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pjw73nh's avatar
pjw73nh
Explorer
May 26, 2013

Dometic Duo therm and furnace come on at same time

Greetings

My Dometic duo therm thermostat has malfunctioned.  I believe. 

The model is the square one with the 2 digit LCD it does fan cool and heat. I tried to put a link here but it didn't work.

 We are out this weekend and it is unseasonably cold here in the northeast.  I turned the furnace on via the tstat and the air conditioner fan started.   Maybe the compressor too, but it was pretty cold outside.  The furnace waited its usual delay, then purged, and fired up just fine.  The only way I could get the air conditioner turned off was to turn the circuit breake for it off

So, in short, when I use my furnace, the air conditioner comes on too.  The trailer is a 2011 bullet premier.  I tried to find the manual for the tstat/ac but it's not with the rest of my manuals.   I am sure I can find one online but I currently have very limited Internet access while on the road.  

If I had my druthers I'd like to get two separate, mechanical tstats one for each system.  I am wondering if this can be done given the electronic involved.   Are all the electronics in the tstat?  Or is there another controller someplace?

Any help or advice welcome.  Thanks.  
  • Is that a Heat Pump A/C?
    If so. There should be 2 settings for Electric heat and gas heat.
    Maybe you did not move slide switch far enough over to gas heat.
  • No, no heat pump. No slide switch either. All solid state/ digital.
    I am having issues uploading a picture. This may work.

    http://tinyurl.com/phacmtd. Picture

    Thanks.
  • Never mind. Once I found the manual online, it was a setting that got inadvertently changed. Thanks anyway. Maybe this will help someone else.

    Thermostat Manual PDF
  • pjw73nh wrote:
    Never mind. Once I found the manual online, it was a setting that got inadvertently changed. Thanks anyway. Maybe this will help someone else.

    http://www.primetimerv.com/download/ComponentManuals/Thermostat-SingleZoneDigital-Dometic-OperatingInstructions.pdf


    AMAZING what reading the owners manual can do:B Just state the obvious.
    On the NEW Dometic LCD Tstat you MUST have the Fan set to AUTO to keep the roof AC fan from running in furnace mode. If set to Lo or Hi, the roof AC fan will run on furnace setting. Doug
  • That seem realy dumb to have to set a t-stat that way, looks like many problems down the road. I would have thought that something wrong with stat, been around this for years never saw a stat that would bring both on if not set correctly, you most likely get nothing in that case. Always something new.
  • The manual should have been given to me with the rest of the manuals for my trailer when I purchased it. I shouldn't have to go looking online for it when out camping on a holiday weekend, in an area with little or no internet access, and no tech support at the dealer or manufacturer.

    The manual is not necessarily WRONG, but is poorly written IMO.

    Thermostat Manual

    See page 9.

    “Furnace” - Furnace Mode
    In this mode there are 3 fan speed selections:
    Lo - (LOW): The fan operates continuously at low speed.
    Hi - (HIGH): The fan operates continuously at high speed.
    Au - (AUTO): The fan will be OFF.

    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.


    Though I have no experience with heat pumps or heat strips in RVs, my presumption is that this thermostat can also be used with AC units that have the heat strip built in to them, thus the thermostat could very well be used to control the speed on the fan as it uses the AC fan for air handling when using the heat strip.

    This is not my case nor many that I have owned. I have, and always have had furnaces and AC units that were separate from each, and they either had separate controls or were CLEARLY marked with a physical heat/cool switch.

    Taken by itself, patently the first paragraph is flat out untrue. My furnace a ducted Atwood model with a single speed fan. Reading through the second paragraph (again poorly written), I was able to figure out the issue.

    My version of the second paragraph would be placed ON TOP of the first one and would read:

    Note: "Some RVs have a heat pump, heat strip and/or furnace. How the thermostat behaves will depend on which heat source you have.

    For external furnaces, in "Furnace Mode", the default setting for the fan is Au (AUTO). This means that AC unit fan will not come on when the furnace comes on. If additional indoor air circulation is desired during "Furnace Mode", by selecting either Lo (LOW) or Hi (HIGH) the air conditioner fan will come on at the selected speed."


    If I knew about heat pumps or strips, I would include that info as well, but I don't have that knowledge.

    Best,

    P.../NH
  • dougrainer wrote:
    pjw73nh wrote:
    Never mind. Once I found the manual online, it was a setting that got inadvertently changed. Thanks anyway. Maybe this will help someone else.

    http://www.primetimerv.com/download/ComponentManuals/Thermostat-SingleZoneDigital-Dometic-OperatingInstructions.pdf


    AMAZING what reading the owners manual can do:B Just state the obvious.
    On the NEW Dometic LCD Tstat you MUST have the Fan set to AUTO to keep the roof AC fan from running in furnace mode. If set to Lo or Hi, the roof AC fan will run on furnace setting. Doug


    It sure helped me. I was camping in Canada one chilly morning and had this happen. I should have checked the manual myself but insted I checked this forum. Thanks Guys/Ladies. I am warm now