So first of all, I've changed nothing: same thermostat and same everything.

I typically run my blower fan 24/7, but a few days ago it stopped working unless the AC is on. Which tells me that the motor itself is fine, the issue is in whatever controls the motor.

I've been trying to understand how it's possible that the fan won't run despite my thermostat settings. My question is: does the fan get started via a different mechanism when the AC is running vs. not?

I'm really hoping that this is something I can fix myself and not pay a specialist, and I really appreciate any help!


  • Does it work when the system is switched to heat? – ThreePhaseEel Aug 2 '19 at 4:15
  • Running the air handler without a/c on is not doing anything beneficial. Why would you want to do that? My thermostat does not even allow that. – Jim Stewart Aug 2 '19 at 20:57
  • Does it have a thermocouple to sense the air temperature, which in turn controls the blower speed? A Fanhandler is such an add-on. – Mike Waters Aug 2 '19 at 21:19
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    Haven't tried heat because it's hot outside, but that is a good question... Running the fan constantly keeps the air circulating, so I don't end up with hot and cold/humid pockets in different parts of the house. Also filters it out constantly... No I don't think I have that. – Vlad Kovryga Aug 3 '19 at 19:50

The contacts in the thermostat for the "fan on" are different than for the "fan auto". If you know what type of thermostat you have you should be able to find a wiring diagram and determine which wires to short out on the mounting block to simulate "fan on". In my thermostat, it's red and green but you'd have to verify that with yours. You could also remove the thermostat with the "fan on" in the "on" position and test for continuity between the contacts.

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  • Thank you! I will look for the wiring diagram and try to figure it out. Knowing that the contacts are different gives me the motivation to work on this. Much appreciated. – Vlad Kovryga Aug 3 '19 at 19:52

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