Interlock arrangement for a stack damper control

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United States Patent (19)

Matthews et al.

54) nterlock arrangement for a

Stackdamper control

75 Inventors: Russell Byron Matthews; Thomas Edward Hayes, both of Goshen, Ind.

(73) Assignee: Johnson Controls, Inc., Milwaukee, Wis.

(52) U.S. Cl. ................................... 236/1 G; 110/163;

2,155,642 4/1939 Dewey ................................ 236/1 G 3,086,583 4/1963 Reichow ... 431/71 3,934,796 1/1976 Smith ..................................... 431/20 Primary Examiner-William E. Wayner

Assistant Examiner-Robert J. Charvat

Attorney, Agent, or Firm-Emrich, Root, O'Keeffe & Lee

A control arrangement for a heating system including a furnace having a fuel-fired burner apparatus and a vent stack with a motor driven damper plate pivotally mounted within the stack, includes a stack damper con trol circuit including first and second limit switches which permit energization of a drive motor in response to a request for heat, premitting the damper plate to be driven to an open position, and for energizing an inter lock switch which prepares an energizing path for fuel supply valves of the system, the first limit switch being operated when the damper plate approaches the open position to complete the energizing path for the fuel supply valves prepared by the interlock switch, and the second limit switch deenergizing the motor when the damper plate is driven to the fully open position. The motor is reenergized at the end of the heating cycle to drive the damper plate to the closed position, and the second limit switch deenergizes the motor when the damper plate reaches the closed position.

15 Claims, 1 Drawing Figure

Drawings

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Another object of the invention is to provide a heat

INTERLOCKARRANGEMENT FOR A STACK ing system including a motor actuated damper appara DAMPER CONTROL tus which is interlocked with the fuel valves of the BACKGROUND OF THE INVENTION system to permit operation of the fuel valves only when the damper is in a fully open position.

1. Field of the Invention Yet another object of the invention is to provide a This invention relates to heating systems including control arrangement for a heating system including furnaces having fuel-fired burners, and, more particu stack damper control apparatus, which prevents the larly, to a control arrangement which provides a safety operation of fuel supply valves of the system in the interlock between a stack damper control apparatus and 10 event of an unsafe failure of the stack damper control fuel supply apparatus for such systems. apparatus.

2. Description of the Prior Art Another object of the invention is to provide a heat Heating systems employing furnaces having fuel ing system of the pilot ignition type including a motor fired burners require a vent stack to conduct combus actuated damper apparatus which is interlocked with a tion products away from the burner. Automatically 15 pilot flame sensing circuit to prevent operation of fuel controlled stack dampers are generally used in the ven valves of the system and the energization of a drive tilation stacks to permit the stacks to be closed when the motor of the damper apparatus in the event of an unsafe furnace is not operating to minimize heat losses when failure of the flame sensing circuit. the furnace is not operating. However, for safe opera These and other objects are achieved by the present tion, it is necessary that the stack damper be open in 20 invention which has provided a control arrangement advance of each operation of the burner. Accordingly, for use in a heating system and which provides a safety systems in which automatic dampers are used generally interlock between a fuel supply means which supplies include a control arrangement which provides an inter fuel to to a burner apparatus of the system for combustion provide heat, and a stack damper control means lock between the damper control mechanism and fuel 25 which controls the positioning of a stack damper plate supply apparatus of the system to assure that the damper is fully open before the burner operates and is which is pivotally mounted within a vent stack. The closed after the completion of the operation of the damper plate is normally maintained in a first position to close the vent stack when the system is deactivated, and burner.

In one known arrangement in which a primary 30 istorotatable open the by way of a drive motor to a second position vent stack when the system is activated.

