AC Humidity - Fiberglass RV


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Old 06-05-2021, 09:47 AM   #1
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Trailer: 2002 13 ft Scamp
North Dakota
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AC Humidity

Hi, everyone-

Hoping someone can help me troubleshoot here. I had my AC replaced maybe two years ago and it worked great, then I started having issues. (It's a Dometic.) Now when I turn it on, it pumps humidity into the camper, rather than removing it, and it's just miserable in there. At the moment, it's 67% humidity and 66 degrees. I know it's hot in the upper midwest and I know about what an AC can and can't do in those conditions, but this has been happening at much lower temperatures, and every time I turn it on, I just watch the humidity climb on the gauge. Yesterday, when I got set up, the gauge said 25% humidity and as I turned on the AC, I watched it climb to 60% as the interior cooled.

So. What I suspect is that it's not draining, but I can't seem to locate drain holes in the AC itself. Suggestions for what to try to fix this? Trouble shooting? Things to try? Things not to do? Has anyone dealt with this before? Any help would be appreciated--thank you!
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Old 06-05-2021, 11:10 AM   #2
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Name: bill
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If you are not seeing water run down the outside of the trailer, something is wrong.

Dometic manuals are on line. There is a drain pan on the bottom of the AC (outside) with drain hole(s). Water then just runs down the roof.
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Old 06-05-2021, 02:58 PM   #3
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Remember that when you say humidity, you mean relative humidity. It's relative to the temperature and because colder air can hold less water vapor, as the temperature drops, the relative humidity increases.

At 67% humidity and 66 degrees (F), the dew point is 55. Consider that at 94 degrees and only 29% relative humidity, the dew point is also 55! Thats fairly dry air and what most people consider comforable.

So it is perfectly normal to see the relative humidity go up when the temperature drops when the amount of water vapor in the air is the same.

I don't think there is a thing wrong with your A/C. If you still think there is then report in the outdoor RH and temp, and the inside values for a few hours from the time you turn on the A/C.

You could have a leak from the condensate but it would trickle in like rain water and not pump moist air in. If there is no leak in the roof then the water will eventually fill up whatever there is and then overflow to the side.

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Originally Posted by Karen B. View Post
... Yesterday, when I got set up, the gauge said 25% humidity and as I turned on the AC, I watched it climb to 60% as the interior cooled....
By the way, 25% RH is really low and not likely to be seen in many areas except in the driest part of the winter. I don't trust that gauge.
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Old 06-07-2021, 08:08 AM   #4
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Name: Alexander
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I know the window unit I use for my trailer, dehumidifies as it cools. Check with Dometic to see if yours is supposed to work the same way. If they say it does, then you have a problem.
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Old 06-07-2021, 08:52 AM   #5
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I know the window unit I use for my trailer, dehumidifies as it cools. ...
Yes, A/Cs dehumidify but a large roof unit on a small camper in colder weather does not dehu very well, and so the humidity goes up with the drop in temp. If OP will report on the RH when the outdoor temp is about 80 F then we will know if it is working OK or not. I still vote that it is OK.. and RH will be around 50 percent if the outdoor temp is in the 80's ... its just not going to produce a very low RH at 66 degrees.
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Old 06-07-2021, 09:06 AM   #6
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PS. Check the inside filter and clean as needed. If only two years old the outside coils should not be very dirty but check them also and clean if needed. Verify the compressor is coming on... the sound is usually pretty hard to miss and the lights might even dim for a second when the compressor comes on.
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Old 06-08-2021, 07:47 AM   #7
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North Dakota
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Yes--I suspect this is the problem. There should be condensation running and there isn't any. I haven't been able to locate the drain holes, so I'll be hunting up a taller ladder to get up there.


Quote:
Originally Posted by thrifty bill View Post
If you are not seeing water run down the outside of the trailer, something is wrong.

Dometic manuals are on line. There is a drain pan on the bottom of the AC (outside) with drain hole(s). Water then just runs down the roof.
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Old 06-08-2021, 07:50 AM   #8
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I did clean the filter inside, which didn't make much difference. I hosed off the outside coils, which also didn't make much difference. The compressor is coming on and I can feel the difference between when the compressor is on, and when the fan is on. So, at least I have eliminated some potential problem points. Onward to the more difficult! Thank you!

Quote:
Originally Posted by gordon2 View Post
PS. Check the inside filter and clean as needed. If only two years old the outside coils should not be very dirty but check them also and clean if needed. Verify the compressor is coming on... the sound is usually pretty hard to miss and the lights might even dim for a second when the compressor comes on.
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Old 06-08-2021, 11:27 AM   #9
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It may be too large of a unit for the camper and short cycles and as a result fails to dehumidify the air.
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Old 06-08-2021, 11:39 AM   #10
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Originally Posted by redbarron55 View Post
It may be too large of a unit for the camper and short cycles and as a result fails to dehumidify the air.
Thats the best answer so far.
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Old 06-08-2021, 11:49 AM   #11
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Quote:
Originally Posted by redbarron55 View Post
It may be too large of a unit for the camper and short cycles and as a result fails to dehumidify the air.
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Originally Posted by Darwin Maring View Post
Thats the best answer so far.
Quote:
Originally Posted by gordon2 View Post
Yes, A/Cs dehumidify but a large roof unit on a small camper in colder weather does not dehu very well, and so the humidity goes up with the drop in temp. If OP will report on the RH when the outdoor temp is about 80 F then we will know if it is working OK or not. I still vote that it is OK.. and RH will be around 50 percent if the outdoor temp is in the 80's ... its just not going to produce a very low RH at 66 degrees.
Best answerS so far...

Unless I missed it the OP still has not told us the outdoor temps. An oversized A/C (which they are because these campers are so much smaller than what the A/C is designed for), with a small temperature differential will not remove much moisture at all. But when it gets into the 80's the A/C should make it comfortable.

And I wonder why you are cooling to 66 degrees (F)... I would need a jacket at that temp!
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