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Thread: Old honeywell thermostat a/c fan won't turn off.

  1. #1

    Default Old honeywell thermostat a/c fan won't turn off.

    I have an old Honewell mercury thermostat with a a/c and heat back plate.
    It's been there maybe 20 years. I can turn the fan off and it still runs where it's on hear or cold. The heat is hot water baseboard. Is there a reset button on these old thermostats?

  2. #2
    DIY Senior Member Runs with bison's Avatar
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    Aug 2009
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    The only time I've had trouble with things not shutting off was with digital types with failing batteries. They would get really screwy when the battery was low.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    If the fan switch is in auto, does this still happen? In auto, I thought the device (furnace/boiler/a/c) controlled the fan. It may be wired wrong. For heat-a/c-fan control, it needs 4-wires. Essentially, hot, heat control, a/c control, fan control. The internal switches connect the hot to either heat or a/c control, depending on the switch position and the thermostat setting, and can also connect to the fan control line independently.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4

    Default I'm leaning towards a relay problem.

    I have a trane model twg025a 140b1. It's about 12 years old. The thermostat wires go into the unit. I suppose when I remove the side panel I'll see what's inside. I had a similar problem with my gas boiler and the thermostat wouldn't turn on the circulator pump. Turned out to be a relay
    on the side of my boiler. To bad the A/c isn't as easy to find.

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