Low speed interlock for a two stage two speed furnace

Claim 1

In a gas-fired forced air furnace for heating air in a comfort space and in which at least one gas burner is disposed in a combustion chamber that is supplied with combustion air and which supplies heated combustion gases to a heat exchanger for heating circulation air that passes through the heat exchanger and returns to the comfort space, in which an electrically actuated gas valve supplies fuel gas to said at least one gas burner and is actuable into a cutoff mode, a partial-flow low-fire mode, and a full-flow high-fire mode, the gas valve having a low-fire input and a high-fire input; and in which a low-fire pressure switch in gas pressure communication with said collector box is operative to connect electrical power to said gas valve low-fire input when said collector box pressure attains a first predetermined pressure level and a high-fire pressure switch in gas pressure communication with said collector box is operative to connect electrical power to said gas valve high-fire input when said collector box pressure attains a second predetermined pressure level; the improvement which comprises interlock means connected in series with said high fire pressure switch to deny electrical power to said gas valve high-fire input when said gas valve is actuated in its low-fire mode in response to a call for low-fire.