dometic thermostat

[G and J]

On my second thermostat...same problems. We've had a couple of cool nights this September and when I try to adjust the furnace temperature, the A/C comes on. Display numbers jumping all over the place. Then removing the cover and reinstalling results with a couple of dashes on the display. Contacted Dometic via website support and their response was to offer another thermostat. To their credit, Dometic has been great with the support, but I want the darn thing to work properly. The cover is now removed and we operate the thermostat by pressing the metal tabs. I don't think the unit itself is faulty but rather the cover.

Does anyone have the HVAC electrical schematics for a 2018 315RLTS? I'm am leaning towards attempting replacing the CT thermostat and control with a different setup.

[klenger]

Does anyone have the HVAC electrical schematics for a 2018 315RLTS? I'm am leaning towards attempting replacing the CT thermostat and control with a different setup.

I have found a simple solution for installing a traditional analog residential T-Stat (Honeywell for example) to control the Dometic AC. I am in the process of putting together a web page with the details, but I lay out the high level details here. The Dometic CT T-Stat communicates with the AC control board in the roof-top unit over a simple proprietary digital bus, making it difficult to replace the T-Stat with an analog model. I have researched the options to replace the Dometic roof-top control board with an analog compatible version, but did not find a good solution. I found many forum posts with people successfully upgrading from the old mechanical analog T-Stat to the CT system by replacing the control board and T-stat as a set, but not going the other way.

In an effort to improve my furnace response, due to the poor location by GD of the CT T-Stat in the hutch wall above the furnace, I installed an additional Honeywell analog T-Stat on the side of the island and connected it to the furnace in lieu of the furnace control wires from the AC unit control board. The furnace is old school analog (+12 VDC to turn on the heat), so the analog T-Stat worked just fine for that.

What I came up with for fixing the AC side is kind of clever if I may so so myself. There are 3 wires between the AC control board and the CT T-Stat: +12 VDC power, Ground, and the digital communication bus. The key to my solution working was that the CT T-Stat has non-volatile memory, so it remembers the previous settings after the power is removed and reapplied. If you leave your T-Stat AC set to ON when you shut down your RV (by turning off the battery switch), the AC will immediately start working the next time you turn on the battery switch and apply external campground power. The CT T-Stat "wakes up" with all the previous settings. Here's the clever part. I took the cooling compressor output from the Honeywell analog T-Stat and connected it to the Dometic CT T-Stat +12 VDC power input in lieu of the +12 VDC power from the AC unit control board. Then I set the Dometic T-Stat temperature to Cool (auto, high, or low as you prefer) and set the temperature to a temperature lower than I expect to use (60° for example). Now I set the Honeywell analog T-Stat to cool with the temperature set to say, 70°. The Honeywell T-Stat will cycle on and off as necessary to control the temperature in the RV, and (drum roll please) the Dometic CT T-Stat simply functions as an analog to digital converter to control the AC cooling, but has no effect on the set temperature. Early testing of this (I did the mod while camping a few weeks ago) mod is very positive and the Honeywell T-Stat has settings to control the number of on/off cycles per hour.

I will follow up with a wiring diagram and additional details soon and will post this on my website at http://www.klenger.net/hvac.html

[klenger]

I have updated the information on my web site at my website at http://www.klenger.net/hvac.html

Please contact me with any questions or corrections.

[Boomer58]

I am hoping someone here can help me out. We are currently staying in our 2018 337RLS while parked next to our house. Long story but we our helping our daughter, sil, and 2 grandkids until they purchase another house. We arrived home last night and the temp was in the 40's. Turned the heat on ( which has worked fine ). knew I was getting low on propane but I was trying to use what was left in the one tank before I switched over to the other. All of the sudden the a/c starts running. I checked the first tank and it showed red on the regulator indicator. I switched over to the other tank and the a/c still ran. Around 3 am I shut everything off ( yes both cats were not happy and found their way under the blankets ). This morning I turned the furnace back on and the a/c started right up also. I shut the breakers off for both a/c units, the furnace seems to be cycling fine and we are warm again. Has this happened to any of you and what was the fix ? I have not read all of the posts on this thread but it seems that there is an issue with the Dometic thermostats. Thanks for any advice or help you can give.

[carnolddsm]

Couple of suggestions:

1. Remove the thermostat cover, it pops off. The original covers cause weird things to happen. You can still operate the system by touching the “prongs.” If this solves the problem contact Dometic customer service for a replacement cover.

2. Make sure the fan is set to auto. When the fan is in manual mode the AC fans will run, but the compressor should not run.

I am hoping someone here can help me out. We are currently staying in our 2018 337RLS while parked next to our house. Long story but we our helping our daughter, sil, and 2 grandkids until they purchase another house. We arrived home last night and the temp was in the 40's. Turned the heat on ( which has worked fine ). knew I was getting low on propane but I was trying to use what was left in the one tank before I switched over to the other. All of the sudden the a/c starts running. I checked the first tank and it showed red on the regulator indicator. I switched over to the other tank and the a/c still ran. Around 3 am I shut everything off ( yes both cats were not happy and found their way under the blankets ). This morning I turned the furnace back on and the a/c started right up also. I shut the breakers off for both a/c units, the furnace seems to be cycling fine and we are warm again. Has this happened to any of you and what was the fix ? I have not read all of the posts on this thread but it seems that there is an issue with the Dometic thermostats. Thanks for any advice or help you can give.

[Boomer58]

I shut everything off and turned it back on with fan on auto and ''Bingo'' it works like new. It worked fine with the fan on low then all of the sudden the A/C fan started running. Thank you for the help. That's what I like about this site, all the people willing to help. 2 weeks until Florida.