How can my furnace exhaust fan run with the power off, the thermostat off, and the breaker off?!

  • If it's running under power, check your breakers again. If it's turning slowly, wind might be blowing in your exhaust vents. Check those two things. – Valkor May 16 '19 at 13:57
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    It can't. You're mistaken. Is there a local power switch near the furnace? – isherwood May 16 '19 at 14:49
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    Shut off (nicely) anything (like a desktop computer) that would mind and turn the MAIN breaker off. Still running, or no? – Ecnerwal May 16 '19 at 15:11
  • What kind of furnace is this? (Make/model would be helpful) – ThreePhaseEel May 17 '19 at 1:15

Some furnaces have more than one breaker

Well, most honestly.

  • The Air Conditioning unit has a breaker
  • If present, electric heating elements (e.g. emergency heat) have one or more breakers
  • The blower will have its own breaker, typically shared with the electronics on a gas furnace
  • A built-in humidifier or dehumidifer may have its own breaker
  • Depending on location, the 24V thermostat transformer may be on yet another breaker

Unrelated, the radon venting system has its own breaker.

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