Community Forums

BuzzWolfAR · 11-04-2017, 10:35 PM

Either my air conditioner is not working correctly (contrary to what my dealer says) or I need another air conditioner (which would make the dealer very happy).

We have a total length of 34 feet, three slides (one large and two smaller), single pane glass windows. Insulation rating on the unit was pretty good.

We live in Arkansas so 90 degree weather is common. I have one AC unit (Dometic 15000 btu). When we get to a site and it is 90 degree, it will only cool to 86 in six hours.

Using an instant read thermometer I get about a 20 degree differential from the outside air, and I don't know how much air it circulates.

If it is 74 in the early morning and 74 in the unit, by late afternoon when it is 90 out, the unit is 81 or so.

Do you think the unit is working correctly and I need another AC or is the unit not cooling correctly. My dealer says everything is fine.

vsheetz · 11-04-2017, 10:40 PM

Not enough cooling from the one unit. Two minimum needed for such a unit, imo.

harleyjt · 11-04-2017, 10:44 PM

I would think a 34 ft RV with 3 slides needs 2 units. Especially in the south.
jt

saddlesore · 11-04-2017, 10:52 PM

Short answer... the more AC units the better,
In your case I would add another unit,
In our 40', single slide rv we have 2 15k's with arctic insulation and multi-pane windows (including the skylight in the shower....really would like to have one more AC unit if we stay south of I-10.

NvGuy · 11-04-2017, 11:44 PM

Yep, time for a second A/C We are at 35' and have a 15 and a 13.

BuzzWolfAR · 11-04-2017, 11:50 PM

Quote:

Originally Posted by NvGuy

Yep, time for a second A/C We are at 35' and have a 15 and a 13.

But you live in Nevada and I'm in northern Arkansas.

That said, your qualifications are impressive, so please respond again. I'm not in the hot south or high desert.

BuzzWolfAR
2014 F350 Diesel
2018 Cedar Creek 29ir

Walt Bennett · 11-05-2017, 03:39 AM

We've a '10 38' Montana that's white that has their "arctic" insulation package. All lighting is LED. We've cut double thick Mylar bubble insulation to fit all the windows and have reflective cushion plugs for the three vents (including the Fantastic Fan). With all lighting off, windows all blocked and no heat generating appliances running, we can get a 28 degree temperature drop with one 15k AC unit. I'm pretty certain that's the best anyone can expect from one of these units.

The problems are that: There is no RV that can have thick enough walls and ceilings for decent insulation and still have decent living space inside. The popular dark color schemes are totally against energy efficiency. Any lighting (including LED) or appliance will generate heat. This is all just math and science.

With the continuing increase in temperatures, you'll find that you'll be increasingly limited in the places and times you can go recreate as you'll need AC power to run two or maybe even three air conditioners. If you do any dry camping and get a second AC, you might also look for an inverter generator that can supply 50 amps as well.

bruceisla · 11-05-2017, 04:16 AM

We had a 37' coach with 2 A/C units and dual pane windows ... running one was not sufficient on a 90 degree day ... 2 was fine.

Delta -T is measured as the difference between the intake and the output of an A/C unit. The outside temp and humidity are relevant but you can disregard while measuring. Expect a difference of about 18-21 degrees.

Walt Bennett · 11-05-2017, 05:53 AM

Quote:

Originally Posted by bruceisla

Delta -T is measured as the difference between the intake and the output of an A/C unit. The outside temp and humidity are relevant but you can disregard while measuring. Expect a difference of about 18-21 degrees.

This is true, but does not reflect your coach, 5th wheel or trailer's overall efficiency. That will be shown by the outside vs. inside temp. That's where your outside color, where you're parked (shade vs. sun), added insulation and what you're doing inside all count. (And those double pane windows don't have enough air gap between the glass to add any appreciative R value.)

MikeD1 · 11-05-2017, 06:25 AM

One more data point: 35' with white roof, double pane windows, full wall slideout, arctic insulation. The rear AC was DOA. In hot weather over 90, the front unit struggled to keep up. We could get by with shades down, keeping the door closed as much as possible etc. It was a world of difference with the rear unit replaced. Can now keep it just about as cool as we want.

bruceisla · 11-05-2017, 06:38 AM

Quote:

Originally Posted by Walt Bennett

This is true, but does not reflect your coach, 5th wheel or trailer's overall efficiency. That will be shown by the outside vs. inside temp. That's where your outside color, where you're parked (shade vs. sun), added insulation and what you're doing inside all count. (And those double pane windows don't have enough air gap between the glass to add any appreciative R value.)

True. But, if you were trying to determine functionality of the unit, the Delta-T is measured.

All the other factors, as you said, are very important to reduce heat load.

Barlow46 · 11-05-2017, 07:55 AM

Copied from Airstream forum:
“The written specifications on both Dometic and RVP (Coleman) clearly state that a properly functioning RV air conditioning unit should display a 'delta T' (commonly known as the temperature differential between the cooled air measured at the duct or air outlet and the return air measured at the return air grill) should be 18*F-22*F.

This measurement is NOT dependent on the outside ambient air temperature. There are no absolute temperatures to measure. The 18*F to 22*F temperature differential is strictly related to the temperature of the air going into the evaporator and the cooled air coming out of the evaporator.

As you unit runs for any length of time, the inside temperature should be dropping as the air is cooling during this process, depending of course on your insulation factors, the outside ambient temperatures, the amount of solar heating the aluminum skin will receive from exposure to the sun and the size and capacity of your A/C unit.”

My opinion is that you need another A/C but at the same time, I don’t think yours is operating properly. Did you or dealer pull the inside shroud and check to see if the block off seal is fully intact and not allowing cool air to be pulled back into the system. Also, you could have a feed trunk to the outlets loose and discharging air into the ceiling. A 20 degree delta is not right if you are basing it on outside air temp. Delta should be measured where air enters filter and at discharge outlet.

sbleiweiss · 11-05-2017, 08:05 AM

Our 38 foot MH has two 13.5 units and they struggle to keep up on hot days. I think you are getting all you should expect from one 15.

BrianGlenn · 11-05-2017, 08:16 AM

On a previous 5ver my AC had the option to divert the cool air directly out the bottom of the AC and into the RV, avoiding the ceiling ducting. I found directing the flow of cool air into the roof ducting cooled the inside of the ceiling really well, not so much the inside of the coach.

Air flow was also greater directly out the bottom of the Dometic 15K - and noisier unfortunately.

I also installed a maxxair vent (fantastic fan) in the vent opening in the upstairs bedroom and drew out hot air while first cooling down the RV.

I choose to NOT install a 2nd low profile 12K unit (Penguin) - we preferred the cool night air from the maxxair powered vent (reversed to draw cool air into the RV). It was very quiet on low speed.

My thoughts,
Brian