burner control is conditional on and subsequent to the The control arrangement of the present invention opening of a stack damper, a drive motor is energized in comprises activate means responsive to a request for response to a request for heat to drive the damper to an heat to effect the energization of the drive motor to open position, and limit switches complete the burner permit the damper plate to be driven towards the sec circuit and deenergize the drive motor when the 35 ond position, damper reaches the fully open position. The drive to prepare an interlock means operable when energized motor is reenergized at the end of the heat run to move and limit switch meanspath energizing for the fuel supply means, the damper to the closed position, a further switch de means to be energized by the permitting for activate the interlock means when the energizing the motor when the damper reaches the damper plate is at the first position. The limit switch closed position. Movement of the damper from the fully means is operable when the damper plate has been open position permits a limit switch to interrupt the driven to the second position to deenergize the drive burner circuit. A time lag is provided between the inter motor and to complete the energizing path for the fuel ruption of the burner circuit and the closing of the damper to allow volatiles to be purged from the furnace supply means. The activate means is operable when the heating demand has been met to deenergize the fuel following operation of the burner. 45 supply means and the interlock means and to effect the When operating properly, systems such as the type reenergization of the drive motor to permit the damper referred to above provide the desired interlock between plate to be returned to the first position. the stack damper and the fuel supply apparatus. How The safety interlock between the fuel supply means ever, under certain failure conditions, such as the weld and the stack damper control means is provided by the ing together of contacts of the limit switches, or, when 50 limit switch means, which includes first and second cam-operated switches are used, the cams becoming limit switches, and the interlock means. In accordance loose and shifting out of place, the fuel supply valves with a disclosed embodiment, the interlock means is may be energized while the vent stack is closed. embodied as a switching device, such as a relay, and the Therefore, it would be desirable to have a control first and second limit switches comprise cam operated arrangement for use in a heating system which provides 55 switches which are coupled to a drive shaft of the mo a safety interlock between a stack damper control appa tor. The interlock relay is energized over the first limit ratus and fuel supply apparatus of the system which switch at the start of each heating cycle and operates to prevents operation of the fuel supply apparatus when close associated contacts which are connected in the ever the damper is in a position other than a fully open energizing path for the drive motor. The second limit position, or in the event of an unsafe failure of the con 60 switch permits energization of the drive motor when trol apparatus. the interlock relay operates, causing the damper plate to SUMMARY OF THE INVENTION be driven to the second or open position. When the damper plate has been driven to the fully open position,

It is therefore an object of the present invention to the first limit switch operates to complete the energiz provide a control arrangement for a heating system 65 ing path for the fuel supply means, and to interrupt the which provides a safety interlock between stack damper energizing path for the interlock relay, which is main control apparatus and fuel supply apparatus of the sys tained by a holding path provided by further contacts of tem. the relay. In the event that the interlock relay fails to 4 operate at the start of a heating cycle, such as due to a The heating system 10 further includes a vent stack malfunction of either limit switch, as by welding of 21 for venting combustion products away from the contacts, or failure of the interlock relay itself, the ener main burner when the main burner is lit. A stack gizing path for the drive motor is interrupted, so that damper plate 22 is pivotally mounted within the stack the damper plate is maintained closed and the energiza by way of a shaft 24 for movement between closed and tion of the fuel supply means is prevented. open positions under the control of a stack damper In an application in a pilot-ignition type heating sys control circuit 30, including a drive motor 32. The shaft tem, including a pilot valve and a main valve, and an 24 is mechanically linked to a drive shaft 34 of the drive electronic pilot flame sensing means, a safety interlock motor 32 which is operable when energized to drive the is provided between the fuel supply means and the O damper plate 22 between the closed and open positions. flame sensing means to permit the interruption of the The damper plate 22 is normally maintained in the energizing path for the drive motor in the event of a closed position, as illustrated in the drawing, when the malfunction of the flame sensing means which would system 10 is deactivated, so that the vent stack 21 is otherwise permit fuel to be supplied to the main burner closed, preventing heat loss via the vent stack 21. In apparatus in the absence of a flame. In accordance with 15 response to a request for heat, the stack damper control the invention, the flame sensing means includes a circuit 30 energizes the motor 32 to permit the damper switching means, embodied as a relay, having normally plate 22 to be driven from the closed position to the closed contacts connected in the energizing path for the open position, represented by the dotted line in the drive motor. The flame sensing means is energized in drawing to permit combustion products to be vented response to the activate means following a request for 20 away from the main burner. The stack damper control heat, so that in the event of a malfunction in the flame circuit 30 includes a limit switch 36 which effects the sensing means, which permits the relay to operate in the deenergization of the motor 32 when the damper plate absence of a pilot flame, the relay of the flame sensing 22 reaches the open means will interrupt the energizing for the drive motor permits energization position.

of the

A further limit switch 38 fuel supply valves 12 and 14 before the damper plate is driven to the open position 25 to enable a flame to be established at the main burner and before the fuel supply valves are operated. More apparatus to meet the heating demand. over, the normally closed contacts of the relay are oper When the heating demand has been met, the stack ated by an armature which also controls normally open damper control circuit 30 permits reenergization of the contacts of the relay which effect the energization of drive motor 32, to permit the damper plate 22 to be the main valve. Thus, should the normally open 30 contacts become welded closed during a heating cycle driven to the closed position. The limit switch 36 effects the normally closed contacts are prevented from reclos the deenergization of the motor 32 when the damper ing at the end of the heat run thereby preventing energi plate 22 reaches the closed position, and the limit switch zation of the drive motor in response to the next call for 38 interrupts the energizing path for the fuel supply valves 12 and 14.

