Electrothermal furnace control

Claim 2

Apparatus for controlling the operation of a furnace in response to the demand of a thermostat sensing the temperature in a zone heated by the furnace, the furnace having a burner and an electrically energizable fuel valve which, when energized, supplies fuel to the burner, said apparatus comprising: ignition means which, upon energization, generates recurrent sparking for igniting the fuel, said ignition means normally being energized when the thermostat demands heat and ceasing to generate sparking after ignition of the fuel; a triggerable semiconductor current-switching device interconnected with the fuel valve and, when triggered, being conductive for energizing the fuel valve, said fuel valve includes an inductive winding, said switching device including main terminals serially connected with the winding; a triggering circuit for said switching device interconnected with said ignition means and operative to supply triggering current to said switching device for initial triggering thereOf upon energization of said ignition means and a capacitor in said triggering circuit cooperating with said inductive winding to supply triggering current to said switching device for continued triggering thereof after said ignition means ceases generating sparking as long as the thermostat demands heat; a thermistor connected in said triggering circuit for preventing said triggering current from being supplied to said switching device when the thermistor is heated above a predetermined threshold temperature; and means for heating said thermistor interconnected with said ignition means and being energized to cause heating of said thermistor when said ignition means generates sparking, said thermistor requiring a predetermined heating time interval to reach said threshold temperature, whereby if the fuel is not ignited within said predetermined interval, triggering of said switching device is terminated to prevent fuel from being further supplied to the burner.

Claim 3

Furnace control apparatus as set forth in claim 1 wherein said ignition means includes a further triggerable semiconductor current-switching device including a pair of main terminals and generates sparking when said further switching device is triggered, said capacitor being connected to one of the main terminals of said further switching device to supply said triggering current for initial triggering of the first-said switching device upon triggering of said further switching device.

Claim 4

Furnace control apparatus as set forth in claim 1 wherein said means for heating said thermistor comprises a further thermistor thermally coupled to the first-said thermistor.

Claim 5

Furnace control apparatus as set forth in claim 3 wherein each of said thermistors has a positive temperature coefficient and a transition temperature above which the resistance thereof increases relatively abruptly, said further thermistor having a higher transition temperature than that of the first-said thermistor, said thermistors together constituting an electrothermal timer.

Claim 6

Furnace control apparatus as set forth in claim 3 wherein said further thermistor is connected in a series circuit with said ignition means for concomitant energization therewith when said ignition means generates sparking, said further thermistor being deenergized when said ignition means ceases to generate sparking.

Claim 7

Furnace control apparatus as set forth in claim 5 wherein the resistance of said further thermistor is increased by energization thereof for said predetermined time interval, whereby energization of said ignition means is reduced for causing sparking to be generated at a reduced sparking rate.

Claim 8

Furnace control apparatus as set forth in claim 1 further comprising a second triggerable current-switching device which is conductive, when triggered, for causing power to be supplied to the first-said switching device, said second switching device normally being triggered when the thermostat demands heat.

Claim 9

Furnace control apparatus as set forth in claim 7 wherein said second switching device includes a triggering terminal and further comprising a second thermistor mounted for sending the temperature in a plenum of the furnace and connected to said triggering terminal of the said switching device for preventing further triggering thereof, thereby preventing further energization of the fuel valve, when heated above a predetermined threshold temperature corresponding to a predetermined maximum permissible temperature in said plenum.

Claim 10

Furnace control apparatus as set forth in claim 8 wherein said second thermistor has a positive temperature coefficient of resistivity and a transition temperature above which the resistance thereof increases relatively abruptly.

Claim 11

Furnace control apparatus as set forth in claim 8 for controlling the operation of a furnace normally having a forced-air draft supplying the burner and further comprising: an additional thermistor mounted for being cooled by the forced draft and connEcted in a series circuit with said second thermistor and said triggering terminal for preventing triggering of said second switching device when said additional thermistor is heated above a predetermine threshold temperature; and means for heating said additional thermistor, said means normally supplying insufficient heat to cause said fourth thermistor to be heated above said threshold temperature as long as there is sufficient forced-air draft, but causing heating of said additional thermistor above said threshold temperatures thereby preventing energization of the fuel valve if there is insufficient forced-air draft.

Claim 12

Furnace control apparatus as set forth in claim 10 wherein said additional thermistor has a positive temperature coefficient of resistivity and a transition temperature above which the resistance thereof increases relatively abruptly, and said means for heating said additional thermistor comprises another thermistor having a positive temperature coefficient of resistivity and a transition temperature higher than that of said additional thermistor and above which the resistance thereof increases relatively abruptly.

Claim 13

Furnace control apparatus as set forth in claim 7 wherein each of said switching devices comprises a triac.

Claim 14

Furnace control apparatus as set forth in claim 1 further comprising a second thermistor, said thermistor being mounted for sensing the temperature in a plenum of the furnace and in said triggering circuit for preventing said triggering current from being supplied to said switching device when said second thermistor is heated above a predetermined threshold temperature corresponding to a predetermined maximum permissible temperature in said plenum.

