My RV roof air conditioner cools for 15 minutes then just blows air. Why?

Dear Dave, 
I have a Dometic roof air conditioner. It puts out cold air for about 15 minutes then just blows air after that. I turn it off for about an hour and the whole process starts over again. —Dave, 2017 Winnebago Micro Minnie, 21 feet

Dear Dave,
From what I can find on walkaround videos, your Micro Minnie has a ducted roof air conditioning system with a wall thermostat. The first thing I would do is verify the temperature using an anemometer. You can find one on Amazon here.

This will tell the actual temperature at the thermostat to verify what it is reading. If you have a ceiling vent that is directly above the thermostat, it could get colder than the rest of the room. Once it hits the set temperature, it shuts off but the rest of the room is still warmer. I believe your thermostat has a fan mode and the high and low setting runs the fan all the time. Auto only runs the fan when the air conditioner or furnace is running. There is also an “off” setting.

Next, remove the return air cover and inspect the opening to see if there is any frost. There is a thermistor or temperature sensor that goes up into the evaporator coil. If it gets too cold, it will shut down and just blow air to try and defrost the unit. You should see a wire going up to the evaporator coil and at the end of it is a sensor very similar to the thermistor of a refrigerator. You should be able to test the temperature in that compartment with the anemometer as well.

Room temperature sensor

One other option could be a room temperature sensor. Some of the models have a remote sensor away from the thermostat that senses temperature. It could also be located directly below a vent and shutting off early. Knowing exact temperatures in these areas should help pinpoint the issue.

I sent a request to my Dometic tech contact asking if there was a way to test the thermostat by jumping wires. I got this from him:

When they are having this issue, if they turn the thermostat OFF, then tap and keep their finger on the UP arrow, the thermostat will display a temp reading. This is what the thermostat sensor detects (or thinks) the ambient temperature is around the thermostat (or wall temp). If it seems WAY off from what the customer thinks the area feels like temp-wise, it could be defective. The only way to really test accurately, though, would be with a laser temp gun pointed at the WALL right next to the thermostat and compare what the thermostat temp is stating.

 You might also enjoy this from Dave 

My RV’s Dometic roof air conditioner isn’t working. How do I check if the thermostat is dead?

Hey, Dave,
I was hooked up to shore power in my 2016 Airstream Interstate 24GT. While changing the mode at the Dometic 10-button A/C thermostat control panel, it suddenly lost power. The panel is completely dead. I checked the breakers and fuses in the distribution center and they were all good. I removed the thermostat to check for power to the unit with a multimeter, but it has a phone-type jack. The A/C is totally dead. Is it possible that the thermostat panel is blown, or is there somewhere else I can check breakers or fuses? Please help. Thanks. —James, 2016 Airstream Interstate 24GT

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

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