heat. 35

Thus, the control arrangement of the present inven The limit switches 36 and 38 each comprise cam tion not only provides for the energization of the fuel actuated switches, the operation of which is controlled supply means only when the damper plate is at the fully by ways of cams CA and CB. The cams CA and CB, open position, but also guards against unsafe failure of which may comprise a unitary cam structure, are me one or both limit switches, or in the flame sensing cir 40 chanically linked to the shaft 34 of the motor 32. The cuit, for systems of the pilot ignition type, preventing limit switch 36 includes a movable contact member the energization of the fuel supply means in the event of CA1 having contacts CA1 and CA1" carried by a resil such occurrence. ient switch arm 37 which is movable by way of cam CA, and a pair of fixed contacts CA2 and CA3. Cam

DESCRIPTION OF THE DRAWING 45 actuator portions 41 and 42 are disposed at diametri The single FIGURE, which comprises the drawings, cally opposed positions along the periphery of the cam is a simplified representation of a heating system em CA. The actuator portions 41 and 42 are normally dis ploying a control arrangement provided by the present engaged from movable contact member CA1, which is invention. biased to normally permit contact CA1 engage contact 50 CA2 when the damper plate 22 is in the closed position.

DESCRIPTION OF PREFERRED When the cam CA is rotated clockwise approximately EMBODIMENTS 90, the actuator portion 41 engages the movable Referring to the drawing, there is shown a simplified contact member CA1 which then moves contact CA1' representation of a heating system 10 employing a con out of engagement with contact CA2 and contact CA1' trol arrangement provided by the present invention. In 55 into engagement with contact CA3. When the cam CA the exemplary embodiment, the heating system 10 is of has been rotated through an additional 90', actuator the pilot ignition type. The system 10 includes a fuel portion 41 is moved out of engagement with the mov fired heating apparatus having a pilot outlet 11 and a able contact CA1, which then moves contact CA1' out main burner 13, a pilot valve 12, a main valve 14, an of engagement with contact CA3 and contact CA1" into igniter circuit 16, and a flame sensing circuit 20. The 60 engagement with contact CA2.

pilot valve 12 is operable when energized to supply fuel Similarly, limit switch 38 includes a movable contact to the pilot outlet 11 for ignition by sparks provided by member CB1 having contacts CB1" and CB1" carried the igniter circuit 16 to provide a pilot flame. The main by a resilient switch arm 39, which is movable by cam valve 14 is operable under the control of the flame CB, and fixed contacts CB2 and CB3. Cam CB has cam sensing circuit 20 to supply fuel to the main burner 65 actuator portion 43, which normally engages the mov apparatus 13 for ignition by the pilot flame to establish able contact member CB1, permitting the contact CB1" a flame at the main burner for providing heat for the to engage contact CB2, and to move contact CB1" into system. engagement with contact CB3 with approximately 90' 5 of rotation of the cam CB and prior to movement of the Limit switch 36 operates to deenergize the motor 32 damper plate 22 to the fully open position. when the damper plate 22 reaches the closed position. Contacts CA1 and CA2 of limit switch 36 provide a The motor 32, which may be an AC synchronous portion of an energizing path for the drive motor 32 motor, operates at a specific speed, such as 1RPM, and when the damper plate 22 is in the closed position and 5 thus provides sufficient time for the interlock circuit to deenergize the motor 32 when the damper plate 22 operate at the start of a heating cycle. The timed closing reaches the fully open position. of the damper plate 22 at the end of each heating cycle Contacts CA1 and CA3 of limit switch 36 provide a allows combustion products to be vented from the vent portion of a return drive energizing path for the motor stack 21 before the damper plate 22 is returned to the 32 and deenergize the motor 32 when the damper plate O closed position.

