Thursday, June 1, 2023


Why does our A/C unit freeze up?

gary-736Dear Gary,
We have a 15 kBTU ducted Duo-Therm roof-mounted air conditioner on our coach. No heat pump. Here in Texas, when temperatures are over 100 degrees every day, our A/C unit will keep freezing up, leaving us without cooling for over an hour while it thaws. It puts out good cold air, but it will freeze up at least twice daily – sometimes even in the middle of the night. This will happen whether we are parked in shade or sun.

Any suggestions as to what we can do to prevent this? We have tried not using the ceiling fan, which is right in front of the return. We have tried having all the air dump into the area right where the unit is located via the shutter vent on the unit itself instead of going through the entire coach via the duct work. We have tried other various things. Much to our dismay, nothing seems to work without, at some point, our A/C freezing up. Any ideas? —Liz S.

Click to enlarge

Dear Liz,
All Duo-Therm roof units have a freeze control device specifically mounted on the evaporator coils. The positioning of the control switch is crucial to proper operation. Have someone check for the proper positioning of the control switch and that it indeed measures no continuity when the ambient temperature is over 69-degrees.

Pictured is a diagram from the service manual to help you understand the location of this switch. It’s entirely possible the switch has come loose from its mounting or is faulty. This freeze control is a thermally activated switch of sorts that closes once the temperature of the evaporator falls below 49 degrees F. Once closed, it will reopen when the temperature rises to between 61 and 69 degrees F. If it fails to open and the temperature is above 69 degrees, it needs to be replaced. A simple continuity test and temperature measurement should quickly reveal the status of your freeze control device.

Read more from Gary Bunzer at the See Gary’s videos about RV repair and maintenance.




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4 years ago

Instead of turning the unit off switch it to fan only. The air flowing through the evaporator fins will defrost it quickly. And yes check for restricted air from either a dirty filter, dirty evaporator or having to many vents closed off.

Reginald Goforth
4 years ago

Oops on the, not or the.

Reginald Goforth
4 years ago

Always check the condensing coil and evaporative coil, if they are dirty or plugged with cotton wood tree cotton your unit can and will freeze the condensation or the evaporative coil.

Philip H. Wood
4 years ago

Before you look for something exotic-remove the shroud and check if the condenser is dirty. The face the fan is on. They get dirty and reduce air flow. This is a common problem. If it is dirty-mix dish washing soap and water ( a little rich) and soak the condenser then rinse it out with a hose but don’t use too much pressure -you do not want to bend the fins. I have worked on this stuff for over 50 years.

4 years ago

I had this problem after having a new roof installed. I called my A/C man and he told me to check the compartment where the A/C fit in the roof hole. He said that it may not be sealed correctly and sucking hot air from the area between the ceiling and roof. I checked and sure enough there was a big gap. I used silver sealing tape and sealed all of the gaps and never had another problem.

Gary D Bogart
4 years ago

With all due respect, a evaporator temp of 49 degrees is marginal, and I prefer a temp of 40 degree evap, and 17 degrees between incoming and outgoing air through the evaporator. Temperatures above 60 degrees means something is not right.

4 years ago

Check the positioning of the unit. Mine kept freezing up too. One night in a hard rain my roof leaked near the ac. Found the ac had been installed improperly to one side partially blocking return air. Thus the freezing. Resetting ac properly solved problem. Good luck

4 years ago

Another possibility is the unit is low on coolant too! I know this because my home A-Coil froze up once and it was due to low coolant. Similar circumstances to Texas, I live in Louisiana. Very Hot summers.

And I have NO Idea how to add coolant or if it can be added at all.

RV Dealers would know. (MAYBE)

Mike Sokol
4 years ago
Reply to  Jeff

Yes, I do believe it’s a low coolant issue. One of the guys who used to work for me was an air-conditioner tech, and he had a test rig with pressure gauges on the low and high pressure side of the compressor. He could then add the correct amount of freon to stop the shop air conditioners from icing up.

Vito Mazzaro
4 years ago


Isnt it possible sthat sthe filters could be dirty restricting air flow over the coil and causing the unit to freeze up

4 years ago
Reply to  Vito Mazzaro

My thought exactly. Usually a dirty filter, dirty evap., or too many grills closed. If not it may be short on charge. Airflow or charge is usually the culprit in freezeups.

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