Claim 15

Furnace control apparatus as set forth in claim 13 wherein said second thermistor has a positive temperature coefficient and a transition temperature above which the resistance thereof increases relatively abruptly.

Claim 16

Apparatus for controlling the operation of a furnace in response to the demand of a thermostat sensing the temperature in a zone heated by the furnace, the furnace having a burner and an electrically energizable fuel valve which, when energized, supplies fuel to the burner, the burner normally being supplied with a forced-air draft, said apparatus comprising: an ignition circuit connected for energization when the thermostat demands heat, said ignition circuit generating recurrent sparking for igniting the fuel and ceasing to generate sparking after ignition of the fuel; a triggerable semiconductor current-switching device having a pair of main terminals and a triggering terminal, said main terminals being connected with the fuel valve for energization thereof when said switching device is triggered; a triggering circuit interconnecting said triggering terminal and said ignition circuit for normally supplying current to said triggering terminal initially upon energization of said ignition circuit and then for as long as the thermostat demands heat, said triggering circuit including: a first thermistor connected for preventing said triggering current from being supplied to said triggering terminal, thereby preventing further energization of the fuel valve, when said first thermistor is heated above a predetermined threshold temperature; a second thermistor mounted for sensing the temperature in a plenum of the furnace and connected for preventing said triggering current from being supplied to said triggering terminal, thereby preventing further energization of the fuel valve, when said second thermistor is heated above a predetermined threshold temperature corresponding to a predetermined maximum permissible temperature in said plenum; and a third thermistor mounted for being cooled by the forced draft and connected for preventing said triggering current from being supplied to said triggering terminal thereby preventing further energization of the fuel valve when said third thermistor is heated above a predeterMined threshold temperature; means for heating said first thermistor interconnected with said ignition circuit for concomitant energization therewith to cause heating of said first thermistor when said ignition circuit generates sparking, said first thermistor requiring a predetermined heating time interval to reach said threshold temperature thereof, whereby if the fuel is not ignited within said predetermined interval, fuel is prevented from being further supplied to the burner; and means for heating said third thermistor normally supplying insufficient heat to cause said thermistor to be heated above said threshold temperature thereof as long as there is sufficient forded-air draft, but causing heating of said third thermistor above said threshold temperature thereof, whereby fuel is prevented from being supplied to the burner, if there is insufficient forced-air draft.

Claim 17

Apparatus for controlling the operation of a furnace in response to the demand of a thermostat sensing the temperature in a zone heated by the furnace, the furnace having a burner and an electrically energizable fuel valve which, when energized, supplies fuel to the burner, the burner normally being supplied with a forced-air draft, said apparatus comprising: an ignition circuit connected for energization when the thermostat demands heat, said ignition circuit generating recurrent sparking for igniting the fuel and ceasing to generate sparking after ignition of the fuel; a triggerable semiconductor current-switching device having a pair of main terminals and a triggering terminal, said main terminals being connected with the fuel valve for energization thereof when said switching device is triggered; a triggering circuit interconnecting said triggering terminal and said ignition circuit for normally supplying current to said triggering terminal initially upon energization of said ignition circuit and then for as long as the thermostat demands heat, said triggering circuit including: a first thermistor connected for preventing said triggering current from being supplied to said triggering terminal, thereby preventing further energization of the fuel valve, when said first thermistor is heated above a predetermined threshold temperature; a second thermistor mounted for sensing the temperature in a plenum of the furnace and connected for preventing said triggering current from being supplied to said triggering terminal, thereby preventing further energization of the fuel valve, when said second thermistor is heated above a predetermined threshold temperature corresponding to a predetermined maximum permissible temperature in said plenum; and a third thermistor mounted for being cooled by the forced draft and connected for preventing said triggering current from being supplied to said triggering terminal thereby preventing further energization of the fuel valve when said third thermistor is heated above a predeterMined threshold temperature; means for heating said first thermistor interconnected with said ignition circuit for concomitant energization therewith to cause heating of said first thermistor when said ignition circuit generates sparking, said first thermistor requiring a predetermined heating time interval to reach said threshold temperature thereof, whereby if the fuel is not ignited within said predetermined interval, fuel is prevented from being further supplied to the burner; and means for heating said third thermistor normally supplying insufficient heat to cause said thermistor to be heated above said threshold temperature thereof as long as there is sufficient forded-air draft, but causing heating of said third thermistor above said threshold temperature thereof, whereby fuel is prevented from being supplied to the burner, if there is insufficient forced-air draft.

Claim 18

Furnace control apparatus as set forth in claim 15 wherein said means for heating said first thermistor comprises a further thermistor thermally coupled to said first thermistor and said means for heating said third thermistor comprises another thermistor thermally coupled to said third thermistor.

Claim 19

Furnace control apparatus as set forth in claim 16 wherein each of said first, second and third thermistors has a positive temperature coefficient and a transition temperature above which the resistance thereof increases relatively abruptly.

Claim 20

Furnace control apparatus as set forth in claim 17 wherein each of said first, second and third thermistors is connected in a series circuit with said triggering terminal.

Claim 21

Furnace control apparatus as set forth in claim 16 wherein said switching device comprises a triac.