Dometic AC stops working at night - Montana Owners Club

Andy Smith · 05-28-2021, 04:12 AM

Hi All,

I have a 2020 3855BR with two vented Dometic AC units, one over the bedroom, and one with a heat pump option over the living area (Not sure exactly what models they are). I have noticed sporadically over the past few nights here in SC that with the bedroom unit set at 70, it will run normally cycling on and off until late at night (after midnight), at which point I will wake up uncomfortably warm. The temps and humidity have been pretty high (90s during the day and 70s at night, 60-80% humidity). The thermostat is still set at 70, but when I check the actual temp, it's usually closer to 75 or 78. Turning the t-stat off for a minute and back on will make the system kick back in. This seems to ONLY happen at night to the bedroom AC. During the day I keep the AC in the bedroom at 77-80, and it always turns back on when the room hits the appropriate temperature. Looking for any recommendations on how to keep from waking up at night to cycle the unit. Thanks!

RMcNeal · 05-28-2021, 04:54 AM

Andy, it seems you are experiencing the same thing I and many others have had with the Dometic CT thermostats. Mine started just as you described. I also would have the LR unit run and never shut off, getting colder and colder.
The only fix we have found is to replace the CT thermostats with Micro Air Easy Touch thermostats.
https://www.microair.net/products/ea...32199143555156
They are pricey, but they do work. Search the forum here for Dometic CT thermostat or Capacitive Touch Thermostat and you'll see numerous threads.

Andy Smith · 05-28-2021, 05:05 AM

Thanks for your response. I'm not understanding why it would be necessary to replace with an entirely different thermostat. Shouldn't the system work properly with the one it came with, or at most indicate a failure with the dometic thermostat to be replaced with the same? I've searched the forums and am not finding anything about this particular issue, where the AC only stops working at night. My thought was that it may be related to humidity more so than the thermostat, but I'll see what others think.

Andy Smith · 05-28-2021, 05:23 AM

I would also like to add that the DometicThermostat that I have doesn't look like one that is interchangeable with the MicroAir one, as it does not seem to have the telephone cable style plug. Here are some screenshots. It re-initialized while I was taking off the cover. I've done this a few times and I'm not sure it makes a difference.

RMcNeal · 05-28-2021, 05:29 AM

I agree. Dometic and/or Keystone should step up and fix this issue. I have had control boards, thermostats and freeze sensors replaced under warranty and none fixed the issue. I think you'll find your issue is not as specific as "only at night". I thought the same thing at first.
I have seen mine do several distinct things.
1. Run continuously getting colder and not shut off no matter how cold it gets.
2. Shut off then never turn back on no matter how hot it gets.
3. Cycle on/off at intervals with fan running hi/low. On for 60 seconds, off for 4 seconds, back on for 60 seconds, etc. At each interval the fan may be on high or low. No rhyme or reason.
Dometic can't seem to figure it out and Keystone puts it back on Dometic. Keystone has gone to the Coleman AC's now. After numerous troubleshooting and replacing parts at random, I finally gave up and bought the Easy Touch thermostats last month. I hated having to spend $500 on new thermostats, but I hated even more that the AC was a constant concern and we could never be far away from the rig. We have 2 English Bulldogs and they can't handle heat well at all.
When I first noticed the problem in my 3791RD, it presented exactly as you stated. Turned off at night and not back on. Over time it went to off at random times and not back on without reset. On rare occasions it would run forever until you could see your breath, but that only happened in the living room. Every time I thought I found a common denominator, it would change. I am firmly convinced this system is female. Every time I think I figured it out, something would change. :-).
Here are a few threads that have info about the CT Thermostats:
https://www.montanaowners.com/forums...tic+Thermostat
https://www.montanaowners.com/forums...tic+Thermostat
https://www.montanaowners.com/forums...tic+Thermostat
https://www.montanaowners.com/forums...ght=Thermostat
https://www.montanaowners.com/forums...ght=thermostat
https://www.montanaowners.com/forums...ght=thermostat
That last one is my original thread a year ago. Since then, I finally gave up on replacing parts at random and just bought the Micro Air thermostats. Problem solved.

