Air Conditioning Problem Found - Maybe

[scottz]

Background: I have been having a problem with the bedroom AC unit (occasionally the main room unit, but mostly the bedroom) when run in auto mode. It will occasionally shut off; I suspect it is freezing up (getting large amounts of water off the front roof). We would notice that the unit was not running (normal in auto mode) and it was getting warm in the room. Checking the temperature revealed that it was much warmer in the room than the thermostat set point. Turn the unit off/on and it would start running. When the unit runs, it runs perfect. The work-around is to leave the fan in high mode; the room stays at the proper temperature.

Dometic suggested I replace the thermostats; did that, no help.

I took it to a dealer, and after a couple hundred dollars testing, they told me they could not get the unit to shut down and that the inlet/outlet temperatures were within specs. Of course I told them that before ever taking it in; my fault for thinking they would be helpful.

I got around to looking at it a few days ago. I pulled the unit off the roof (easy, four bolts) to take a look. I was thinking I might find an issue in the roof plenum between the cold air side and the return side, but it looked good. Just for good measure I re-taped the ducts and was getting ready to put it back together when I noticed something. The bottom of the air conditioner is sealed to the roof with foam insulation; I suspect installed on the AC unit at the Dometic factory. There is a strip around the outside of the of both holes and another strip between the cold side and return side. Measuring the center strip, I found that Keystone did not center the AC unit over the cutouts. The center strip was 2" forward of the part of the roof it was supposed to seal against; leaving about a 3/4" opening where the cold air output could go directly into the return air duct (photos 1 & 2). The fix was simple, relocate the foam piece so it would seal against the roof (as opposed to moving the whole AC unit back 2") (photos 3 & 4).

After putting it all back together, I ran the unit for three days without a shutdown. I am not ready to call the problem solved because it is not very hot or humid here; time will tell. If it does continue shutting down, I'm ready to give up and run it in high mode all the time.