22 reaches the closed position. Contacts CB1 and CB2 The position of the damper plate 22 is mechanically of limit switch 38 provide an energizing path for an interlocked with limit switch 38, allowing actuation of interlock relay R1 when the damper plate 22 is in the the limit switch 38 only when the damper plate 22 closed position. Contacts CB1 and CB3 of limit switch 15 aspectreaches the fully open position. This provides a safety 38 provide an energizing path for the fuel supply valves dampersuch that it is impossible to manually operate the plate 22 to any position other than fully open 12 and 14 when the damper plate 22 is in the open posi tion. and simultaneously cause the fuel valves 12 and 14 to be electrically actuated. The energization of relay R1 is

In accordance with the present invention, the stack prevented damper control circuit 30 is interlocked with the fuel 20 38, in the event of a malfunction of limit switch such as ignition valves 12 and 14 by way of the limit switches 36 fuel supply valves welded contacts, thereby maintaining the and 38, the interlock relay R1, and a relay R2 of the event 12 and 14 deenergized. Also, in the of a malfunction flame sensing circuit, which permit energization of the plate 22 is either maintained of limit switch 36, the damper fuel supply valves 12 and 14 only when the damper ously fully open or is continu driven between open and closed positions with plate 22 is in the fully open position. The interlock relay 25 the fuel valves 12 and 14 being energized R1 is energized over limit switch 38 following the oper damper is open. Moreover, the proper only when the ation of thermostatically controlled contacts THS in quence of the relays R1 and R2 and the limit switchesse operating response to a request for heat. Relay R1 is operable and 38 must be maintained in order to permit the fuel when energized to close associated contacts R1A which valves 12 and 14 to operate as will be shown hereinafter. provide a holding path for the relay R1, and to prepare 30 Considering the heating system 10 in more detail, an energizing path for the fuel supply valves 12 and 14. power is supplied to the system 10 over input terminals In addition, contacts R1B close to complete an energiz 51 and 52 thereof which are connectable to a 24 VAC ing path for motor 32, and contacts R1C open to inter source. Terminal 51 is connected over normally open rupt the return drive path for the motor 32. When the thermostatically controlled contacts THS to a conduc damper plate 22 has been rotated to the fully open posi 35 tor L1, and terminal 52 is connected directly to a further tion, limit switch 36 deemergizes the motor 32, and limit conductor L2.

switch 38 operates to complete an energizing path over The energizing path for relay R1 is provided over limit switch 38, permitting power to be applied to the limit switch 38 which has contact CB2 connected to pilot valve 12 and the igniter 16, permitting fuel to be conductor L1 and contact CB1 connected to one side of supplied to pilot outlet for ignition by sparks provided 40 the operate coil 57 of relay R1 at point 59, the other side by the igniter circuit 16. of which is connected to conductor L2. Accordingly, When a pilot flame is established, the flame sensing when contacts CB1 and CB2 are closed, the operate coil circuit 20 effects the energization of the main valve 14 57 of relay R1 is connected between conductors L1 and through the operation of associated relay R2, which L2 for energization whenever contacts THS close. closes contacts R2A, permitting fuel to be supplied to 45 Relay R1 has normally open contacts R1A connected the main burner apparatus for ignition by the pilot between point 59 and conductor L1 to provide a hold flame. In addition, contacts R2B open to disable the ing path for the relay R1, and to prepare an energizing igniter 16, and contacts R2C, which are connected in path for operating solenoids 12' and 14 of the fuel the energizing circuit for the motor 32, open. Contacts valves 12 and 14, which is completed by limit switch 38 R2C of relay R2 prevent operation of the motor 32 in 50 when the damper plate is in the fully open position. the event of a malfunction in the flame sensing circuit 20 Contact CB3 of limit switch 38 is connected to a con which would otherwise permit operation of the main ductor L1', permitting power to be supplied to the fuel valve 14 in the absence of a pilot flame. When relay R1 valve solenoids 12 and 14 when the limit switch 38 is operates at the start of a heating cycle, the motor 32 is operated to move its switch arm CB1 into engagement energized over normally closed contacts R2C of relay 55 with contact CB3.

R2, contacts R1B of relay R1 and limit switch 36 to Relay R1 has normally open contacts R1B connected drive the damper plate 22. Failure of a relay R1 to in series with normally closed contacts R2C of relay R2 operate prevents the energization of the motor 32 so between conductor L1 and point 61 which is connected that the limit switch 38 cannot operate thereby prevent to movable contact CA1 of limit switch 36. Contact ing operation of the pilot valve 12 and the main valve 60 CA2 of limit switch 36 is connected to one terminal 54 14. of the motor 32, which has a second terminal 55 con When the heating demand has been met following a nected to conductor L2, permitting the motor 32 to be successful heating cycle, the thermostatically con energized when relay R1 operates. Relay R1 also has trolled contacts THS open, causing relay R1 and the normally closed contacts R1C connected between point fuel valves 12 and 14 to be deenergized. When relay R1 65 61 and terminal 54 of the motor 32 for providing a drops out, contacts R1C complete the energizing path return drive energizing path for the motor 32 at the end for the motor 32 over limit switch 36, permitting the of each heating cycle over contacts CA1 and CA3 of damper plate 22 to be driven to the closed position. limit switch 36. Relay R1 is a double-pole, double throw 6 relay, with contacts R1B and R1C employing a com In addition, contacts R1C open, and contacts R1B mon armature of the relay R1. Thus, should contacts close completing an energizing path for the drive motor R1B become welded together, contacts R1C cannot 32 which extends from conductor L1 over normally reclose. closed contacts R2C of relay R2, contacts R1B, Referring to the fuel supply apparatus, the pilot valve contacts CA1 and CA2 of limit switch 36 and the motor solenoid 12, and the igniter circuit 16 are connected in 32 to conductor L2.

