Induced draft fan control for use with gas furnaces

The system (10) has a controller (14) controlling the supply of gas through a gas valve (12) and air for combustion by using an induced air draft fan (28), ignition of the gas by using ignitor (22), the delivery of heated air from a heat exchanger (20) by using an air blower (34) in response to signals from a thermostat (42). A selected constant flow of air for combustion is provided by controlling the speed of the motor driving the induced motor fan (28) despite changes which may occur in back pressure. Induced draft fan motor parameters proportional to motor torque and motor speed are read on an ongoing basis and inputted to controller (14) which computes a desired voltage and compares that with referenced data stored in the controller memory and makes corrections to the speed of the induced draft fan motor to maintain the constant air flow. The motor speed and motor torque are also monitored to ensure that they are within selected limits indicative of safe operation and responsive to this input energisation of a relay (KM1) is controlled to deenergise the gas valve and ignition.