Observations on Air Conditioner Function

GulfCoast

We have not yet camped with air conditioning in the 2017 T@G we bought last fall. Our July North Carolina trip to escape the coastal heat and humidity had to be cancelled. So, to satisfy my curiosity and alleviate my frustration about not being in the mountains, I've been tinkering with the air conditioner and running it under various control conditions. The objective is to understand they system better and figure out how to get the best performance from it. I'll offer a few observations from what I've seen.

Afternoon conditions here on the Mississippi Coast during the past two week have typically been 91 - 92 F and 6-65% RH. It's been dry here. The T&G is parked beneath a high, open carport and it's shaded from direct solar gain. For each operating point, I allowed the interior temperature and humidity to stabilize, usually requiring an hour between changes. In some cases, multiple runs were made with the same settings. No lights or electronics were on inside and there were no occupants. I'm not aware that the compressor ever cycled off during any of the runs; it was always running when I checked. Condensate was collected in a basin below the camper, but not measured.

• I began with one window open about one inch, the overhead fan cover opened about 3/4", and the 12V air conditioner condenser vent fans on. I set the air conditioner to the high cool mode. Interior temperature pulled down to the mid-70s with humidity in the mid-80s. Cooler, but not very comfortable. Disappointing. A slight improvement was obtained by removing the rolled up socks I put in the cowbells to keep insects and dust out during storage. (Checklist updated.)

• Several progressive reductions in window and fan vent openings substantially improved interior temperature and humidity. Condensate production declined noticeably, since less humid outdoor air was being cooled.

• The best performance was achieved with the windows and fan vent completely closed. The air conditioner consistently cooled the interior to 25 to 26 degrees below the outdoor ambient, stabilizing at 66 - 68 F and 55-65% RH at about 92 F and 65% outside. Very nice. (Air at 65F/65%RH contains about 13% as much moisture per pound as air at 92F/65% RH.)

• Taping the two vents on the sides of the air conditioner cabinet to completely block airflow reduced the interior temperature by about two degrees and reduced humidity by a couple of percent RH. Less cool air was being drawn out of the interior and being replaced by leakage with hot, humid outdoor air.

• Switching the 12V condenser vent fans off caused the interior temperature to climb a degree or two with no noticeable effect on humidity. The comparatively more powerful air conditioner condenser fan is responsible for most of the flow through the air conditioner condenser chamber.

• After the interior was initially cooled, switching from high cooling to low cooling made only a small difference in the interior conditions. Temperature increased a degree or two, while humidity declined by a percent or two.

Disclaimer: I realize that this is not a scientifically rigorous evaluation - varying outdoor conditions, basic instruments, ... Having a significant direct solar load might change things. Also, I suspect that there are great differences between T&Gs in how well the air conditioner cabinet is sealed and that might affect things strongly. Two adult humans releasing water by respiration into the interior may have a significant effect on interior humidity.

I'm curious about what problems and solutions others have found with their systems. I know some folks have had problems with high humidity while running the air conditioner. Does that happen only when your in the camper? With windows closed or cracked open? Has anyone else tried taping the air vents to the air conditioner cabinet? How has the system behaved during prolonged foggy or rainy weather?

The_Rigger

I'm not at all surprised that you found the greatest reduction in RH% with the windows & vent fan completely closed; that's the equivalent of having your car's A/C set on full-recirculating mode, which prevents (or at least reduces) the addition of any humidity from the outside air being drawn into the camper.

I would suspect that if your T@G was parked in open sunshine for testing instead of in the shade of your carport, your interior air temperature would be higher but the RH% would remain the same. By what percentage the temps would increase is, of course, an open question.

Good post, though. Lotsa food for thought.

GulfCoast

Quite a few T@G owners have posted here and on other forums that having a vent and/or window cracked open is required for the air conditioner to work properly, particularly for humidity control. I agree that it's not logical, and the runs I did confirm that cooling is better with windows and vents closed.

I'm curious about the explanation for their experiences. Perhaps, the the air conditioner isn't properly sealed inside the cabinet? Blockage in the airflow from the cowbells?

Sharon_is_SAM

@GulfCoast - if I remember, the A/C is oversized for the TaG. It will quickly cool down and turn off before dehumidifying. I think venting helps to circulate the air in the small area and may allow for the A/C to cycle for longer periods. I will try to find the discussion.

Sharon_is_SAM

Yep. See the final comment on page 2 of this link: https://teardrop-trailers.vanillacommunity.com/discussion/680/a-c-settings-for-t-g/p1

GulfCoast

That discussion is mostly related to the direction of flow for the three fans that help cool the air conditioner condenser. In through the cowbells and out through the vent on the bottom.

Air conditioners dehumidify the interior air only when the evaporator is below the dewpoint temperature of the air. If the system is oversized, the compressor cycles more often and the evaporator doesn't stay cool enough to condense moisture from the air. The air conditioner does more sensible cooling (temperature reduction) and less latent cooling (moisture removal).

If oversizing is the issue, we can effectively downsize the air conditioner by switching to to the low cool setting, forcing the unit to run longer and increasing the latent cooling fraction.

Introducing more outside air would help dehumidification only if the outdoor air has less moisture than the interior air, as with high outdoor temperature and lower humidity. That would be more likely in the Southwest US than in the Southeastern US. Outside ventilation might also be beneficial if there was a significant latent load (moisture source) in the camper - say two adults and a couple of dogs.

Real world conditions, such as leaky ducts and seals can throw a wrench into the science, though. I'd be interested in what people have experienced, particularly if they have some measured inside and outside temperature and humidity figures, or just good guesses.

I'm also curious about why NuCamp put the vents in the inside of the camper to let air move from the conditioned space to the air condenser condenser plenum. Anyone have any thoughts?

The_Rigger

"I'm also curious about why NuCamp put the vents in the inside of the camper to let air move from the conditioned space to the air condenser condenser plenum. Anyone have any thoughts?"

Strictly a SWAG here, but maybe to draw humid interior air out of the interior, to be replaced more quickly by drier, conditioned air? Sort of equivalent to running your car's A/C in non-recirculating mode?

Hell, I dunno...

GulfCoast

Good thought. But the interior air has already been cooled, and it's being replaced by outdoor air that leaks into the cabin to replace what flowed out.

I'm at a loss to figure out the design rationale behind those vents.

ras

The ac fan, cowbells and under trailer vent are in place to exhaust heat from the area behind the air conditioner since it's not hanging out a window as it does in homes. I'm sure that closing the overhead fan cover and the windows significantly speeds cooling but once you add actual breathing bodies to the equation, we crack the windows and run the fan to provide air exchange and prevent condensation.

danp

My wife does not like white noise or the sound of the AC, I am kinda screwed but i told her to wear ear plugs becasue there is no way I an sweating all night. AC is loud but what can you do? Anyone thought about adding more insulation around the AC box? Did it help?

We try running it a while before sleeping but two pair of lungs breathing heats it up in a heartbeat. She is wearing earplugs and I am a butthead

GulfCoast

@ras, so you've found it necessary to run the overhead vent fan while using the air conditioner, nut just open a window and the vent?

Did you also try running the air conditioner on the low cool setting?

ras

Yes, I run the fan too. But at a very low (and quiet) speed. I've added the PWM fan speed controller. (Info available on this site, Inexpensive and easy to install) We've never had a problem with condensation using this method. Very often, we end up running on low and even shutting the AC off after a few hours but maintaining the venting. I also have the front of the trailer raised about half a bubble and the passenger side raised half a bubble to ensure good drainage from the AC unit since the drain on our tag is in the right rear. Can't prove it's all necessary, but we've never had a moisture problem employing this method.