parallel between conductors Lil' and L2. The main When the motor 32 is energized, the motor shaft 34 valve solenoid 14 is connected between conductors L1' rotates, moving the damper plate 22 toward the open and L2 over normally open contacts R2A of relay R2. position. Cams CA and CB are also driven, and when The igniter circuit 16 may, for example, be the type 10 cam CB has been rotated a few angular degrees less disclosed in the U.S. Pat. application, Ser. No. 698,161 than 90, and as the damper plate 22 approaches the of G. E. Dietz, which is entitled, "Fuel Ignition System fully open position, cam CB permits contact CB1 to Including An Igniter Which Provides A Lingering move out of engagement with contact CB2, and into Spark.' The operation of the igniter is disclosed in de engagement with contact CB3, completing the energiz tail in the referenced application. Briefly, when power 15 ing path for the pilot valve solenoid 12 and the igniter is applied to conductor L1, the igniter circuit 16 is 16 from conductor L1 over contacts R2A of relay R1 energized to provide ignition sparks for igniting fuel and contacts CB1 and CB3 of limit switch 38. supplied to the pilot outlet. When a pilot flame is estab lished, the relay R2 operates, opening contacts R2B, to When cam CA has been rotated 90', corresponding the fully open position for the damper plate 22, which are connected in an enabling circuit for the ig contact CA1 is moved out of engagement with contact niter circuit, to thereby disable the igniter circuit 16. CA2, interrupting the energizing path for the motor 32, The igniter circuit 16 continues to provide sparks for a and predetermined time following the operation of the relay returnintodrive engagement with contact CA3 to prepare a energizing path for the motor 32.

R2, assuring ignition of the fuel in the event of a mal When the pilot valve solenoid 12' is energized, fuel is function in the flame sensing circuit 20 which allows 25 supplied to the pilot outlet 11 for ignition by sparks relay R2 to operate in the absence of a flame. provided by the igniter 16. When a pilot flame is estab The flame sensing circuit 20 is energized over a trans former T1, which has a primary winding 62 connected lished, the flame sensing circuit 20, senses the pilot between conductors L1 and L2, and a secondary wind flame and effects energization of relay R2. When relay R2 operates, contacts R2C are opened, interrupting the ing 63 connected between conductors L3 and L4 which 30 energizing path for the motor 32. In addition, contacts are connected to input terminals of the flame sensing circuit 20. The flame sensing circuit 20 may be the type R2B of relay R2 are opened, disabling the igniter 16, disclosed in U.S. Pat. No. 3,902,839 of Russell B. Mat and contacts R2A are closed, energizing the main valve thews, which was issued on Sept. 2, 1975. As disclosed solenoid 14, causing fuel to be supplied to the main in detail in such patent, the flame sensing circuit 20 35 burner 13 for ignition by the pilot flame to provide heat includes a flame sensor 65 which is disposed in proxim to satisfy the heating demand for the system 10. ity to the pilot outlet and is responsive to a pilot flame When the heating demand has been met, contacts to effect the operation of relay R2 of the flame sensing THS open, interrupting the supply of power to conduc circuit 20. The relay R2 operates to open contact R2C tor L1, causing the deenergization of the fuel valves 12 to interrupt the energizing path to point 61 for the drive and 14, permitting the main burner flame and the pilot motor 32. Relay R2 also opens contacts R2B to disable flame to be extinguished. The flame sensing circuit 20 the igniter circuit 16, and closes contacts R2A to com and relay R1 are also deemergized, and when relay R1 plete the energizing path for the main valve solenoid 14 drops out, contacts R1C close completing the return between conductor L1' and L2. Relay R2 is a double drive energizing path for the motor 32 over contacts pole, double throw relay, with contacts R2A and R2C 45 CA1 and CA3 of limit switch 36 for energizing the employing a common armature. Thus, if contacts R2A motor 32 to drive the damper plate 22 to the fully closed become welded together, contacts R2C cannot reclose position. As the motor shaft 34 is driven, cams CA and when relay R2 is deemergized. CB are rotated. When cam CB has been rotated approx Operation imately 5, switch arm CB1 is moved out of engagement 50 with contact CB3, interrupting the energizing path for

