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Old 09-23-2012, 01:17 PM   #1
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Dometic rooftop AC "Short Cycling"

All, I just purchased a brand new Dometic 457915 rooftop unit (with inside controls) a couple of months ago from an online retailer (RV Wholesaler). When I installed it and camped with it the first time in August, it seemed to do "pretty good" in the 90+ degree heat (brought the camper to maybe 65, probably closer to 70). I've never owned a pop-up, so I had no clue what to expect until I felt another small pop-up's AC... and you could "hang meat in it"... most everybody I talked to echoed this sentiment. We just went camping last weekend again, and even though it was only mid-80s outside, the camper was just barely cool (I'd guess mid-70s inside).

When I checked it out, I noticed the AC would come on full blast (compressor kicked in) when I first turned the unit on, and it was REALLY cold. However, after 5-10 minutes, it would cut off and just run the fan, even though in my opinion, it's not cold enough to justify the "High" and "Coldest" settings. After that the AC begins to short cycle... I'm not sure how often, but I'd guess the fan runs 75% of the time by itself, with the compressor kicked in the other 25% of the time. It's just not getting cool enough in the camper for me. However, it seems to do better at night... maybe because it doesn't need to cool as much?

I wrapped the thermostat wire in a cloth, thinking it could just be getting cold, but that didn't help. Also, there is NO water leaking out of the AC unit at all... no matter how long we leave it on all day. Any ideas? Thanks!
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Old 05-13-2013, 03:22 PM   #2
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Hello again... well, it's been nice and cool outside, so I haven't needed this post answered yet... but I took my pop-up camping this weekend and the same thing happened... turned into an ice box for just a couple of minutes, and then the AC started going to fan 75% of the time, then compressor/AC, then back to fan and so on. I popped the cover off and did not see any ice build-up on the fins. I also noticed that when we just had the fan on by itself (no AC), when we went to leave and I tilted the camper forward, water ran out of the AC unit itself, but never ran out during the whole time the fan was on (probably 2-3hrs).

Any ideas? It's about to be hot outside, and I got an itch to go campin! Thanks!!!
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Old 05-14-2013, 03:25 PM   #3
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I had the same situation. I went into the thermostat and found an adjustment screw to set when it would tell the compressor to come on. Sorry for the quick explanation but I only had a minute.
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Old 05-14-2013, 03:27 PM   #4
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Also sounds like your drain is plugged up above.
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Old 05-17-2013, 04:38 PM   #5
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Biggrigg - Was this on a Dometic Brisk Air? I haven't popped up the camper again yet, but all I recall seeing was a wire for the thermostat, no actual adjustment screw.

F1100turbo - Dumb question, but where the heck would the drain be at on this thing? Is there a drain plug on it? I searched the parts listing, but I wasn't able to find anything.

Thanks!
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Old 05-18-2013, 07:36 AM   #6
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Mine is a Brisk Air. What I did to fix mine might be what you shouldn't do but it worked for me. When you remove the filter you see a copper wire. It leads to the manual thermostat. To get to the manual thermostat you have to go into the box that houses the wires and that the control knobs are in. Once in the box follow that copper wire from behind the filter to the manual thermostat and remove the thermostat. Mine had an adjustment screw on the side.
I knew mine was not set correctly because when the temperature inside the camper was about 76 degrees, with the A/C off, I could turn the thermostat knob and it would not click until it got all the way to the coldest setting. After my adjustment it will now click (indicating that it is turning on) at about the middle of the thermostat setting. Try adjusting the termostat knob first. If it doesn't click until close to the coldest setting with the A/C off in a warm camper then this may help.
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Old 05-18-2013, 09:28 AM   #7
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Biggrigg, mine is a Brisk Air too, so that definitely gives me some hope. I don't see an issue pulling out the thermostat. How did you know which way to turn the adjustment screw? I'll do trial and error with the cover to the box off. This definitely sounds like what is going on with mine, as I agree... you have to crank the temperature almost all the way up to get it to cut on, and then it cuts off at probably around low 70s... definitely not cold enough for me.

If it'll ever stop raining, I'll pop it up and see what happens. Thanks!
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Old 05-18-2013, 10:46 AM   #8
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I had the same type thing happen on our Rockwood. It turned out to be that an ac vent was blowing on the thermostat. Just ensure you don't have a vent blowing cold air towards the thermostat. Mine was not blowing right at it but it was blowing toward the refrigerator and the thermostat was on the wall adjacent to it.
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Old 09-17-2013, 01:14 PM   #9
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Just an update on this thread, since I never like to leave one unfinished: I took my PUP into a Dometic dealer to have the AC serviced under warranty (still have another year on it). He cranked up the AC and stated that it never got warm on him, and that it ran fine for hours. I don't believe that at all, considering that I just took it back out this weekend in mid-80s weather and we were sweating half of the time (the other half of the time the compressor was kicked in and cooling properly). I believe he would walk into the PUP when the compressor was kicked in, and therefore he thought it was "always" kicked in. The dealer also charged me $150 to check it out, since there was "nothing wrong with it".

Here's where I'm at now: If I take it back to this guy and he states that this is "normal" for the rooftop unit to have the compressor kick in for 5 minutes or so, then cut back off for 5 minutes or so (leaving my PUP humid and warm when the compressor is NOT kicked in), then I may be out another $150 with nothing to show for it. I wanted to try bigrigg's suggestion, but I didn't want to void the warranty and I figured it would be easier just to let the warranty dealer fix it. I'll take any suggestions or comments on this, as I'm about to drop it back off over there tomorrow if I can't find another solution.

BTW, I don't have a wall thermostat, so that would not be part of the problem. I've also noticed water draining out of the AC, so I don't think it's plugged, and I've tried the AC on Low/Med/High and all of the temperature settings to ensure it' not "freezing up". Thanks!
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Old 09-17-2013, 01:42 PM   #10
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Sounds to me like:
1) the dealer wouldn't know if you worked on it or not
2) the warranty is worthless anyway

Open 'er up and give that screw a twist!
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