Help! A/C Comes On When Heat Comes On


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.


  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 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.

  4. Thanks For This INFO!
    I’ve asked SEVERAL RV Dealers, and they had ZERO info on this!
    Like others said with their makes manual, our Minnie heater manual also did not cover this . . .
    gamgator2000 in Bishop, CA

  5. Thank you! Found ourselves in this situation on our 2nd night of our new-to-us Work and Play. You saved our weekend!! It worked!!

  6. Thank you so much for this! We shivered through the night for four days. I was prepared to go to the dealer tomorrow and tell them the air conditioner was defective. Your instructions were easy for me to understand and worked perfectly.

    I wish I had thought to do an internet search from the campsite.

    Thanks for the help.

  7. Thank you so much. I was stymied by the digital thermostat in my 2019 camper. The thermostat in my 5th wheel definitely does not work that way. Your hint saved me a lot of time!

  8. I’m new to RV! This was the greatest gift today! Learning that age 65 and divorced I can do it on my own, yes I can! Thank you so much!


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