Tuesday, October 3, 2023


Dometic A/C keeps changing fan speeds. Why?

Dear Dave,
My RV’s main Dometic A/C unit will sometimes keep changing fan speeds. It sounds like the compressor hesitates and starts with the fan speed being higher or lower. This will continue for some time, or I can raise the thermostat temp to stop it. Is this normal or could it be a bad thermostat? The internet has all kinds of misinformation. —Carl, Keystone Cougar MKS333

Dear Carl,
You did not give the year or model, but I believe this is a normal operation for a 15,000 BTU roof air with a wall thermostat and zones. I posed the question to my contact at Dometic and here is what he stated:

Depending on the controls of the A/C, this is likely operating normally. Below is how our single zone 3 button thermostat would act in the cool mode setting…      

In the cool mode, the system will cycle the compressor on and off based on the room air temperature and the temperature set-point on the thermostat (max 90º min 55º). The fan will turn on first followed by a delay of approximately two minutes before engaging the compressor.

LOW / HIGH: The fan operates continuously at speed. The compressor cycles on and off.

AUTO: When auto fan is selected, the fan speed will vary depending on the difference between the temperature set-point and the room air temperature. In auto fan mode, the compressor and the fan will cycle on and off with the thermostat.

When the difference is: Greater than 5º, the fan operates on HIGH.

When the difference is: Less than 5º, the fan operates on LOW.

Example: If the actual room temp is 80 degrees and the thermostat is set to COOL mode, AUTO fan, with a set point of 70 degrees, we have a difference of 10 degrees and the fan will automatically run on HIGH. As the temp at the sensor begins to drop and gets to around 75 degrees, the fan will automatically switch to LOW and continue to run at that speed until the unit is satisfied or the sensor temp begins to rise. In that case, the fan would switch back to HIGH.

One thing you might try is installing the SoftStartRV product, which will reduce the initial amp draw and should quiet it a little. I don’t think it will do much for the cycling, but it will make it a little more tolerable.

 You might also enjoy this from Dave 

What does an E7 error code mean on Dometic roof A/C?

Dear Dave,
The front Dometic air conditioner stopped working on my 2008 Airstream Classic 34’ and is showing an E7 error code. I can reset the breaker and turn the A/C on and it will run for a minute, but then it trips the breaker and shows the E7 error again. I am full-time RV living and cannot find any mobile service help in Melbourne, Florida. Thanks. —Larry

Read Dave’s answer.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here


We have a popular forum for Ask Dave. Please be as brief as possible. Attach a photo or two if it might help Dave with his response. Click to visit Dave’s forum. Or send your inquiries to him using the form below.

Click or drag a file to this area to upload.


Dave Solberg
Dave Solberghttp://www.rv-seminars.com/
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.


5 4 votes
Article Rating
Subscribe to comments
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments
Bob P
2 months ago

To Carl, I had a 2020 Mesa Ridge 23RLS with one 15000 BTU A/C that did the same thing.I thought it was defective and did several diagnostics until I finally broke down and read the manual and found it was doing exactly what it was supposed to do. That was the difference in technology from my previous 2002 motorhome and the 2020 TT. Technology is great but I had 18 years between A/C’s to catch up on.

2 months ago

To the original poster:

If YOU do not know what the problem is or lack there-of a problem how can you say the internet is “Full of Misinformation” concerning your question?

Sign up for the

RVtravel Newsletter

Sign up and receive 3 FREE RV Checklists: Set-Up, Take-Down and Packing List.