RMcNeal · 05-28-2021, 05:33 AM

The one in the pictures above is the exact same thermostats I had. I ran for several days with the faceplate off and thought it was fixed, but alas, the problems returned.
You can try that and see if it works for you for a while. There are several models on the Micro Air website. Just choose the one that matches your thermostat. If memory serves, mine was the 351.
Start here to find the one that's right for you...
https://www.microair.net/products/ea...56#CompatChart

JDDilly · 05-28-2021, 05:34 AM

That looks just like the one I had and I replaced with the MicroAir. I hope the link works, it is to a selection tool on the MircoAir site.

http://www.micro-air.com/kb-easytouc..._Do_I_Need.cfm

Andy Smith · 05-28-2021, 06:17 AM

Yep, link looks good, thanks all! I'm going to get up on the roof later today and just make sure there's nothing reducing airflow on the outside (already cleaned the internal ducting). I'm not so much concerned with the cost of the new thermostat (money is only useful if you use it to improve life!), and the features are great, just frustrated that the original is such junk. I'll see if cleaning the coils helps, and then if the wife approves the new t-stat.

phillyg · 05-28-2021, 07:20 AM

Mine have always been flaky (2016 3711FL). Does anyone know if I can replace just one to see if it really fixes the problem?

432bartman · 05-28-2021, 11:35 AM

The Micro Air thermostats are awesome. Ours would not run on Auto and the Micro Airs solved it. Another great feature is you can control it with your smart phone. We replaced both thermostats and eventually put in the Micro Air Easy Starts.

Harrisden54 · 05-28-2021, 01:10 PM

Similar issues here with the Dometic thermostats. Replaced both with Micro-Air and now I can sleep through the night without being wokeup by the bedroom A/C cyling on and off every 15-60 seconds.

RMcNeal · 05-29-2021, 04:36 AM

Quote:

Originally Posted by phillyg

Mine have always been flaky (2016 3711FL). Does anyone know if I can replace just one to see if it really fixes the problem?

Yes you can replace them one at a time. Gordon (GHallof) did just that. He told me that he intends to buy the second one when he can.

**scottz** · 05-29-2021, 11:58 AM

Quote:

Originally Posted by Andy Smith

Hi All,

I have a 2020 3855BR with two vented Dometic AC units, one over the bedroom, and one with a heat pump option over the living area (Not sure exactly what models they are). I have noticed sporadically over the past few nights here in SC that with the bedroom unit set at 70, it will run normally cycling on and off until late at night (after midnight), at which point I will wake up uncomfortably warm. The temps and humidity have been pretty high (90s during the day and 70s at night, 60-80% humidity). The thermostat is still set at 70, but when I check the actual temp, it's usually closer to 75 or 78. Turning the t-stat off for a minute and back on will make the system kick back in. This seems to ONLY happen at night to the bedroom AC. During the day I keep the AC in the bedroom at 77-80, and it always turns back on when the room hits the appropriate temperature. Looking for any recommendations on how to keep from waking up at night to cycle the unit. Thanks!

My rig is three years older than yours, but I had (have?) exactly the same problem. It is not my thermostats; they have been replaced two or three times.

You did not mention what fan mode you are using, but the simple solution for me is to leave the fans on high (not auto) all the time. The units never quit (freeze up in my opinion) in this mode.

You may also want to make sure your air flow is correct and your return air ducts are not crushed.

I may have corrected the problem, but not sure since it does not occur very often.

Search the forum, there is a lot of discussion on this issue. Here is one thread to start with.

