Check transformer and control circuit

What is going on

If the furnace is completely dead, no lights, no blower, the first question is whether the 24 VAC control power is there. This check answers it fast and tells you if the transformer is the problem. The transformer drops line voltage down to 24 VAC for the board and thermostat, and the power also has to pass the blower door interlock switch. So a dead furnace does not always mean a dead transformer. Take it step by step and you will know in a couple of minutes.

Likely causes

• Faulty transformer
• Open fuse
• Bad wiring or splice in the control circuit
• Open door interlock switch because the blower door is not seated
• Short in the thermostat wiring burning out the transformer

Check in this order

1. Remove the blower compartment door and disconnect the thermostat low voltage wires at the control terminals, so the thermostat wiring cannot load the transformer
2. With power on and the door interlock closed, measure between R and C. It must read 24 VAC
3. If there is no voltage, check the transformer primary at the line connections, fuse, splices and door interlock
4. If line voltage reaches the transformer primary but not the secondary, replace the transformer
5. Be careful, line voltage is live while you measure
6. If you replace the transformer, find out why it failed, such as a shorted thermostat wire
7. Reconnect the thermostat wires, reinstall the blower 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)