How cold should the air conditioning get the RV?

[Bionic_Man]

We've had the trailer since new. It is about 32'. Air conditioning is original. It is the 15k model (Dometic?). I don't remember having problems in the past, but this year, the AC doesn't keep up as well as I would like.

Trailer was in full sun. I had all the blinds down. Not running any appliances, the AC would keep the inside of the trailer about 10* lower than the outside temp. So, at 95* outside, it was about 85* inside.

I have always heard that the air coming out the vents should be about 20* cooler than ambient indoor temp. I didn't check that, but we couldn't get anywhere near 20* cooler than outside temps.

Is this to be expected, and I just have a bad memory of how it worked in prior years? Is there a way to add coolant to the unit? Would replacing the unit make a significant difference? If it is up to me, we are keeping the trailer, so I am ok spending some $.

[smthbros]

mr_andyj wrote:
humidity is a very very small factor of the physics of heat exchange on the condenser part of the AC unit, but not enough to waste a sentence on. Humidity does matter as far as how cool will feel comfortable though. More humid = needs to be cooler.

All my campers have had one AC unit and have been more than enough to cool, even in 113 desert sun, though they ere not big spaces either.

So, what about humidity of the air across the evaporator?

[MFL]

I agree with above two posters. A properly installed, properly maintained 15K AC can cool a reasonable sized RV, yes, even more than 27'. Also many RVs are much better insulated than others. Just another reason you can't make a blanket statement, that is not credible! Also, the number of years a person does a particular job, is not as important as the tech's quality of workmanship.

Jerry

[mr_andyj]

humidity is a very very small factor of the physics of heat exchange on the condenser part of the AC unit, but not enough to waste a sentence on. Humidity does matter as far as how cool will feel comfortable though. More humid = needs to be cooler.

All my campers have had one AC unit and have been more than enough to cool, even in 113 desert sun, though they ere not big spaces either.

[ktmrfs]

dougrainer wrote:

MFL wrote:

whjco wrote:

dougrainer wrote:
1. 32 RV. A 15k will NOT cool the RV adequately in temps over 85 degrees. PERIOD. You need 2 AC units in RV's over 27 foot. Especially since Slide Outs are almost standard in Trailers. Do you have a slide out?

We have a Cherokee 274vfk that's just short of 34'with a super slide that has a single 15k BTU AC unit. We have hot humid summers here in Kentucky and I've had it in high humidity 98 degree weather parked in full sun in Alabama and I've never had any issues keeping the trailer very cool and comfortable.

Yup, once again, Doug has made too broad of a statement! All RVs are not created equal! My FW also cools great, with a single Dometic 15K AC, even in direct sun.

Jerry

Not making a Broad statement! I am giving you 41 years as an RV tech in HOT north Texas summers. Plus the scores of unhappy RV'ers that cannot keep their Longer than 27 foot RVs comfortable in the heat with 1 AC (even a 15k and ducted). Doug

I'd say that most larger single AC trailers with factory setup, AC is very marginal. However, my experience with my trailer and some other campers I've helped about $50 and a few hours work will take the AC from "almost adequate" to "adequate".

The ceiling plenum runs I've seen have big air gaps between the plenum and vents, meaning much of the cold air goes to cooling the ceiling. A bit of time with aluminum duct tape, problem solved.

Next they seldom install enough ceiling vents, and AC blowers aren't designed to push against much head pressure. So, next step is a 5 inch hole saw and roughly doubling the number of vents, again do a good job of sealing them up. In many cases the ceiling plenum runs in a straight line all the way to the end of the trailer. so it's easy to see where you can add a vent. just make sure the hole doesn't interfere with a ceiling joist.

Then open up the AC inside cover. You'll likely see lots of open spaces around the holes to the plenum again letting cold air go into the ceiling. tape those up so it is well sealed.

Then, I've found that the aireport vents are a direct replacement for many ceiling vents and provide less restriction and better air distribution than the factory vents.

All that makes a big difference. Still in hot weather don't expect the AC to cool the trailer down, but you likely will see it will hold a set temp that is comfortable. We can hold 74F even into near 100 if I set the AC to come on at 74. Wait till it's 78 inside, and it will just stay at 78, really can't cool it down.

Will a second AC be better? depends. most places we go camping either have only a 30A hookup or more often no hookups. So then without a BIG generator, practicality limits us to one AC unit.

If you have 50A access, another story.

[dougrainer]

MFL wrote:

whjco wrote:

dougrainer wrote:
1. 32 RV. A 15k will NOT cool the RV adequately in temps over 85 degrees. PERIOD. You need 2 AC units in RV's over 27 foot. Especially since Slide Outs are almost standard in Trailers. Do you have a slide out?

We have a Cherokee 274vfk that's just short of 34'with a super slide that has a single 15k BTU AC unit. We have hot humid summers here in Kentucky and I've had it in high humidity 98 degree weather parked in full sun in Alabama and I've never had any issues keeping the trailer very cool and comfortable.

Yup, once again, Doug has made too broad of a statement! All RVs are not created equal! My FW also cools great, with a single Dometic 15K AC, even in direct sun.

