Sunday, December 4, 2022


RV ‘Gremlins’, Part 4: RV roof air conditioners: Sometimes they work, sometimes they don’t


We have had quite a number of questions come in concerning the efficiency and overall operation of RV roof air conditioners. Some don’t cool, and some just don’t work at all. Once again, it’s important to understand how the roof air conditioner works to help find the “gremlin.”

It starts with the thermostat (t/stat), which is powered by 12-volt DC from the house batteries. When the desired temperature is set, the t/stat creates a “closed” situation, which sends power to the module board up in the air conditioner. This is also powered by 12-volt DC.

The motor starts, which then turns the fan blade. The compressor also starts up, typically with a huge amp draw and thumping or grinding sound. This initial amp draw can trip the campground breaker if plugged into a 30-amp circuit with other appliances on.

Aha! We’ve found the RV “gremlin”

It does this several times and you take it to the dealer. As “gremlins” work, everything appears to be working fine to the dealer but you get charged for diagnostic time. You go through all this only to go out on the next trip and it does it again! Typically, what is happening is that the dealership plugs the unit into their 30-amp outlet with nothing else on in your coach and it works fine. The “gremlin” here is the other appliances that you might be running at the time and possibly a low voltage situation at the campground source.

It’s what I call “When The Stars Align Syndrome”. Meaning the RV roof air conditioner starts up while the refrigerator is running and the converter is charging the batteries. Keep in mind that most appliances are not running all the time. Rather, they cycle on and off just like the roof air conditioner. So, many times there is no issue until a few of them kick in at the same time. That is why it is a good idea to have a device at the pedestal such as the Surge Guard that shows the amp draw and voltage. The best is an Energy Management System (EMS) that not only tests, but monitors by shutting down some appliances during critical usage.

Insufficient cooling

Now let’s look at insufficient cooling. Again, we go back to how the roof air conditioner is designed to work. When the motor kicks in and the fan spins, it draws air from the inside of the RV up through the air intake on the ceiling and draws the warm moist air through the evaporator coil. At that time the compressor is pumping what I call the “coolant” through copper tubes zig-zagging through the evaporator. Technically, it’s probably not coolant. But what it does is more important, as it flashes and draws heat and moisture out of the air. It’s similar to when you sweat and even warm air blowing across your arm cools you down.

This process can only decrease the ambient temperature of the inside of your RV by 15 degrees. If the temperature outside is 80 degrees and you are parked in the shade, the air conditioner works great. However, if your RV has been sitting sweltering in the sun and is 120 degrees inside, it can only “cool” it down to 105 degrees and not right away. If the ambient temperature of the inside of your rig is more than 100 degrees and has high humidity, it could take most of the day to get it even below 80 degrees. The “gremlin” in this situation is ambient temperature, humidity and the ability of the unit.

Other factors that play a role with your RV roof air conditioners

Other factors that could affect the efficiency are how many windows are open or roof vents are running, which will help move air and even get rid of some of the hot/humid air inside the rig. On one occasion you might have a good breeze coming into the big slider window or be parked in the shade, while the next day your RV is parked in the sun with no breeze at all.

Evaporator coils

Let’s look a little further into the operation of the unit after the warm, moist air is drawn into the evaporator coils. These coils need to be cleaned and ventilated for the air to pass through freely. This unit had a bad compressor and would not cool. You can see the entire evaporator coil is caked with body powder!

A very dirty RV roof air conditioner that needs to be cleaned

Not much air was getting through this and the harder the compressor worked, the higher the amp draw until it eventually stopped working. This typically would not be what I consider a “gremlin” as it would not work well from this point forward. But it does explain why something would have worked for a while and then not.

Your RV roof air conditioner’s thermostat

And finally, the thermostat. Typically the t/stat used for the RV roof air conditioner is powered by the 12-volt house battery, so there is that “gremlin” we discussed earlier with sulfation and 12-volt power available. Then there is the wiring used not only to the t/stat but internal connections, and especially solder points. Temperature changes can do some strange things to wiring, solder points, and other electronics. A hairline crack in a solder connection can expand and contract with temperature changes. This can sometimes create an open situation and then later a closed situation and it will work.

More RV “gremlins”

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

Read more from Dave here


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Dave N
2 months ago

We had the old intelletic climate control in our 01 Bounder.
there is a guy that makes a replacement for it in Ohio.
it is a led smart unit which is plug and play.
his name is Jon Waiter.
you can find him here.

4 months ago

Wrong on 1 point. It’s not that the ac can only cool the rv 15deg fr9m outside temp, the difference between ac intake &output should be 18-20 deg. So when air in is 110 air out should be around 90 &so on down as it’s running. Whether that cools your rv depends on btu size, insulation, windows&doors, people & pets generating heat & others. Older or cheaper rvs mean more limitations.

Bob p
4 months ago

I had a gremlin in the bedroom A/C, it leaked water into the distribution duct work and dripped on the corner of the foot of my side of the bed. RV dealer couldn’t find the leak 3 different service appointments to the tune of $175. SIL and I finally found it was a small crack in the plastic drain pan. Waterproof epoxy sealed it, no more leaks the remainder of the 2 more years we had it. It was perplexing, we replaced the roof seal, then the RV dealer replaced it again even though I told him it was new. Never will I return to that dealership.

Steve G
4 months ago

Dave, first off thank you for your time and expertise. I own a 15 foot travel trailer and live in the South. When we purchased our trailer the dealer convinced us to opt for the 13500 btu unit.
It was a huge failure, even on the lowest fan speed the unit would cool the trailer so fast it would not dehumidify the air. Leaving us cold and clammy feeling. My solution was to remove the roof top unit and add a 12000 btu inverter style mini split system. It works fantastic! Near silent operation and the humidity problem was solved. Five years on and we have no regrets.

Randy Bitner
4 months ago

Great Article. Now if I can only convince my wife about cooling. I.E. 15° cooler, she keeps turning the thermostat down, and down. And I keep saying, that will not help, time will tell the story. And then at night, it’s cold in here!

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