Check induced draft blower motor

What is going on

The induced draft blower is the motor that pulls flue gases through the heat exchanger and starts the whole ignition sequence. If it does not run, nothing else happens, so it is one of the first things to look at on a no-heat call. The test is two parts. First you check the motor windings with the power off, for continuity and for a short to the frame. Then you put power back on and see whether the motor is getting 115V during the ignition trial. Those two readings tell you whether the problem is the motor or something upstream of it.

Likely causes

• Open winding in the motor
• Winding shorted to ground
• No 115V to motor during ignition trial, pointing at the control or wiring
• Seized or dragging motor bearings that trip the motor or the control

Check in this order

1. Shut off power, remove the burner door and disconnect the motor leads
2. Check continuity between each pair of motor leads with an ohmmeter. An open pair means a bad winding
3. Check each lead to the motor frame. Any reading to ground means replace the motor
4. If windings are good, reconnect the wires and power the furnace
5. Call for heat and measure 115V at the motor terminals during the ignition trial. Be careful, this is live line voltage
6. If 115V is present and the motor does not run, replace the motor
7. If there is no 115V, work back to the control and harness. Reinstall the burner door and verify operation

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)