Jerry

Not making a Broad statement! I am giving you 41 years as an RV tech in HOT north Texas summers. Plus the scores of unhappy RV'ers that cannot keep their Longer than 27 foot RVs comfortable in the heat with 1 AC (even a 15k and ducted). Doug

[MFL]

whjco wrote:

dougrainer wrote:
1. 32 RV. A 15k will NOT cool the RV adequately in temps over 85 degrees. PERIOD. You need 2 AC units in RV's over 27 foot. Especially since Slide Outs are almost standard in Trailers. Do you have a slide out?

We have a Cherokee 274vfk that's just short of 34'with a super slide that has a single 15k BTU AC unit. We have hot humid summers here in Kentucky and I've had it in high humidity 98 degree weather parked in full sun in Alabama and I've never had any issues keeping the trailer very cool and comfortable.

Yup, once again, Doug has made too broad of a statement! All RVs are not created equal! My FW also cools great, with a single Dometic 15K AC, even in direct sun.

Jerry

[whjco]

dougrainer wrote:
1. 32 RV. A 15k will NOT cool the RV adequately in temps over 85 degrees. PERIOD. You need 2 AC units in RV's over 27 foot. Especially since Slide Outs are almost standard in Trailers. Do you have a slide out?

We have a Cherokee 274vfk that's just short of 34'with a super slide that has a single 15k BTU AC unit. We have hot humid summers here in Kentucky and I've had it in high humidity 98 degree weather parked in full sun in Alabama and I've never had any issues keeping the trailer very cool and comfortable.

[smthbros]

poppa wrote:

joshuajim wrote:
The other factor to consider is humidity. The higher it is the less cooling you will get.

less sensible cooling, still removing latent heat

Since you mentioned "sensible cooling" and "latent heat", and that you agree that the only correct way to evaluate the performance of an air conditioner is to measure the amp draw on the compressor and compare that to the expected amp draw for the temperature it is operating at, perhaps you can explain how to account for "latent heat" removal with that method?
As far as I have noticed on this forum, nobody else has been able to do so.

[ktmrfs]

time2roll wrote:
Analog meters have their place and work quite well actually.

We fought WW2 and went to the moon with analog equipment and slide rule measurements.

Digital just makes us lazy, not smarter. Why learn to navigate by the stars when you have GPS?
Never underestimate how smart our ancestors were.

agree that analog meters still have a place. I have a NIST traceable digital voltmeter (AC,DC, current, C, I etc.) and an old NIST traceable Simpson analog meter. While the digital meter is the one I grab most of the time. (It's more robust and less likely to be damaged by dropping or incorrect hookup) But there are times when an analog meter gives me a better qualitative picture of what's going on. I protect the old simpson with kid gloves and do NOT loan it out to anyone. The simpson is over 50 years old and if I was to damage it a replacement would be real hard to find.

[time2roll]

Analog meters have their place and work quite well actually.

We fought WW2 and went to the moon with analog equipment and slide rule measurements.

Digital just makes us lazy, not smarter. Why learn to navigate by the stars when you have GPS?
Never underestimate how smart our ancestors were.

[Weldon]

Bite the bullet and install another ac. My suggestion would be a 15 btu rather than 13.5.

[mich800]

dougrainer wrote:

mich800 wrote:

smthbros wrote:

Sunnysidebeach wrote:


One key point is; AC units need to cycle. That's why in a couple places we camped this summer, we had to raise the inside temp to 75 instead of 73 a couple days.

Why?

I think they may have that backwards. Short cycling is bad. It needs to have adequate run times to pull out the humidity. That is why too big a system can be bad.

AS it relates to Home systems, YES, having too large a system for a house, means the system does NOT RUN long enough to remove the humidity/moisture in the house. As it relates to RV's, it is a RARE RV that has too many roof AC's to overcool a RV. If the system is continually running(compressor) in hot weather, that is a good thing. It is doing its job. While it may be best to allow the Compressor to cycle, so you do not shorten its lifespan, that is the RV'ers choice. But there are Thousands of AC units that never shut off in temps above 85 degrees. BUT, having the correct amount of AC BTU's will allow the RV to cool down enough to cycle off. Doug

Thank you. Mine is the same. Runs constantly to keep up. Never had a need to cycle it off in the RV. And I agree, never been in an RV that was over spec'd on the AC.

[dougrainer]

mich800 wrote:

smthbros wrote:

Sunnysidebeach wrote:


One key point is; AC units need to cycle. That's why in a couple places we camped this summer, we had to raise the inside temp to 75 instead of 73 a couple days.

Why?

I think they may have that backwards. Short cycling is bad. It needs to have adequate run times to pull out the humidity. That is why too big a system can be bad.

AS it relates to Home systems, YES, having too large a system for a house, means the system does NOT RUN long enough to remove the humidity/moisture in the house. As it relates to RV's, it is a RARE RV that has too many roof AC's to overcool a RV. If the system is continually running(compressor) in hot weather, that is a good thing. It is doing its job. While it may be best to allow the Compressor to cycle, so you do not shorten its lifespan, that is the RV'ers choice. But there are Thousands of AC units that never shut off in temps above 85 degrees. BUT, having the correct amount of AC BTU's will allow the RV to cool down enough to cycle off. Doug

[mich800]

smthbros wrote:

Sunnysidebeach wrote:


One key point is; AC units need to cycle. That's why in a couple places we camped this summer, we had to raise the inside temp to 75 instead of 73 a couple days.

Why?

I think they may have that backwards. Short cycling is bad. It needs to have adequate run times to pull out the humidity. That is why too big a system can be bad.