How much below outside temperature does your air conditioner cool?

[TXiceman]

AS I have state many times, the A/C performance is based on dropping the interior air 20 degF, not the out side air temperature.

 

Run the A/C on high fan, and take a temperature reading as close to the unit as possible, Get the reading at the air return and the closest outlet duct.

 

The outlet air should be 20 degF below the inlet air in the RV (+ or - 2degF).

 

A unit is rated to produce 13,500 or 15,000 BTUH under certain conditions. The load consist of a sensible load and a latent load. The latent load is removal of moisture from the air. The higher the latent load, the less cooling is available for sensible cooling. SO, on a humid day, you may not see quiet as much of a sensible temperature drop.

 

When measuring the air temperatures, use a thermometer and not a infrared temperature gun. It measures the temperature of the grill (metal or plastic) and not the true air temperature.

 

Stay in the shde, keep window shades closed and enjoy the rV.

 

Ken

[Kirk W]

I've seen people get into arguments over the 20 degree estimate less that outside air(but we all know now that it's ambient air going into the conditioner). If I had to camp/RV at a mere 20 degree's less than OUTSIDE temp's, I wouldn't RV in any hot area's at all!

I have been RVing for a long time and have never heard of the 20° less than outside theory. I do know quite a few RV techs and also HVAC techs who will tell you that the norm of cooling ability for most a/c units is an 18° to 22° temperature drop across the cooling coils when measured by the temperature of the air entering the cooling coils compared to the temperature of the air leaving the cooling coils. Outside temperatures impact is only due to the insulation of the RV, the amount of outside air penetration of the RV and things of that nature. The air conditioners can't control those things and they are not a part of air conditioner efficiency but of RV construction quality and maintenance.

[acampingwewillgo]

I agree KIrk.....so as I said, I guess I have pretty good insulation in the Vogue and not having slides(no outside air penetration) certainly helps with keeping things cool. Maybe using the word "theory" was a tad strong, maybe commonly but mistakenly thought by many is the 20 degree's less than the outside temp fable. Heck, even this thread started out with that premise, didn't it?

[pugsly]

I just finished giving our rooftop unit a good cleaning. One thing I found, to my surprise, was a lot of debris on the cold air return heat exchanger. Turns out my foam air filter had been shedding little pieces into the return. Vacuumed everything out real good.

 

Always pays to give things a good doing over if you can.

[acampingwewillgo]

What a difference a Good cleaning makes!!! I was up on the roof a couple weeks ago and figured I may as well take the shroud's off and clean some. Glad I did, at least for the front one.

[pugsly]

What a difference a Good cleaning makes!!! I was up on the roof a couple weeks ago and figured I may as well take the shroud's off and clean some. Glad I did, at least for the front one.

 

I just did fairly major roof work - after 10 years the UV had done quite a lot of damage. I replaced the plastic shroud on the AC and also the fridge vent, along with re-sealing every roof opening and screw hole with ProFlexRV (the brush on stuff). It was a major job, but decided that the time had come to clean all the old caulk off and start fresh rather than just go over the cracks.

 

Was a good opportunity to blow out everything and make sure there weren't squirrels or raccoons living in the AC before I started it up for the season.

[Legendsk]

This is quite a thread with such a mix of good information, myths, physical facts, misunderstandings and opinions. So here are some opinions of mine, based on thermodynamics and other engineering courses and a lot of years experience.

 

First, an air conditioner is fundamentally 1/2 of a heat pump. The only real operational difference is that a heat pump can be operated pumping heat from the outside to the inside or from the inside to the outside, whereas an air conditioner can only pump from inside to outside.

 

Outside air temperature does indeed have an impact. Most heat pumps can only pump heat across about a 40º differential, maximum. If you are using a heat pump (h/p) to heat with, at around 30º it won't get the coils any warmer than 70º or so. As Kirk and others pointed out, you can only get around a 20º air temperature change between input air and output air going through the coils, so it will only put out 50º or so air and at the end of the duct, even less. Below 30º heat pumps begin to poop out for other reasons and in many climates if your heat exchange coils outside are just sitting out in the weather, the system has to be supplemented with something else like electrical resistance or gas heating.

 

Likewise if the temperature outside is 120º and the outside heat exchanger is sitting out in the weather, the coolest the air conditioner (a/c) or heat pump (h/p) will get the coil temperature is about 80º. If you start the a/c up when the trailer has been sitting out in the sun and it is at least 120º or more in the trailer, again the 20º differential applies and the air temp out of the a/c should be in the 100º or so range. But with good insulation, after grinding a long time, the a/c should be able to get the air temp in the trailer down to, say 110º. So now the a/c output air temp should be 90º and ever so slowly the poor old a/c should be able to get the temperature down to fairly close to the 80º. But THAT IS AS COLD AS IT IS GOING TO GET! Because a/c's simply aren't able to pull more than about a 40º differential.

 

There are lots of other things that get involved. If the outside heat exchange unit is in the shade and there is a nice breeze blowing and humidity is high (which allows the moving air to absorb more heat), it is possible to get the a/c to operate as though the outside temperature was only 100º? Then it can cool those coils down to 60º and the output air temperature would cool the inside of the trailer much better. Once the temp inside the trailer (and the a/c input air temp) gets down to 80º then the output air temp of the a/c could begin to approach 60º and in a well insulated trailer, perhaps get the temperature down to 70º or so. The stars would have to align perfectly to get much below that.

[learn_as_you_go]

In addition to all this good advice, we have about 3 strategically placed fans that helps push the cool air to other parts of the RV.

This weekend we blocked off the air ducts in the living room area as we don't spend time up there.
That made a big difference for getting more cold air into the ducts in the bedroom area.

It has been up to 105 degrees outside and it has been comfortable inside, less than 80 degrees.

Hope this helps someone.

[mptjelgin]

You'll want to be careful about blocking off too many ducts. The A/C needs a certain amount of air moving over the coils to operate correctly. If you block off several ducts the airflow will decrease too much and the coils will freeze. I will sometimes close off our bathroom duct as it gets very cold in there, but I don't close more than a single duct. What I don't know is exactly how many you can block off without potential problems.

[Legendsk]

I have a 35 ft HitchHiker with darkly tinted windows and a 15,000 BTU heat pump. I mostly live in far (I can see Mexico from here) south Texas with lots of days over 100º and many over 120º in direct sun. If I closed the door to the bath and bedroom and open the heat pump to dump all output into the kitchen and living area, it would keep it quite comfortable. If the h.p. output were routed to the ducts cooling all of the trailer and the basement and water tanks, the temperature would get to 90º or more. It worked okay if in the day I only cooled the living area and switched to the ducts about an hour before bedtime.

 

But I have my faceting workbench built into the bedroom closet and wanted to use it during the day. I added sun shade screens that just snap on, to the windows on the sunlit sides. Made all the difference in the world. Now the h.p. can cool the whole trailer easily. I believe the difference in electric bill will pay for the shades in about a year. I really recommend shades or awnings for anyone in the south.