For the purpose of illustrating the operation of the the fuel valves 12 and 14, and into engagement with control arrangement for the heating system 10, it is contact CB2 preparing an energizing path for relay R1. assumed initially that the system 10 is deactivated with When the damper plate 22 reaches the closed posi the damper plate 22 in the fully closed position, and that tion, cam CA has been rotated approximately 90', and the limit switches 36 and 38 are operated to the posi 55 contacts CA1 and CA3 open, deemergizing the motor tions shown in the drawing. In response to a request for 32. Also, contact CA1 reengages contact CA2, prepar heat, contacts THS close, extending power to conduc ing an energizing path for the motor 32 over contacts tor L1 for energizing relay R1 over contacts CB1 and R1B of relay R1. Accordingly, the system 10 is pre CB2 of switch 38. The flame sensing circuit 20 is also pared for the next heating cycle.

energized over transformer T1. Safety Aspects When relay R1 operates, contacts R1A are closed providing a holding path for the relay R1 between con As indicated above, the proper sequencing of the ductors L1 and L2, and preparing an energizing path limit switches 36 and 38 and relays R1 and R2 is re for the pilot valve solenoid 12' and the igniter circuit 16 quired to enable the fuel valves 12 and 14 to operate. which extends from conductor L1 and contacts CB1 65 That is, relay R1 must operate before R2 operates, and and CB3 of limit switch 38 to conductor L1'. At this the limit switches must be operated to the positions time, the energizing path is interrupted by limit switch shown in the drawing at the start of a heating cycle. 38 since contact CB1 is still engaging contact CB2. Also, contacts R2C of relay R2 must be closed at the 7 start of a heating cycle to permit eventual energization ducting combustion products away from said burner of the fuel valves 12 and 14. apparatus, and stack damper means including a drive For a failure of relay R1, such as an open coil 57, then motor, and a stack damper plate pivotally mounted when contacts THS close, relay R1 remains disabled within said vent stack, said damper plate being normally and contacts R1B remain open preventing energization maintained at a first position to close said vent stack and of the motor. 32. Also, should contacts R1B become being rotatable to a second position to open said vent welded closed, then contacts R1C remain open at the stack, said drive motor being operatively coupled to end of a heating cycle, preventing reenergization of the said damper plate for driving said damper plate between motor so that the damper plate 22 is maintained in the said first and second positions, a control arrangement fully open position. 10 comprising activate means responsive to a request for Moreover, for a failure in the flame sensing circuit 20 heat to effect the energization of said drive motor over which permits relay R2 to be operated in the absence of an energizing path to permit said damper plate to be a flame, then upon the closure of the contacts THS, driven from said first position towards said second posi relay R2 operates, opening contacts R2C and the ener tion, switching means, limit switch means having first gizing path for the motor 32 is interrupted, preventing 15 contacts connected in an energizing path for said eventual operation of the pilot valve 12 and the system switching means and second contacts connected in an 10 is maintained in a lock out condition. energizing path for said fuel supply means, said limit In the event of a failure condition following a suc switch means being coupled to said stack damper means cessful start up, such as the welding together of the for operation thereby to close said first contacts provid contacts R2A which control the operation of the main 20 ing an energizing path for said switching means and to valve 14, then when the heating demand has been met, open said second contacts, interrupting the energizing and contacts THS open, the pilot valve 12 and the main path for said fuel supply means when said damper plate valve 14 are deemergized, extinguishing the flame. The is at first position, to permit said switching means to be flame sensing circuit 20 responds to the loss of flame to energized by said activate means when said first deenergize relay R2. However, since contacts R2A are 25 contacts are closed, said activate means causing said welded together, contacts R2C cannot reclose since switching means to operate to close third contacts such contacts employ a common armature of the relay which are connected in said energizing path for said R2. Accordingly, when contacts THS close on the next fuel supply means, said stack damper means operating call for heat, the energizing path for the motor 32 is said limit switch means in response to said drive motor interrupted since contacts R2C are open. Thus, the pilot 30 driving said damper to said second position to open said valve 12 and the main valve 14 are maintained deener first contacts to interrupt said energizing path for said gized. switching means and to close said second contacts to In the event contact CB1 of limit switch 38 becomes complete said energizing path for said fuel supply means welded to contact CB2, then, following activation of for energizing said fuel supply means, said activate the system 10 through operation of contacts THS, relay 35 means being operable when the heating demand has R1 operates on the motor 32 moves the damper plate 22 been met to deenergize said fuel supply means and said to the open position and stops. However, the pilot valve switching means and to effect the reenergization of said 12 and main valve 14 remain deemergized because mov drive motor to permit said damper plate to be driven able contact CB1 cannot engage fixed contact CB3 to from said second position to said first position. complete the energizing path to the fuel valves 12 and 2. A system as set forth in claim 1 wherein said limit 14. The motor 32 remains deenergized as long as THS switch means includes a first limit switch having said are closed. When contacts THS open, relay R1 drops first and second contacts, and a second limit switch for out, and the notor returns the damper plate 22 to the preparing said energizing path for said drive motor closed position. when said damper plate is at said first position, and For the condition where contact CB1 of limit switch 45 operable when said damper plate has been driven to said 38 becomes welded to fixed contact CB3, then, on the second position to interrupt said energizing path for said next call for heat, the relay R1 cannot be energized drive motor.

