Thursday, January 20, 2022


Ask Dave: What’s the weird clicking in RV’s roof AC when furnace starts?

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. Today he discusses what might be causing a clicking in the RV’s roof AC when the furnace starts.

Dear Dave,
I have a No Boundaries 16.8 with a Dometic AC, thermostat, and propane heater. I am getting this weird arrhythmic clicking that sounds like it’s coming from the roof where the AC unit is. It lasts for 5 or 10 seconds before the heater comes on. The clicking seems to continue after the heater starts for a few seconds but eventually stops. This happens when boondocking with 12v only. Not sure if [it would happen when] I’m plugged in. What gives? —Mark

Dear Mark,
I have heard of this before. However, when I was at Winnebago we used Coleman and never had an issue with it. So I contacted my Dometic rep and he suggested changing the differential temperature setting. Sounded a little like “tighten the spandex nut on the differential plate behind the quinzy housing”! Or add blinker fluid!

Thermostat operational manual

So I got a copy of the thermostat operational manual and, sure enough, it is a “thing.” According to my Dometic rep, the temperature differential setting is the difference in the temperature setting on your t/stat and ambient temperature. According to the manual, it is preset at 2 degrees. That means the furnace will start when the temperature drops 2 degrees below what you set on the t/stat.

He also indicated that when the differential is set at 1 degree, the relay in the roof air conditioner has a tendency to click several times thinking it is supposed to turn on the fan for better air circulation. Here is the procedure to reset the t/stat to 2 degrees:

Click to enlarge.

I would suggest setting the fan to auto and take a look at the differential setting. I would guess it is set to 1 degree, which seems to be a fairly common issue with that. If it is on 2, maybe try resetting to 1.

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1 month ago

That is interesting. Wasn’t aware some units ran the AC fan to circulate furnace heat. Love it when I learn something knew.

1 month ago
Reply to  Kyle

Yes, I just learned this too- thanks Dave!