Dealing with RV air conditioner “freeze-ups” 

By Steve Savage, Mobility RV Service

This time of year we get calls for air conditioner service. Often the caller is all worked up because he or she thinks that since their air conditioner has stopped working, there must be something seriously wrong. That’s often not the case — it’s often just a freeze-up.

The scenario for an air conditioner coil freezing up goes something like this. Your air conditioner or air conditioners were working and gradually, over the course of a hot day with high humidity, airflow out of your ceiling vents first drops off and then falls to zip. Finally the compressor may stop running and it seems as though the air conditioner has quit. What is going on?

The giveaway for the diagnosis in this case is the airflow slows and then quits entirely. That sequence of events is not the signature of fan motor failure — it signals ice buildup on the evaporator coil inside your air conditioner. For those who have never looked up into their air conditioner, if you removed the filter you see from inside your RV and look up into it, you will see a finely finned cooling coil. That is the evaporator coil and it gets very cold — so cold that it approaches the freezing point of water. It is the water running down that coil and into a drain pan that drips off the roof of your RV when the air conditioner is running. You can only see it from inside your RV so there is no need to go up onto the roof and take the cover off the air conditioner. You can fix this problem from inside your RV and best of all, you do not need any tools.

First, a little bit more about what causes coil icing. The first thing an air conditioner does is remove what is called “latent” heat, also known as the humidity from inside your RV. It does this by circulating air into the return and across the cooling coil, where humidity condenses out and runs off the roof of your RV. On a humid day there can be so much moisture, if the fan is running on low speed, some of the water on the coil will start to freeze. When this happens and the coil becomes partially blocked, the freezing is accelerated and more and more of the coil freezes, and less and less air passes over it until the coil is completely iced over.

When the coil is iced over, air delivery to the vents is blocked and, if enough of the coil freezes up, the freeze protector that serves to protect the compressor from operating with an iced coil closes or opens a circuit (depending on which manufacturer’s air conditioner is on the roof) and the compressor quits running. So there you are on a blazing hot day with no air coming out of the vents and no compressor operation.

If the above is you, do this first. Remove the filter cover so you can see the coil. It will be sheeted over with ice. If you cannot see the fins, it is surely iced over, but you can also put your hand on the coil and feel the ice. The coil cannot harm you, and you will not harm the coil by touching it.

Feel the ice? If you can, you just got lucky. Go to your thermostat and switch the “mode” to “fan high” instead of “cool” and give it a few minutes (how many minutes depends on how much ice has built up). Two things should happen. The air coming out of the ceiling vents will gradually increase and you will hear the ice dropping off the coil and into the pan. You might also get some water dripping out of the filter cover as the ice melts. Once the coil is clear of ice, you can switch the thermostat back to “cool,” but leave the fan on “high,” not “auto” or “low.”

Coil icing seems to be a more common problem in large diesel motorhomes than other types of RVs. In my opinion, this happens because duct work is so poorly designed that the air conditioner has a harder time moving air than in other types of RVs, but that is just a guess. It can happen in any RV, so the rule of thumb is when the temperature and humidity are both high, the fan should be running on “high” also.

Now for the naysayers: Yes, coil freeze-up can occur with a loss of refrigerant, but it is self-correcting, as a system which loses refrigerant will continue to lose it until the system is empty since it is a pressurized system. That being said, an AC system leaking refrigerant will run with good air delivery but ultimately will not blow cold, so it is easy enough to tell a leaker from a system with a frozen coil.

Stay cool!