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KATOOM's avatar
KATOOM
Explorer
Oct 23, 2015

HVAC thermostat wiring question

I'm trying to help a friend diagnose a non-functional AC and heater in their TT. Not a heat pump either.

Both components were working this summer running off a generator. At one time the generator circuit kept popping but at this stage I'm blaming that more on the generator than anything in the HVAC. I suspected possibly the thermostat first but when researching wiring for thermostats, I'm confused as to why I dont see a (C)common wire. :h A previous owner changed the factory thermostat over to a residential unit too.

Its difficult to find info regarding RV setups too as most is about residential unit. I was going to jump the hot lead to the heater fan and then the compressor in order to see if the thermostat was faulty but I'm leery since I dont see a common wire in the C location. Any advice? Thank you.

Here's a pic of what I'm looking at too.



  • Joe417's avatar
    Joe417
    Explorer
    Oct 23, 2015
    Not sure how yours should be hooked up but the paper work on the one I have in my home is by color. If the other end is wired per standard convention then you have black in the wrong location. Obviously green isn't connected where it belongs.

    Orange
    Black
    Yellow
    White
    Red Heat
    Red Cool
    Green

    For anyone to be sure, you would need to post the model of the ceiling unit.

    There are a lot of different combinations of wiring on the internet.

    Search on "rv thermostat wiring" to get more ideas how 12V thermostats wire.

    Also the reason you don't have or need the "C" or common connection is because it's a digital that is run off battery power and not a transformer from the HVAC unit.
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    Oct 23, 2015
    Need some info

    First off go back and 'EDIT' your post and make picture sizes no wider than 600
    It will keep pics on page.

    Now:
    What t-stat is this........Brand/Model (Hunter/Honeywell/Lux etc)

    What RV furnace..brand/model

    What A/C Unit...brand/model


    THEN how to wire it can be posted........otherwise shot in the dark

    'C' is not used
  • wnjj's avatar
    wnjj
    Explorer II
    Oct 23, 2015
    All the thermostat does is connect RH to white to run the heat or RC to yellow for cool. It also connects RC to green to run the fan. You should be able to short those pairs of terminals to bypass the thermostat. I'm assuming the wiring is fine since it used to work ok. The OP was asking how to bypass to test the heater and A/C.

    The only difference between residential and RV use is that residential uses 24V AC and RV uses 12V DC. The thermostat uses relays to connect "incoming" power (RH/RC) to whatever it needs to run (fan, heat, cool).
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    Oct 23, 2015
    Really need brand/model of RV Furnace and A/C Unit plus which t-stat that is.

    RVs use 12V DC

    Replacement 'digital' t-stats may or may not use the 12V DC positive.

    White is Furnace and goes to 'W'
    Yellow is A/C Compressor and goes to 'Y"
    Red is 12V DC positive.......may be used..maybe not DEPENDS
    Green can be ground and go to RC/RH
    Green can be High Fan and go to 'G'
    Blue can be Low Fan and go to 'G'
    Black can be Ground and go to RC/RH

    SO............what do you have?
  • KATOOM's avatar
    KATOOM
    Explorer
    Oct 23, 2015
    wnjj wrote:
    All the thermostat does is connect RH to white to run the heat or RC to yellow for cool. It also connects RC to green to run the fan. You should be able to short those pairs of terminals to bypass the thermostat. I'm assuming the wiring is fine since it used to work ok. The OP was asking how to bypass to test the heater and A/C.

    The only difference between residential and RV use is that residential uses 24V AC and RV uses 12V DC. The thermostat uses relays to connect "incoming" power (RH/RC) to whatever it needs to run (fan, heat, cool).


    Thats pretty much what I was looking for. Thank you.

    That said, I will try to deliver all the other information but just note that this is your "a typical" TT with probably a Suburban or Dometic AC unit on the roof and either one or the other for a heater down in the floor. Therefore I'm sure all the wiring you see in the picture (which for some reason fits in my screen) is what the trailer came with. How it was hooked up.....I can only guess that the wiring is in the same locations since everything worked. The only difference I see in the residential thermostat being used is there's no fan "low/high" switch, and thats more than likely where there unused green wire comes in.

    No matter, I'm going to check today with a test light for conductivity and see if I can get specific functions to run. Thank you again all. :B
  • Chris_Bryant's avatar
    Chris_Bryant
    Explorer II
    Oct 23, 2015
    This looks like a Coleman- they run the furnace directly from the thermostat, whereas DuoTherm runs it from the a/c circuit board.
    So, what I see is:
    Red and White from furnace- Red (+12 volt) goes to RH, White goes to W
    Red from rooftop air conditioner to RC (+12 volt)
    Y to Y (compressor, or cool)
    G to black, which should be high or low fan.

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