Patent ID: 6851948

Claim:
In a gas furnace that is responsive to a thermostat, and which contains a power supply, a gas valve, and an autoresettable thermal switch having a thermal switch probe that is responsive to an over-pressure condition, and a microprocessor responsive to said thermostat and said thermal switch for controlling said power supply and said gas valve, and wherein said gas furnace includes a furnace vent system, a process for detecting a blockage in said furnace vent system, comprising the steps of: mounting said autoresettable thermal switch adjacent to an entrance to said furnace vent system so that said thermal switch opens due to a flue pressure exceeding a certain level and causing hot flue gasses to pass over said thermal switch probe; electrically connecting said thermal switch in series between said power supply and said thermostat; programming said microprocessor to carry out the following steps upon said thermostat calling for heat; (a) instituting a combustion cycle; (b) sensing a condition of said thermal switch and detecting when said thermal switch opens; (c) determining, when said thermal switch is open, whether the number of openings of said thermal switch or the duration that said thermal switch remains open exceeds a preprogrammed criterion; (d) disabling, when the number of openings of said thermal switch or the duration that said thermal switch remains open exceeds said preprogrammed criterion, said furnace and reinstituting said combustion cycle after said furnace has been disabled for a specified period of time; and (e) allowing, when said thermal switch is open and said furnace is not in a disabled state, said thermal switch to reset and reinstituting said combustion cycle upon resetting said thermal switch.