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CMunrock's avatar
CMunrock
Explorer
Aug 08, 2015

Why does my A/C fan on when I run the furnace?

My fifth wheel has two temp controllers. One has a digital display that controls my A/C and blower. This thermostat also controls my gas furnace.

THe other thermostat is the type that I've had on other RVs, with the manual slider on the top.

In order for my furnace to run, I have to put the digital thermostat on the 'heat' setting. The fan blows air thru the overhead ducts at the same time that the furnace fan blows warm air thru the floor vents and wall vents. Is this a normal setup?

I'm asking because both systems are pretty noisy and when I want to run the furnace, I don't want or need to run the overhead system fan, but it doesn't seem possible for me to run only the heating system fan.

Hope this makes sense. I'm accustomed to sliding the manual thermostat to the desired temp, and waiting for the furnace and fan to kick on and then shut off once the temp is reached. The overhead fan seems unnecessary and makes additional noise.
Seeking Advice

9 Replies

  • CMunrock's avatar
    CMunrock
    Explorer
    Aug 09, 2015
    Thanks all, very helpful. I visited the RV to take some pics and played around with the thermostats a little. There is no "auto" setting on the Dometic digital thermostat, so I put it on 'furnace' and the furnace kicked on, but the overhead fan did not, which is what I was hoping.
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    Aug 09, 2015
    C7XR7 wrote:
    No, this is not a "normal" set up.

    With the furnace you need to have the fan set to AUTO, this is because the heat exchanger in a gas unit needs to heat up to a certain temperature prior to engaging the fan so that it does not blow cold air on you. Secondly the gas will shut off when the thermostat is satisfied but the heat exchanger needs to cool to a certain temperature before the fan shuts off. Not the same for electric heat.

    Older (much older) style furnaces the propane fired off and then a temp switch turned fan on.

    NOT anymore........
    T-stat calls for heat
    Fan comes on to 'purge' combustion chamber AND makes up a 'sail switch' which then allows DC voltage to flow for circuit board to open gas valve/ignition occur.
    Fan continues to run after t-stat opens to cool heat exchanger down.
    Safety devices/operating scheme!



    CMunrock wrote:
    My fifth wheel has two temp controllers. One has a digital display that controls my A/C and blower. This thermostat also controls my gas furnace.

    The other thermostat is the type that I've had on other RVs, with the manual slider on the top.

    AS suggested by Chris Bryant....

    The 'other' t-stat most likely is for a roof vent....fantastic vent that is temp controlled.

    The digital t-stat would be for A/C Unit AND Furnace

    Set FAN to 'Auto' and then only furnace will operate when you 'heat' vs having A/C Unit Fan run also.
  • C7XR7's avatar
    C7XR7
    Explorer
    Aug 09, 2015
    No, this is not a "normal" set up. Let me ask this question; do you get heat from the overhead vents when the heat is on? If not then it isn't going to hurt anything for that fan to run but it probably isn't going to help much either. AC requires really good airflow while heat is less dependent on good airflow. You could run the AC with the fan in the ON position and the compressor would cycle and fan would keep the air moving in the home helping to keep it cooler and the air "mixed". With the furnace you need to have the fan set to AUTO, this is because the heat exchanger in a gas unit needs to heat up to a certain temperature prior to engaging the fan so that it does not blow cold air on you. Secondly the gas will shut off when the thermostat is satisfied but the heat exchanger needs to cool to a certain temperature before the fan shuts off. Not the same for electric heat.


    CMunrock wrote:
    My fifth wheel has two temp controllers. One has a digital display that controls my A/C and blower. This thermostat also controls my gas furnace.

    THe other thermostat is the type that I've had on other RVs, with the manual slider on the top.

    In order for my furnace to run, I have to put the digital thermostat on the 'heat' setting. The fan blows air thru the overhead ducts at the same time that the furnace fan blows warm air thru the floor vents and wall vents. Is this a normal setup?

    I'm asking because both systems are pretty noisy and when I want to run the furnace, I don't want or need to run the overhead system fan, but it doesn't seem possible for me to run only the heating system fan.

    Hope this makes sense. I'm accustomed to sliding the manual thermostat to the desired temp, and waiting for the furnace and fan to kick on and then shut off once the temp is reached. The overhead fan seems unnecessary and makes additional noise.
  • Uually's avatar
    Uually
    Explorer
    Aug 08, 2015
    Set your thermostat on fan . You should have a + - & auto setting. Try setting it on the auto and try that. Worked for me.
  • WyoTraveler's avatar
    WyoTraveler
    Explorer
    Aug 08, 2015
    If you have A/C heat pumps they will heat and blow air through the ceiling ducts. Newer RVs the heat pumps are interconnected with the furnace. When it is too cold for the heat pumps to maintain the set temp the furnace will come on. At times mine cycles back and forth between the A/C heat pumps and furnace. Not sure which system you have. Posting a picture of your thermostat would be helpful.
  • DrewE's avatar
    DrewE
    Explorer II
    Aug 08, 2015
    Is there a (possibly inoperative) heat strip on the air conditioner unit? Perhaps the heat setting for the digital thermostat is controlling that. More a stab in the dark than anything.
  • Chris_Bryant's avatar
    Chris_Bryant
    Explorer II
    Aug 08, 2015
    Don't know about the second thermostat, but to keep the a/c blower from running with the furnace, set the fan to auto.
    Second thermostat sounds like a roof vent/ventilator 'stat.
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    Aug 08, 2015
    Don't really understand why you have 2 thermostats....:H
    1 digital and 1 analog.

    Sounds like someone added the digital and didn't remove/disconnect analog


    FAN setting should be in AUTO position (not ON). In ON position A/C fan is turning on when t-stat calls for furnace.
    (A/C Fan can be turned ON by itself...furnace blower will only run when heat is called for)




    Would be nice to see picture of these 2 thermostats

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