because the energizing path provided over contacts 3. A system as set forth in claim 2 wherein said sec CB1 and CB2 is interrupted. ond limit switch is operable to prepare a return drive Considering limit switch 36, should contact CA1 50 energizing path for said motor when said damper plate become welded to contact CA3, the motor 32 continues is driven to said second position, said return drive path to run during the time contacts THS are open. When being completed by further contacts of said switching contacts THS close, relay R1 is energized when means which close when said switching means is deen contacts CB1 and CB2 close. This causes contacts R1C ergized, and said second limit switch being operated to to open stopping the motor 32 and terminating the heat 55 interrupt said return drive path for said motor when ing cycle. said damper plate is driven to said first position. If contact CA1 becomes welded to contact CA2, then 4. A system as set forth in claim 2 wherein said first when contacts THS close the motor 32 will continue to and second limit switches are coupled to said drive run, driving the damper plate 22 between the open and motor for operation thereby.

closed positions, permitting the valves 12 and 14 to be 5. A system as set forth in claim 1 wherein said fuel energized each time contacts CB1 and CB3 close at supply means includes pilot valve means and main valve which time the damper plate 22 is in the fully open means, said pilot valve means being connected to said position. fuel supply means energizing path for operation, when We claim: ever said fuel supply means energizing path is com 1. In a heating system including a furnace having a 65 pleted, to supply fuel to a pilot outlet for ignition to fuel fired burner apparatus, fuel supply means operable establish a pilot flame, and flame sensing means, includ when energized to supply fuel to said burner apparatus ing further switching means energized when a pilot for combustion to provide heat, a vent stack for con flame is established, to effect the energization of said 8 main valve means permitting fuel to be supplied to a for said drive motor, and a second actuator means cou main burner apparatus for ignition by the pilot flame. pled to said drive motor for operating said third and 6. A system as set forth in claim 5 wherein said flame fourth contacts.

sensing means is energized responsive to said activate 9. A system as set forth in claim 8 wherein said first means, said energizing path for said drive motor being actuator means includes a first cam member coupled to interrupted whenever said further switching means is said drive motor and operable to permit said first operated thereby preventing energization of said fuel contacts to be maintained normally closed and said supply means in the event said further switching means second contacts to be maintained normally open when operates prior to said damper plate being driven to said said damper plate is at said first position, and for open second position. 10 ing said first contacts and closing said second contacts 7. In a heating system including a furnace having a when said damper plate is at said second position, and fuel-fired burner apparatus, fuel supply means operable said second actuator means includes a second cam mem when energized to supply fuel to said burner apparatus ber coupled to said drive motor and operable to main for combustion to provide heat, a vent stack for con tain said third contacts normally closed and said fourth ducting combustion products away from said burner 15 contacts normally open when said damper plate is at apparatus, and stack damper means including a drive said first position, and for permitting said third contacts motor, and a damper plate pivotally mounted within to open and said fourth contacts to close when said said vent stack, said damper plate being normally main damper plate is at said second position. tained in a first position to close said vent stack and 10. A system as set forth in claim 9 wherein said first being rotatable to a second position to open said vent 20 and second contacts are operated by a common switch stack, said drive motor being operatively coupled to member whereby the energization of said switching said damper plate for driving said damper plate between means is prevented, thereby preventing the energization said first and second positions, a control arrangement of said fuel supply means in the event of a malfunction comprising switching means for controlling the energi of said first limit switch which permits said second zation of said fuel supply means and said drive motor, a 25 contacts to be closed when said damper plate is at said first limit switch for controlling the energization of said first position.

