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Old 07-27-2016, 03:25 PM   #1
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What should I expect from AC Unit?

I have a one year old 25FB with a single Dometic 15,000 BTU unit. In full sun I will get about 78 degrees interior on a 90 degree day with 40% Humidity and all awnings out. Is this good or bad?
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Old 07-27-2016, 04:22 PM   #2
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I've been told that at most you will see a 20 degree drop in temperature between inside and out. Full sun, asphalt or concrete vs. grass, humidity and the like will affect your performance. A 12 degree drop is disappointing but may be about right. Also, a "hot" Airstream will have a lot of built up heat in the aluminum and wood structural elements that could take overnight to cool down. The AC in your Airstream is not like a car where you can expect to go from deadly heat to comfortable inside a half hour or so.
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Old 07-27-2016, 04:31 PM   #3
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That seems marginal to me. 90 and 40% isn't that bad in my area. I've never measured mine but we regularly camp in hotter temps and we like it COLD inside. You should see 15+ degrees difference between the intake temp and the cool outlet directly at the AC unit.
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Old 07-27-2016, 05:56 PM   #4
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The way to test an AC unit is to measure the return air temperature just before the inlet and the delivered air temperature at the out flow grill. The best an AC can do is 20 degrees, more likely 15 to 18 degrees. If you are getting that the unit is working as best it can.

People just believe AC is AC and forget the factors of unit size and load on the unit. When you first get into a hot car you will note that the delivered air temperature is quite high. As the AC cools the car the delivered air temperature appears to get cooler. That is only because it has brought the in let air temperature down at a faster rate than the sun can heat the ca
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Old 07-27-2016, 06:59 PM   #5
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What should I expect from AC Unit?

The accepted measure on an A/C unit's performance is known in the industry as 'delta-T' which is defined as the temperature differential between the cooled air being exhausted from the evaporator when directly compared to the air temp measured AT THE RETURN AIR GRILL.

Note that this has nothing to do with ambient temperatures measured outside the trailer.

The delta-T for a properly operating unit should be measured after the compressor has been operating for 15 minutes, and should show a differential between 18-22 degrees F.

If your unit fails to reach a delta-T of at least 18 degrees F. , then you have operational issues which could range from a bad start capacitor that will not allow the compressor to initiate and run to a refrigerant leak in your system.

Both issues are easily diagnosed by a competent and experienced RV tech.
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Old 07-27-2016, 08:43 PM   #6
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Quote:
Originally Posted by rocco52 View Post
I have a one year old 25FB with a single Dometic 15,000 BTU unit. In full sun I will get about 78 degrees interior on a 90 degree day with 40% Humidity and all awnings out. Is this good or bad?
My 2001 Safari 25 had an 11,000 BTU AC and only the curbside awning. In my driveway in east central Florida with full sun on the roof and road side and afternoon temperatures in the low 90s, it kept the temperature and humidity comfortable after 30 minutes or so of run time. It would get down to 74 or 75 as I recall. It had the "mouse fur" walls and padded vinyl ceilings. I think that provides some additional insulation over the bare aluminum skins.

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Old 07-28-2016, 11:54 AM   #7
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That seems a bit weak unless you waited until the trailer was heat soaked before turning on the AC. It makes a difference if you start the AC early in the day so it can try to keep up with the suns heating of the skin. A friend used to leave his AC on all night while bundled up in winter clothes so that it would be cool enough in the afternoon in his old 30 footer. He was at an RV park with no trees but electricity was included. You just can't beat parking in the shade in the summer! You can check to make sure that your AC has unrestricted airflow on both hot and cold sides, and check the delta T as suggested.
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Old 07-28-2016, 12:08 PM   #8
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My 2008 Classic 25fb has a Dometic 13.5k. This past weekend it was 96F and humid at the local lake. Trailer sat in front of the house with A/C before we left and it was 96F in there. When I got set up at the lake I turned on the A/C. Our A/C thermostat is located in the front bedroom and was set to 72F. Our site was in full sun and as the sun went down it blasted onto the rear (living area) windows. I deployed the rear and street awnings but not the main as it didn't provide any shade from the setting hot sun. Put the Fantastic Fans on for 10 minutes to vent hot air then closed them. I have a thermometer across from the fridge and it cooled down to the mid 80s. The next day with all awnings deployed the A/C was able to maintain upper 70s until late afternoon and it bumped up to the low 80s. The front bedroom cools down faster than the rear dinette area as cool air is channeled in the hallway to the bedroom. Through the day the A/C compressor cycled on/off until late afternoon. We turn Auto off to keep the A/C fan running when the compressor cycles off. I may try lowering the bedroom thermostat to a lower temperature to see if the rear cools down better but in this particular site the rear was getting all the afternoon sun. We have the mouse fur and vinyl ceilings too. Water was pouring out fast from the drain tube by the street wheel well, it was so humid. I feel the A/C is working the best it can.

