Saturday, September 30, 2023


RV roof air conditioner efficiency – Teaching an old dog new tricks

Recently I posted a question from an reader and my response regarding RV air conditioner troubleshooting (see below). I suggested that the typical unit could only cool the incoming air down 16 degrees. I got quite a few comments from readers that had experienced a difference of 20 degrees or more and wondered where I got my data from.

For the past 20 years I have been working with both Dometic and Airxcel (Coleman) troubleshooting units, and was told in the past that 16 degrees is the normal cooling cycle. Even the RV Technical Institute (RVTI) Level 1 training program indicates 16 degrees is normal, and I have not heard any upgrades to that specific point. I have tested a few A/C’s with an anemometer and found that to be the range.

However, when I received the following comment it made me curious to see what the techs at Dometic and Airxcel had to say. This came from John and was the comment that really got me scratching behind my ears:

“The roof air conditioner can only condition or cool the air in your coach by 16 degrees.” I thought this was curiously specific and wondered where the number came from. In AZ late spring (100F outside) I got 27F drop. Today I asked ChatGPT 3.5 the following: “What is the maximum temperature reduction of a rooftop RV air conditioner?” The response was: “As of my last update in September 2021, rooftop RV air conditioners typically have a maximum temperature reduction capability of around 20 to 30 degrees Fahrenheit (11 to 17 degrees Celsius) from the outside ambient temperature.” I asked for sources, and the response included, “It is commonly known in the RV community and can be found in various RV-related forums, user manuals of RV air conditioners, and expert articles.” This helps to explain the discrepancy. Still curious where that 16F came from.

What do the air conditioner experts have to say

So I sent up my Batman Beacon and was surprised it still worked! I got the first response from Dometic:

On average, our rooftop units drop the temperature around 18-20 degrees F between the inside return air and cold air discharge. Now, this is at 50% humidity. The more dry the environment, the better the split difference (I’ve heard of them producing a 30+ degree difference in really dry conditions); the higher the humidity, the less of a split difference.

So, a customer could be camping in Arizona and get a 27 degree difference between the return and discharge. That same customer packs up and travels from Arizona down to Florida (where it is normally very humid), and suddenly now they are only getting a 15 degree split. This is considered normal.

Keep in mind, these temp readings would be DIRECTLY off the bottom of the AC, and if the cold air has to travel through ducting, the cold air could diminish via the heat gain in the ducting.

Personally, I would say any AC producing less than 15 degree split (again, when taking temps directly off the bottom of the AC) would be considered faulty and need replaced.

Then came the response from Airxcel:

High pressure AC systems typically will have the same characteristics. This is true of RV AC’s as well as residential systems.

The AC is designed to have a 16 to 22 degree drop from what is drawn in through the filters to what is coming out of the closest register.

How well it cools the application will depend on several factors (some listed below)…

      • Grade of insulation used in the application
      • How many windows exposed to sun/heat
      • Heat generating equipment inside the application
      • Amount of bodies generating heat inside the application
      • Humidity

These are all small pieces of what is called the heat load.

If an AC capacity is closely matched to the heat load, it will perform very well in most conditions.

When you have excessive heat loss in a coach, the AC will struggle as the heat is getting into the coach faster than it can be removed.

As the AC runs through a cycle (cycle = amount of air pulled through the AC and conditioned equal to the calculated amount of air in the coach space), the temperature in the coach drops.  This would then drop the temperature of the air coming out of the AC.


      • Temps before cycle
      • Outdoor = 95°F
      • Indoor = 80°F
      • Supply air from AC = 60°
      • Temps at next cycle
      • Outdoor = 95°
      • Indoor = 75°
      • Supply air from AC = 55°
      • Temps at next cycle
      • Outdoor = 95°F
      • Indoor = 70°F
      • Supply air from AC = 50°F

As the cycle continues the temperature should continue to come down if there is no significant heat loss.

When the customer is in the shade, they have removed a lot of the heat load by removing the direct sunlight.

This old dog learned some new tricks

So I guess the 16 degrees I have been using was a minimum drop that should be expected depending on the factors listed. I guess an old dog can learn a few new tricks.

This is an old picture of my good pal, Lola. She loved to go camping with Grandpa, but I think she only heard half of what I said. Miss her and that floppy ear!

 You might also enjoy this from Dave 

It’s 89 out and my RV’s air conditioner only “cools” it to 86. Help!

Dear Dave,
My air conditioner is working and blowing cold air but can’t keep up with outside temperatures. Both compressors seem to be working. I just had the Freon checked and that’s good. We are camping now, and the outside temperature got to 89 and the thermostat in the camper said 86. The unit didn’t turn off all day until about 10:00 at night, when the outside temperature dropped. I’ve also changed filters and cleaned the condenser. Any suggestions? —Tim, 2006 Winnebago Adventurer 37

Read Dave’s answer.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here


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Dave Solberg
Dave Solberg
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.


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2 months ago

I just recently did a recoating of my roof. Before it was a tan or beige color. You couldn’t hardly touch the roof if the sun was shining. I changed the color to white using Liquid Rubber solar reflective coating. Immediately upon even with just the first coat of the white the roof was noticeably cooler. The heat gain inside was greatly reduced and the ac is able to keep the inside much cooler and even runs at lower speeds or even turns off even when outside temperatures are 95 to 105. Best change I’ve made to my RV.

Steve Hericks
2 months ago

What is not well explained is that the heat transfer in the evaporator has two components but most users only assume one. Cooling capacity = simple heat (reducing air temperature) + Latent heat (condensing moisture). Since you can’t see/feel ‘latent heat’ being consumed, all you experience is the reduced simple heat. Latent, by definition, means hidden. Latent heat is the energy required to condense humid air (the water that runs off your roof). This is a ‘side effect’ of cooling air. In humid environments, latent heat can easily consume a third of the ‘cooling power’ just to condense the humidity. The ‘hidden’ energy loss is real and reduces the capacity to reduce the air temperature.

Steve Hericks
2 months ago

There is a lack of understanding about SINGLE PASS temperature drop (the difference between return air temperature and supply air temperature) being nominally around 20F and the ability to achieve and maintain a temperature difference between exterior air temperature and interior air temperature. The last example does a good job of dispelling that showing that 20F is achieved each time the cooler return air goes through.

2 months ago
Reply to  Steve Hericks

These explanations are the clearest ones I have seen that explain the difference between humid and dry. Thanks!

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