My RV’s roof air conditioner drips inside when running. How can I fix it?

Dear Dave,
When running my air conditioner on hot days, it begins freezing up and drips water or “snow”. What is the cure for this little issue? Thank you. —Jeff, 2024 Coleman Lantern 274BH

Dear Jeff,
The water or snow that you are experiencing could be two different things. First, let’s look at how the roof air conditioner works.

How a roof air conditioner works

The thermostat is set to a desired temperature and when the ambient inside temperature rises above that, it sends 12-volt power to the control module in the air conditioner and the unit kicks in. The fan draws warm, moist air from the inside of the rig through the air return in the ceiling and pulls it through the evaporator coil. There are copper tubes incorporated into the evaporator coil which “flash” the air and remove moisture and heat, and the cooler air is then directed down to the vents or ductwork. It looks like your rig does not have ducted vents, but is rather a single unit with the air coming out of the vents on the unit.

As the air passes through the evaporator coil, moisture is drawn out and flows down to the drip pan which has two drain holes, one on each side. The pan is slanted so the moisture should flow to the sides, out the drain holes, and down the roof.

Here is an example of the “rain” you might be experiencing.

If the drain holes are plugged, or you have applied silicone around the roof air conditioner, the moisture will build up and pool in the drain pan and the fan will splash it around and out the vents.

There may be much exposed metal near exhaust area

Another issue could be an abundance of exposed metal around the exhaust area. As the cold air coming out of the evaporator coils hits the hot metal, condensation forms and will splatter out the vents.

Pull off the interior return air cover and you will see the air intake and exhaust areas. Look at the exhaust chamber and you most likely will find not only bare metal areas, but gaps that would create inefficient air flow.

I use Thermo-Pan^® insulated board with HVAC tape to cover any bare metal spots and plug gaps or holes that allow air to flow anywhere other than out of the vents. The Thermo-Pan board is a foil-faced cardboard-type material that will not have the same heat exchange as bare metal and will reduce condensation.

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My RV’s roof air conditioner smells like mildew. What should I look for?

Hi Dave,
I’m wondering if you have any ideas on why, when my AC is on, it smells like mildew in our 2018 Fox Mountain. This is the first time it has happened. I took a look at the air conditioner itself, but I don’t see anything really. Wonder if you can help. Thanks, Dave.  —Andrew, 2018 Fox Mountain

Read Dave’s answer.

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

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