Modulating furnace with two-speed draft inducer

A modulating, forced draft, fuel-fired air heating furnace is provided with a two-speed draft inducer fan, and a fuel valve which is fully modulatable between a maximum firing rate and a low firing rate of approximately forty percent thereof. Incorporated into the furnace control system are normally closed low and high fire pressure-electric switches which sense and are sequentially closed by increasingly negative pressure in the draft inducer fan. Upon a call for heat from a thermostat located in the conditioned space served by the furnace, the draft inducer fan is energized at its high speed setting, and a signal is sent to the fuel valve to set it at its full firing rate flow when opened by an ignition switch portion of the control system. The control system functions to open the fuel valve at this maximum flow setting, and permit light-off of the burner, only if (1) both of the low and high fire pressure-electric switches are closed, and (2) the draft inducer fan is operating at its high speed setting. In this manner, in addition to providing a wide range of furnace heating output modulation, reliable burner ignition is facilitated and heat exchanger warm-up corrosion is reduced due to the full firing rate start-up of the furnace. Moreover, the incorporation of the two-speed draft inducer fan substantially improves the overall fuel efficiency of the furnace.