Troubleshooting a Low Voltage Short
Indications: A Blown Low Voltage Fuse or a Bad Transformer
1. Prepare to Diagnose Short
- Turn off line-voltage power to the air handler or furnace.
- Install a resettable low voltage breaker (if not already on the transformer).
- At the furnace or air handler, disconnect the low voltage wiring to the thermostat. (Leave all other low voltage wiring connected.)
- Turn line-voltage power back on.
- If low voltage breaker trips - go to Immediate Short.
- No short, proceed with Isolating the Short.
2a. Isolating the Short (Heat Pump)
Use a jumper wire to test each output wire at the air handler as follows:
- Connect R to G = Energize fan relay.
- No short - proceed to next test.
- A short would indicate a bad fan relay or control board.
- Connect R to O = Energize reversing valve.
- No short - proceed to next test.
- A short would indicate shorted wiring between the air handler & reversing valve or a shorted solenoid coil.
- Connect R to Y = Energize compressor. Wait until compressor contactor energizes.
- No short - proceed to next test.
- A short would indicate shorted wiring between the air handler & contactor or a shorted contactor coil.
- R to W = Energize heat (aux heat on heat pumps).
- No short - proceed to next test.
- A short could indicate a shorted limit, shorted heat contactor, shorted heat relays or sequencers, or miswired heaters.
- If no short, go to Test Thermostat.
2b. Isolating the Short (Furnace & A/C)
Use a jumper wire to test each output wire at the furnace or air handler as follows:
- Connect R to G = Energize fan relay.
- No short - proceed to next test.
- A short would indicate a bad fan relay or control board.
- Connect R to Y = Energize compressor. Wait until compressor contactor energizes.
- No short - proceed to next test.
- A short would indicate shorted wiring between the air handler & contactor or a shorted contactor coil.
- R to W = Energize heat.
- No short - proceed to next test.
- A short could indicate a shorted limit, shorted heat contactor, shorted heat relays or sequencers, or miswired heaters.
- If no short, go to Test Thermostat.
3. Test Thermostat (Heat Pump)
- Turn off line-voltage power to the air handler.
- At the air handler terminal connections, re-connect R and C to the thermostat.
- At the thermostat, set the System switch to "Off" and the Fan switch to "Auto".
- Turn on line-voltage power to the air handler.
- No short - proceed to next step.
- An immediate short indicates either a bad thermostat or shorted thermostat wire.
- At the thermostat, disconnect R.
- Start over at Step 1.
- No short = Bad thermostat.
- Short = Shorted stat wire.
- At the air handler, jump from R on the air handler to each thermostat wire from the stat:
- R on the air handler to G going to the stat.
- R on the air handler to O going to the stat.
- R on the air handler to Y going to the stat.
- R on the air handler to W going to the stat.
- R on the air handler to L going to the stat.
