Furnace (oil) wouldn't come on at any setting (fan works). Tried reset button on controller, no noise, nothing. Then noticed the white wire to one of the thermostat terminals was hanging loose, possibly from when serviced last Spring?

Just want to know if I can connect that wire safely (power switch off natch) and not create any other problems. Could there be any reason they left it loose on purpose? We do have air conditioning that is run by the same thermostat (old circular kind) and it ran fine all summer. But now we need heat. Does it make sense that the other things run but not heat with that one wire off? Thanks in advance.

  • Just to update, I reconnected the wire and the furnace is working just fine. Michael's explanation was spot on. Thanks again.
    – kbear
    Oct 19, 2017 at 3:38

1 Answer 1


The W terminal for a thermostat connection (normally also mated with a white wire) is the one used to initiate a "heat call" from the thermostat. If that wire was left open then it makes sense that the "cool call" and "fan" functions would have worked just fine during the summer months but that now heating does not.

It is possible that a furnace checkout person had opened the white wire so that they could manually trigger a heat call with a jumper lead during the testing and checkout of the furnace. This would have been done to prevent any surprises from the thermostat when service was in process.

My take is that it was an oversight to not have reconnected the white wire as the service call was wrapping up. You should have no problems connecting the wire back up when the furnace power is Off.

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