switching means and said fuel supply means, said first 11. A system as set forth in claim 8 wherein said fuel limit switch having first contacts connected in an ener supply means includes pilot valve means and main valve gizing path for said switching means and second means, said pilot valve means being connected to said contacts connected in an energizing path for said fuel 30 energizing path for operation whenever said energizing supply means, said first limit switch being coupled to path is completed to supply fuel to a pilot outlet for said stack damper means for operation thereby to close ignition to establish a pilot flame, and flame sensing said first contacts and to open said second contacts means including further switching means energized when said damper plate is at said first position to permit when a pilot flame is established to effect the energiza the energization of said switching means, and a second 35 tion of said main valve means permitting fuel to be limit switch for preparing an energizing path for said supplied to a main burner for ignition by the pilot flame. drive motor when said damper plate is at said first posi 12. A system as set forth in claim 11 wherein said tion, activate means responsive to a request for heat to further switching means comprises a further relay hav energize said switching means causing said switching ing normally closed contacts connected in said energiz means to operate to close contacts connected in said ing path for said drive motor and normally open energizing path for said fuel supply means and to close contacts connected in an energizing path for said main further contacts to complete said energizing path for valve means, said normally open contacts and said nor said drive motor for energizing said drive motor to mally closed contacts being operated by a common cause said damper plate to be driven to said second armature whereby said normally closed contacts are position, said second limit switch interrupting said ener 45 prevented from reclosing whenever said normally open gizing path for said drive motor to deemergize said drive contacts are welded together, thereby preventing ener motor when said damper plate has been driven to said gization of said drive motor in response to a request for second position, said stack damper means operating said heat.

first limit switch when said damper plate has been 13. A system as set forth in claim 7 wherein said driven to said second position to open said first contacts 50 further contacts of said switching means comprise nor to interrupt said energizing path for said switching mally open contacts connected in said energizing path means and to close said second contacts to complete for said drive motor and normally closed contacts con said energizing path for said fuel supply means, said nected in said return drive path for said drive motor. activate means causing said fuel supply means and said 14. A system as set forth in claim 13 wherein said switching means to be deenergized when the heating 55 switching means comprises a relay having said normally demand has been met, permitting said drive motor to be open contacts and said normally closed contacts oper reenergized over a return drive path, including said ated by a common armature whereby said normally second limit switch, to cause said damper plate to be closed contacts are prevented from reclosing whenever driven to said first position, said second limit switch said normally open contacts are welded together, being operated to interrupt said return drive path when thereby preventing the reenergization of said drive said damper plate has been driven to said first position motor so that said damper plate is maintained at said to deemergize said drive motor. second position.

8. A system as set forth in claim 7 wherein said first 15. In a heating system including a furnace having a limit switch has a first actuator means coupled to said fuel fired burner apparatus, fuel supply means operable drive motor for operating said first and second contacts, 65 when energized to supply fuel to said burner apparatus and wherein said second limit switch has third contacts for combustion to provide heat, a vent stack for con connected in said energizing path for said drive motor ducting combustion product away from said burner and fourth contacts connected in said return drive path apparatus, and stack damper means, including a drive 9 motor, and a stack damper plate pivotally mounted and to open said second contacts when said damper within said vent stack, said damper plate being normally plate is at said first position, activate means operable in maintained at a first position to close said vent stack and response to a request for heat to connect power to said being rotatable to a second position to open said vent energizing path for said switching means causing said stack, said drive motor being operatively coupled to 5 switching means to operate to close third contacts said damper plate for driving said damper plate between which are connected in said energizing path for said said first and second positions, a control arrangement fuel supply means, said stack damper means operating comprising first limit switch means coupled to said said second limit switch means in response to said drive stack damper means for operation thereby for control motor driving said damper plate to said second position ling the energization and deemergization of said drive O to open said first contacts to interrupt said energizing motor to permit said damper plate to be driven between said first and second positions, switching means opera path for said switching means and to close said second ble when energized to permit energization of said fuel contacts to complete said energizing path for said fuel supply means, second limit switch means having first supply means, said switching means being prevented contacts connected in an energizing path for said 15 from operating to close said third contacts in the event switching means and second contacts connected in an that said first contacts are open, interrupting the ener energizing path for said fuel supply means, said second gizing path for said fuel supply means when said limit switch means being coupled to said stack damper damper plate is at said first position. means for operation thereby to close said first contacts Sk