I am amazed this is still a problem on 2020 rigs.

https://www.montanaowners.com/forums...ad.php?t=75245

Ram Montana High Country · 05-29-2021, 07:34 PM

Oh yes it is. My 2920 373RD has the bedroom tstat problem. Might agre with freezing but **** it off, turn it on, it works. I think the main unit might freeze up when outside temps are low but works fine when hot.

**scottz** · 05-29-2021, 09:10 PM

Quote:

Originally Posted by Ram Montana High Country

Oh yes it is. My 2920 373RD has the bedroom tstat problem. Might agre with freezing but **** it off, turn it on, it works. I think the main unit might freeze up when outside temps are low but works fine when hot.

My theory is that it has more to do with humidity. Mine also start working just by turning them off/on. I suspect that by the time I notice it getting warm, any freeze up issue would have melted. I think the units have a sensor that detects ice and shuts them down. I've had two dealers look at it and the Dometic pros at the national rally. No help because, of course, it never fails when they are looking at it.

This is a very frustrating problem for everyone. I'm tired of working on it. If I have the problem solved; great, if not, I'll run the fans on high.

RMcNeal · 05-30-2021, 06:32 AM

I tried the "run on high" fix some time ago. I found on mine that the fan would run constantly but the compressor would never kick back on. After a while it would only blow hot air around. After replacing both freeze sensors, one of the techs suggested removing the freeze sensors in case that was the problem. Ran without freeze sensors installed for a while and same problem. The unit was not freezing up. The compressor would shut down then never turn back on without a reset.

**scottz** · 05-30-2021, 08:07 AM

Quote:

Originally Posted by RMcNeal

I tried the "run on high" fix some time ago. I found on mine that the fan would run constantly but the compressor would never kick back on. After a while it would only blow hot air around. After replacing both freeze sensors, one of the techs suggested removing the freeze sensors in case that was the problem. Ran without freeze sensors installed for a while and same problem. The unit was not freezing up. The compressor would shut down then never turn back on without a reset.

Interesting; I've thought for a long time that these units have multiple problems. If I still have issues, my next step (after pulling my remaining hair out) will be to purchase Micro Air t-stats.

Andy Smith · 06-02-2021, 03:47 AM

Thanks Scottz! I had been running the fan on auto, we don't want it running constantly and burning anything out. I've checked the return ducts, and they're not obstructed. There's no icing issues that I can tell, and it always comes back on after resetting. After inspecting the coils, they look brand new.

I ended up running the fan on high the other night, by about midnight it was cool but I was sick of the fan constantly running so I switched back to auto, don't recall if it had any other issues. I'm waiting on a micro air thermostat to replace the dometic with to see if that helps.

RMcNeal · 06-02-2021, 04:59 AM

Andy, and anyone else interested, Chad from Changing Lanes did a really good video on changing out his thermostats for the Micro Air ones. After watching his video, I put both of mine in and got them updated over wifi in about 20 minutes. Really easy process.

https://www.youtube.com/watch?v=1UT2DZ1eacM

firestation12 · 06-02-2021, 01:30 PM

Quote:

Originally Posted by Andy Smith

Hi All,

I have a 2020 3855BR with two vented Dometic AC units, one over the bedroom, and one with a heat pump option over the living area (Not sure exactly what models they are). I have noticed sporadically over the past few nights here in SC that with the bedroom unit set at 70, it will run normally cycling on and off until late at night (after midnight), at which point I will wake up uncomfortably warm. The temps and humidity have been pretty high (90s during the day and 70s at night, 60-80% humidity). The thermostat is still set at 70, but when I check the actual temp, it's usually closer to 75 or 78. Turning the t-stat off for a minute and back on will make the system kick back in. This seems to ONLY happen at night to the bedroom AC. During the day I keep the AC in the bedroom at 77-80, and it always turns back on when the room hits the appropriate temperature. Looking for any recommendations on how to keep from waking up at night to cycle the unit. Thanks!

Try swapping the thermostats and see if the problem transfers to the living room a/c. If not, putting a new thermostat isn't going to give the result you are looking for.

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