This is for all you guru's out there, and you know who you are.We pulled our 5-wheel from Washington State to Michigan to relocate. By the way, we had no problems what-so-ever. Once we got here to Michigan, we realized our A/C wasn't working. The fan and the compressor worked, but no cold air what-so-ever.Ordered a new A/C unit, and since it was newer then the one I had, I also ordered a new thermostat. As was stated in my research, there were no directions on installation or wiring of either unit. I was able to find a local company who sent a tech to install and wire both. The new thermostat has three "modes", fan, cool, and heat. I can set the fan speed to High, Low, or Auto while in the fan mode. The other two modes, I can set the temp. I desire.Now, here comes my problem.I have the fan speed set to Low, and according to the operating instructions, the fan should stay on Low with the compressor shutting off and on to regulate the temp. But, in the evening, night and morning, when the outside temp is somewhat cool, everything shuts off. Both the compressor and the fan. I must then put it in fan mode, and after running the fan for a time, I must put it back in Cool mode.Isn't the fan supposed to run continually? Why does everything shut off? If the coils are freezing up outside, isn't the compressor supposed to shut off and leave the fan running? It is really bad at night because we can't keep the A/C running because everything shuts off.Do I have a bad thermostat? Do I have a bad A/C unit?That is why I am turning to all of you, for some help solving this problem.Thanks in advance for any and all help.NWTraveler

Actually it sounds like it's working as it should. You set the temp with the thermostat on Auto/Cool, the A/C runs to get the ambient air to that temp, it then shuts off, when the ambient air gets above the thermostat setting the A/C turns back on and the cycle repeats. Auto means the fan will run until the A/C gets the ambient air to the temp set on the thermostat, then it shuts off.

That's just it. According to the operating instructions, when I set the fan on Low or High, the fan is supposed to run continually. It isn't.And, if we leave the A/C on while we are gone, it will shut off and never come back on again, until I run the fan while in Fan Mode, then turn it back to Cool.

I'm no expert on RV A/C systems. But, I think you're right that the fan should stay on if set to Low or High. The only time the fan should go on/off in concert with the compressor is when it is in the AUTO position. Since the AC cools and the fan runs, I would focus on the thermostat wiring both where it connects to the AC unit and where it connects to the thermostat. Hopefully, someone took pictures of the wires connected to the old T-Stat before disconnecting. If so, maybe try putting the old T-Stat back on and see if it does the same thing.

Hello, I have fought with thermostats too. It's sounds as if the thermostat and the AC unit are not compatible. For the stage of operation when the low speed fan and compressor shut down the control signal is not present at the AC to turn that function. However the problem could be almost anywhere. Especially check with the schematic drawing and be sure that there are enough conductors between the thermostat and the AC unit. It is important to verify all connections are good. If that doesn't help find a professional.

Sorry, I misread the original post. I missed the part where you have a switch position for just the fan.

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14 Replies

transamz9
Explorer
Aug 17, 2016
I agree with others that something is not compatible between the two. Ill give you something that is weird too and others probably have this feature on their too but my T-stat also has an auto mode for the heat and cool side too. When mine is in auto mode on the HVAC side it will do what ever it needs to keep the set temp. It's screwed up when the A/C is on upstairs and the furnace is running at the same time.:S

My wife has problems working the t-stat and sometimes turns the auto on. Sitting there watching tv or something and the rv gets cooled out and the air kicks off and then a few seconds later the furnace will fire. HAHA!!! Makes no sense to me. LOL!
Njmurvin
Explorer II
Aug 17, 2016
Probably best to call Dometic and make sure that T-stat is compatible with your roof top unit and control box.
momentum_rv
Explorer
Aug 17, 2016
Is there ALSO a place to set the fan speed while in "cool" mode? (not just "fan" mode)
fred42
Explorer
Aug 17, 2016
I have a new unit the same as yours. With the fan set on low and cool mode, the fan does continue to run after the compressor stops.

One odd thing unrelated to your situation. If the unit is off with the fan set on low, when you turn it on to cool mode, it will not start the fan until the temperature is hot enough to make the compressor come on. So, even though the fan is set to low, the unit starts in auto mode. Then it works as you would expect low fan to work after the compressor is started.
Pop-Pop_C
Explorer
Aug 17, 2016
Guess my question is, why do you wasn't the fan to run all the time?
We don't like the noise.
nwtraveler
Explorer
Aug 17, 2016
My first attempt at putting a link in a post.

copy and paste this https://www.amazon.com/Dometic-3313192-000-White-Single-Thermostat/dp/B00C9OEZK6/ref=sr_1_1?s=automotive&ie=UTF8&qid=1471458135&sr=1-1&keywords=rv+thermostat
fj12ryder
Explorer III
Aug 17, 2016
Link doesn't work.
nwtraveler
Explorer
Aug 17, 2016
Maybe I wasn't clear in the original post. My fault.

Our old thermostat was analog. It had a separate switch for fan operation (On and Auto) and another switch for Heat/Off/Cool, with a third switch to control fan speed (High and Low). The old thermostat had six wires going to it. Fan High, Fan Low, heat, cool, 12 volt hot, and a common. The new thermostat only has three wires, a common, a 12 volt+ and a 12 volt-. It is a Dometic (same at the A/C unit) and it is a digital with different Modes (Off, Fan, Cool, and Heat).

According to the operating instructions, once the Fan mode is selected, one can choose between High, Low, and Auto. When the fan mode is set to either High or Low, the fan is supposed to run continuously, no matter whether I choose Modes Cool or Heat. And that is where my problem lies. The fan shuts off with the compressor.

Everything on the new set-up is controlled through a controller box (new, with the new unit) mounted just below the A/C unit and accessible from inside the trailer. I have checked the wires and they seem to be hooked up correctly.

As for installing the old thermostat, it already got thrown out.

It is a single A/C unit, with a single zone thermostat.

Here is a link to the new thermostat https://www.amazon.com/Dometic-3313192-000-White-Single-Thermostat/dp/B00C9OEZK6/ref=sr_1_1?s=automotive&ie=UTF8&qid=1471455031&sr=1-1&keywords=RV+Thermostat

NWTraveler
sayoung
Explorer
Aug 17, 2016
Sounds to me without looking at wiring the tech put a modern tstat on that will not work the fan as the old stat did. Did the old AC/tstat have the same auto/low/hi setup ? You may be short a wire to the hi/lo fan relay.
fj12ryder
Explorer III
Aug 17, 2016
Sorry, I misread the original post. I missed the part where you have a switch position for just the fan.