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Old 07-28-2016, 02:35 PM   #9
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I wonder why noone has mentioned roof seal, window tinting or uva rated screens, curtains, etc. There's only so much a rooftop A/C unit can do to cool a hot metal box in full sun with 90+ temps. You may have to help it along. I'm in full sun in Tampa, Fl. Thise are my solutions for sweat free afternoons. Good luck!
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Old 07-28-2016, 04:43 PM   #10
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What you describe is why we sold our 25' Flying Cloud. When camping in NM in the summer, it would be 95-100+ and we could get the front living area after a long while down into a comfortable temperature, we could never get the rear bedroom cool. We ended up with a 30' with 2 A/C's and even with temperatures 100+ this summer in AZ and NM we could get the whole trailer quite chilly, even to the point where we had to turn off one of the A/C's as it got too cold. Awnings all the way around definitely help when there is no shade. The ducted air definitely helps with distribution and noise reduction.
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Old 07-28-2016, 05:55 PM   #11
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If one end of the trailer is cooler than the other, I find the use of a small fan to push the air around is prudent.
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Old 07-28-2016, 06:18 PM   #12
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OP has a 2015 and I would think the A/C would distribute better for even temperatures. If I could maintain 78 in the rear living area with full sun I'd be more than happy. Mines is the old style air distribution box in the middle of the trailer.

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Old 07-28-2016, 06:30 PM   #13
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Reading this post, we are sitting in full sun outside Santa Fe NM in our 2012 30 FC with two ACs (13.5 and 15). With the full sun and daytime highs mid 90s every camper with less than the max ACs his rig will carry is having trouble keeping cool. We talked to a couple with a 2011 30 FC, one 15 K AC unit, that couldn't get their unit cooled down during the mid day even using a strong fan blowing from the read bedroom. They left a week early the next day for cooler Colorado. My two ACs will keep us at the mid 70s selected if I start the units running early in the morning but by mid day they are pretty much running full time. Like stated above, awnings, shade of any kind makes tremendous difference. To get an idea of the heat you are fighting, feel both the outside and inside skin that is sitting in direct sunlight at mid afternoon.
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Old 07-28-2016, 07:37 PM   #14
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Thanks everyone for your input! I will run the test on my next outing and see if it is within specs. Even at 78 degrees it was nice to have a place to cool off after hikes. Being new at this I was not sure what to expect.
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Old 07-28-2016, 09:19 PM   #15
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Seems pretty poor if that's the lowest it will go.

As its a year old 25' is it ducted?
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Old 07-29-2016, 06:21 AM   #16
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These things are aluminum storage sheds on wheels, don't expect them to cool like your house. A gently blowing fan to circulate air, door kept closed as much as possible and minimal cooking producing as little heat and humidity as possible can keep it comfortable inside unless you are used to 72 degrees in your house. We are camping, not trying to duplicate our house.
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Old 07-29-2016, 06:31 AM   #17
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To build on Lewster's earlier comments, a delta-T of 18 degrees or so at the cold air outlet will not cool the trailer by the same amount since (especially in full sun) the trailer is continuing to absorb heat from the outside environment as you are trying to cool it. In bad outside temperature and humidity conditions, a single AC Airstream might only see a 15 degree drop in the inside temperature. And if you started with a "fully baked" Airstream it might actually take 12 hours or more to reach the full cooling potential.

If you want the ability to cool the inside of your trailer by more than 15 degrees in extreme conditions, you will need two ACs and even then, you'll have to be patient if you start out with an overheated trailer such as you get when towing all day in the sun on a 90 plus degree day.
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Old 07-29-2016, 06:43 AM   #18
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I have a one year old 25FB with a single Dometic 15,000 BTU unit. In full sun I will get about 78 degrees interior on a 90 degree day with 40% Humidity and all awnings out. Is this good or bad?
I don't know whether it's good or bad but it's the best you'll do with a single A/C under those circumstances. These trailers pick up a great deal of heat in the sun, even with awnings out, even with blinds closed.
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Old 07-30-2016, 10:29 AM   #19
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We were wondering if a second AC would be overkill for our 25'. It has been soooo hot this summer. Any thoughts?
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Old 07-30-2016, 11:25 AM   #20
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Is the 40% humidity outside or inside cooled?

Something I learned living in FL and replacing 4 house ac systems AND having a friend in the business is that ACs like humid air. As they draw in the hot humid they pump out the dry cool. If the room is dry and hot then an AC unit is challenged and will be less efficient. If your trailer is in direct sun and heating up the heat radiation could overwhelm any ac unit in with a cool dry interior. Consider if the sun is heating along the side or an end. Shade the side if needed.
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