Patent ID: 8146584

Claim:
A gas-fired induced-draft furnace that has low, medium, and high firing rates, the furnace comprising: a furnace controller having a first pressure switch input and a second pressure switch input; a heat exchanger; a pressure switch assembly configured to sense low, medium, and high pressures across the heat exchanger corresponding to the low, medium, and high firing rates and to generate pressure signals that vary in accordance with the differential pressure sensed across the heat exchanger, wherein the pressure signals are provided on the first and second pressure switch inputs; wherein the pressure switch assembly comprises: a low-heat pressure switch configured to actuate at a low combustion pressure level and connected to the first pressure switch input; a medium-heat pressure switch configured to actuate at an intermediate combustion pressure level; a high-heat pressure switch configured to actuate at a high combustion pressure level; and a medium-high-heat switch arranged to connect the medium-heat pressure switch to the second pressure switch input at the intermediate combustion pressure level and to connect the high-heat pressure switch to the second pressure switch input at the high combustion pressure level.