Dometic rooftop AC "Short Cycling"

[heygreene]

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!

[heygreene]

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!!!

[biggrigg]

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.

[TURBS]

Also sounds like your drain is plugged up above.

[heygreene]

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!

[biggrigg]

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.

[heygreene]

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!

[dalford]

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.

[heygreene]

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!

[wrvond]

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!

[heygreene]

Thanks to Bigrigg's instructions, and Wrvond's encouragement, I've been "giving the screw a twist" for the last few days, while I play around with a few settings. I want to report back on my progress when all is said and done (I've even taken some pictures). However, right now I need to know the following: On the coldest setting, and on high, how cold does your AC keep your pop up? Right now I have mine down to around 62 degrees... which seems like it would be insanely cold, but once you're in there for a while, it almost seems like it could go down a few more degrees (I know, I know, my house is nowhere near that cold). Please let me know if you've ever tested with a thermometer what your AC keeps your PUP at on the coldest setting. Thanks!

[heygreene]

OK, so I've made some good progress... it's pretty much fixed. My only regret is not listening to Biggrigg prior to taking the PUP in for "warranty" work and paying $150 for them to do absolutely nothing. The one thing I didn't do was test the ambient temperature in the PUP prior to adjusting the screw, but I'd assume it was set to come on at 80, and cut off at around 75 at the coldest... it was very warm in there almost all the time.

So I took Biggrigg's advice and removed the thermostat. I found a Torx T8 star-shaped screw in the end of it. Adjusting this screw clockwise appears to make the ambient temperature threshold go down (get colder in the camper). I kept making random adjustments, but for now I've settled on 1.5 complete clockwise rotations, which moved me from around 75 degrees when the compressor finally kicked off, to around 65 when it kicks off now. One thing I've noticed is that if I remove the cover from the AC, it will get down to around 60 degrees, but when I put it back on it only gets down to around 65 degrees... I guess because of the airflow inside of the box hitting the thermostat. I might crank it down just a little more... I think I'd like to be at around 60 degrees when it cuts off... but maybe I'm getting greedy and just asking for it to freeze up. I'm open to opinions on this, and open to hear about what temperatures others are seeing in their PUPs.

To help others some, I attached some pictures. The wire leading to the round metal piece and the white block is the thermostat. The picture where the blue wires are coming in shows the T8 Torx, which is is on the right-hand side of the white thermostat housing.

Thanks again for the advice guys, I could NOT have done it without you!

[biggrigg]

I am glad you got it working.

[LSUTiger]

Heygreene,

Do you access this screw from inside unit or on outside?


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[heygreene]

Hey LSUTiger, you take the plastic outer shell off of the unit (that says Heat, Cool, etc), then take the metal cover off of the thermostat and wiring housing. Once you are inside of the housing, it looks like the pictures I posted above. You then have to remove the thermostat, and turn the Torx screen that I have pictured above. It took a lot of playing around with the screw to get it exactly right (number of rotations, counter vs clockwise, etc), but it cools great now. Ironically, Dometic told me these can't be adjusted when I called them! ;-)

Good luck, and let me know if you need anything else.

[LSUTiger]

Hey greens,

Did you have pull the whole control unit down to get to it or once you remove the main cover that has AC/Heat you can access the thermostat box with it still mounted to the ceiling?


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