Blower control circuit for a furnace

Claim 1

In a furnace having a fuel burner supplied by a fuel regulator, a heat exchanger fired by the burner having a flue gas passage and a heated air passage, a blower powered by an electric motor for displacing the heated air, a space thermostat for determining when heating is required, a limit thermostat for determining when a high temperature limit is exceeded, a power source for supplying current to the furnace blower motor, the space thermostat and the limit thermostat, and a blower control circuit, the blower control circuit including a time-delay relay circuit, and a blower relay having contacts for connecting and disconnecting the blower motor with the power source, said time-delay relay circuit being energized by the power source and being connected to the space thermostat such that when heating is required an electric signal is received from the space thermostat, the time-delay relay circuit including timing means for creating a first electrical relay control signal upon elapse of a delay period after the receipt of the space thermostat siqnal and for maintaininq said first signal until elapse of the delav period after the discontinuance of the space thermostat siqnal, the improvement comprising: said blower relay contacts being normally-open; a safety sense relay circuit energized by the power source and being connected and responding to said limit thermostat for providing a second electrical relay control signal while said high temperature limit is exceeded for controlling the blower relay contacts; and relay drive means responsive to either of said first and second electrical relay control signals for closing said blower relay contacts.