating at its rated speed only, a switch. in said open generated voltage circuit having a holding coil connected to be energized concurrently with said ignition circuit whereby when the ignition circuit is closed the pump motor circuit switch holding coil is energized to close the pump motor circuit, the last named switch also closing a shunt circuit from the said starting winding to its own holding coil, and means to de-energize the ignition circuit and holding coil therein whereby the pump motor cannot be restarted after failure of the blower motor to maintain its rated speed without energizing the ignition circuit and holding coil.

11. The combination with a heating device having a blower motor and a fuel pump motor, independent line circuits to each motor, a temperature actuated electrical mechanism to control starting and stopping of said blower motor, an ignition circuit in said mechanism, a switch in the fuel pump motor circuit, a holding coil to actuate said switch, a circuit connecting said holding coil with the starting winding of the blower motor, a cascade switch in said holding coil circuit movable into make position only while the blower motor is operating at its rated speed, a second switch in the said holding coil circuit, a holding coil for said switch connected to be energized concurrently with the ignition circuit whereby the fuel pump motor switch is closed to start the fuel pump only when both holding coils are energized, and a shunt circuit around said second switch closed when the pump motor switch is in closed position to keep the pump motor running after the ignition circuit is deenergized.

12. A burner control including a blower motor and an ignition circuit and control means therefor, a fuel supply line, a pump in said fuel supply line, a motor for said pump, a circuit for said pump motor, a normally open switch in said pump motor circuit, a holding coil for said switch; a circuit connecting said holding coil with the startingwinding of said blower motor, a normally open switch in said holding coil circuit, a holding coil for said last named switch connected to be energized concurrently with said ignition circuit to close its switch to complete the starting winding holding coil circuit whereby generated voltage impressed on said circuit upon full speed operation of the blower motor will energize the holding coil therein and close the pump motor circuit switch to operate the pump motor, and a shunt across the holding coil circuit switch made upon closing of the pump motor circuit switch to hold said pump motor circuit switch closed after the ignition circuit is de-energized.

13. In a control system for liquid fuel burners including a blower motor and an ignition circuit, a temperature controlled means to start said blower motor and energize said ignition circuit, a starting winding in said blower motor normally in series with the running winding of said motor, a centrifugal switch operable when the motor reaches its rated speed to cut out said starting winding, a fuel pump motor, a circuit for said pump motor, a switch in said circuit, an open control circuit for said switch including a holding coil and said starting winding, and a normally open switch in said control circuit movable into make position when the ignition circuit is energized, whereby the holding coil in said control circuit is energized by generated voltage in said starting winding to close said pump motor circuit.

14. A burner control including a blower motor and an ignition system, means to control operation of the blower motor and energize said ignition system, a fuel supply line, a pump in said supply line, a motor for said pump, a circuit for said pump motor, and means energized by voltage generated during full speed operation of the blower motor and energization of said ignition system to close the pump motor circuit to operate the pump, temperature actuated means in said control means to deenergize the ignition system, and means in said pump circuit closing means to maintain the pump motor circuit closed during full speed operation of the bolwer motor after the ignition circuit is deenergized.

15. In a liquid fuel burner, in combination, an electric blower motor, an electric circuit for said motor, a, coil in said motor adapted to be im-.

pressed with voltage generated upon full speed operation of said motor, a fuel pump motor, a relay switch for controlling a circuit to said fuel pump motor, a normally open electric circuit connecting said coil and relay, an electrically actuated switch operative to close said circuit during initial operation of the blower motor whereby the relay is excited upon full speed operation of said blower motor to close the pump motor circuit, and a shunt circuit around said electrically actuated switch closed upon operation of said relay to retain the relay circuit closed after said electrically actuated switch is opened.

16. A control system for a multiple fired heating system, said system including a plurality of burner units each including a motor operated blower and fuel delivery means, thermostatic control means and an ignition'system common to all of said units, said thermostatic control means being operable to energize said ignition (5 system and one of said blower motors, means energized concurrently with said ignition system effective upon operation of said one blower motor at its maximum rated speed to initiate operation of the associated fuel delivery means to deliver fuel to the burner unit in operation and start operation of the next successive blower motor, similar means also energized concurrently with said ignition system effective upon operation of said second blower motor at its maximum rated speed to institute delivery of fuel to said second burner unit and start operation of the next successive unit, and means to automatically shut down any one of said burner units and stop delivery of fuel thereto upon failure of its blower motor however slight.

WILLIAM F. FALKENBERG. I