Abstract: An improved, induced-draft, gas-fired furnace which is adapted to operate at any of a high, medium and low firing rate. A pressure sensing assembly generates differential pressure signals that vary in accordance with the pressure across a heat exchanger assembly. A gas flow control assembly is responsive to the differential pressure signals, and to signals received from the furnace control circuit, to supply gas to the furnace at any of its three different firing rates, the lowest of those said firing rates corresponding to a gas-air mixture which cannot be ignited. The furnace control circuit applies to the circulating air blower, and to the inducer blower, speed controls signals that are so related to one another that the fuel utilization efficiency of the furnace is at least as great during steady state operation at its low firing rate as during steady state operation at its medium firing rate.

Abstract: An improved, induced-draft, gas-fired furnace which is adapted to operate at any of a high, medium and low firing rate. A pressure sensing assembly generates differential pressure signals that vary in accordance with the pressure across a heat exchanger assembly. A gas flow control assembly is responsive to the differential pressure signals, and to signals received from the furnace control circuit, to supply gas to the furnace at any of its three different firing rates, the lowest of those firing rates corresponding to a gas-air mixture which cannot be ignited. The furnace control circuit applies to the circulating air blower, and to the inducer blower, speed control signals that are so related to one another that the lowest primary wall temperature in the heat exchanger is maintained above the condensation temperature of water during steady state operation of the furnace at its low firing rate.

Abstract: A method and apparatus for increasing the circulating airflow of induced-draft, gas-fired multi-stage furnaces without creating conditions that result in cold spot corrosion therein. Control circuitry is provided to selectably increase the circulating airflow of the furnace during low stage operation. The control circuitry is arranged so that increases in the magnitude of the circulating airflow of the furnace are accompanied by predetermined increases in the magnitude of the combustion airflow of the furnace. The magnitudes of the circulating and combustion airflows are so related to one another that the temperature at the output of the heat exchanger is maintained at a value which is approximately constant, and which is high enough to assure that water cannot condense thereon. The control circuitry may include a relay or relay-like device that is connected to prevent the high stage solenoid of the gas valve from becoming actuated during low stage operation.