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Page 1 INDUCED DRAFT BLOWER MOTORS PURPOSE; The induced draft blower assembly (fig.1) provides combustion air to the furnace burners. 80% furnaces do not need as powerful of a motor to drive the induced draft blower motor assembly as 90% efficient models. 80% efficient furnace models are draft furnaces and use a shaded pole motor. 90% efficient models use a PSC type of motor because the furnace must use the induced draft blower motor assembly to provide both fig. 1 combustion air and vent the products of combustion. The motor will take the furnace through a pre-purge cycle, trial for ignition cycle, burner cycle, post purge cycle and will be energized any time there is a call for heat. A TYPICAL INDUCED DRAFT MOTOR CIRCUIT In most cases, the motor is energized by the furnace IFC board. As you can see in this wiring diagram (fig.2) of a typical furnace induced draft motor circuit, the induced draft motor receives 115 volt potential on the IND H terminal of the IFC. The motor’s neutral lead is connected to the IND N terminal. There should be 115 volt potential present at these terminals anytime there is a call for heat. If the furnace is a two stage model, the induced draft motor will be a multispeed model with a low and high speed tap (Fig.3). The IFC will control the speed of the motor by either applying 115volts to the high speed tap or low speed tap depending on which stage of heat is being called for. fig. 3 fig. 2 Page 2 INDUCED DRAFT BLOWER MOTOR CHECK TOOL NEEDED; MULTIMETER PROCEDURE; SINGLE SPEED PSC MOTOR 1. Disconnect power to the furnace and make sure the induced draft motor is cooled down to room temperature before beginning. IF THE MOTOR IS HOT, THE MOTOR’S INTERNAL OVERLOAD MAY BE OPEN. ALLOW UP TO 2 HOURS FOR THE INTERNAL OVERLOAD TO RESET. 2. Unplug or disconnect the induced draft motor electrical terminals or leads from the motor. The lead terminals may be at the motor, IFC board, or relay. 3. Using an ohmmeter, check for resistance between the common and run Windings, and between common and start winding of the induced draft blower motor. You should measure resistance between each winding (fig.4). If you measure infinite resistance, (AFTER A REASONABLE COOL DOWN PERIOD) from both common to run and common to start windings, the internal overload is open and the motor must be replaced. fig. 4 4. Check the resistance from the run and start motor winding leads to the motor case (fig.5). You should measure infinite resistance. If you measure any amount of resistance, the motor is shorted to ground. Replace the motor. If all of the motor tests check out OK, proceed to the next step. 5. Connect the induced draft motor leads to their correct terminals on the relay or IFC board. fig. 5 Page 3 6. Using a voltmeter, check for 115 volts to the induced draft motor. (fig.6). If the furnace uses a relay to energize the induced draft motor, check for 115 volts at the relay terminals. If the furnace has a IFC board that controls the motor, check for 115 volts at the IFC board inducer motor terminals. (fig.7) If no voltage is present, check for any other safety circuit lockout conditions that may be preventing the IFC from allowing the fig. 6 induced draft motor from energizing. fig. 7