Help! A/C Comes On When Heat Comes On

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As it gets colder, one of the common questions asked in forums is, “I turned my furnace on and the heat came on, but why did the air conditioner (A/C) also come on?” The shortest answer is, your fan in the air conditioner is turned on, you have to turn it to “auto” so that it doesn’t come on. For a Dometic Single-Zone Thermostat, here’s how.

Here’s How

Thermostat showing Fan indicator

  1. Press the On / Off Mode button until Fan is selected
  2. Press the + or button so that the setting changes from “Hi” (High) or “Lo” (Low) to “Au” (Auto).
  3. Press the On / Off Mode button until Furnace is selected

In Dometic’s Words

For reference, here is the full Dometic Single-Zone Thermostat User Manual and this tidbit is on page #9:

Note: If additional indoor air circulation provided by the air conditioner is not desired during Furnace Mode of operation, select Au (AUTO) in the Fan Mode to shut the air conditioner fan off. If Lo (LOW) or Hi (HIGH) is selected the air conditioner fan will continue to operate on the selected speed.

24 COMMENTS

  1. Thankyou so much. Currently freezing my butt off in the trailer because I couldnt figure out how to run the heat without the ac on… thanks to this I fixed it. Bring on the heat

    • Just the fan. I actually think the idea is to suck in warm air from the ceiling where the air intake is and then redistribute it using the ceiling duct work. Meaning, it’s by design to do this.

      But most folks don’t like that it does it.

  2. Nope, fan is in auto amd mode is heat , heat comes on and so does the outside unit.not a heat pump , also i am getting 22 v at C and Y …at C and W i am getting 24.7v … it should be a bad tstat..but when i jump off at board with tstat in off mode.Y to C , heat also comes on with out side unit.
    May ne board and tstat because i am getting voltage coming back from tstat when i shouldnt. But when i take tstat out same responce.so when i pull the w wire.cooling comes on correct.if i pull Y wire heat comes on by itself.whats your thinking.andnits not j7st the fan the compressor is comming on.thanks .bp

  3. My ac comes on when the thermostat is on no matter which fan setting and with all zones in the off setting. I was hoping your recommendation would work but no such luck.

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