An RV air conditioner that was not broken

The most common air conditioning call we take in a Texas summer is not a failed compressor and it is not low refrigerant. It is a unit running flat out, all day, moving air that cannot get through the coil.

Why “it needs a recharge” is usually wrong

Start here, because it is the single most expensive misunderstanding in RV air conditioning.

A rooftop RV air conditioner is a sealed system. Unlike the one in your truck, it is manufactured closed, with no service ports and no way to connect a set of gauges to it. There is nothing to top up. If a rooftop unit has genuinely lost its charge, it has lost it through a failure in the sealed system, and the answer is a new unit — not a recharge.

So if somebody offers to recharge your Dometic, Coleman or Advent rooftop, the reasonable question is where they intend to attach the hoses. In practice, the overwhelming majority of “low on refrigerant” calls turn out to be one of three things: airflow, a capacitor, or a fan motor. This one was the first.

What restricted airflow feels like from inside

It rarely presents as “the A/C is dead.” It presents as an A/C that is clearly trying:

• It runs constantly and never satisfies the thermostat, especially from mid-afternoon onward.
• The air coming out is cool, but not cold, and there is not much of it.
• It ices up — sometimes visibly at the interior assembly — then works briefly after it thaws.
• It trips the breaker on the hottest day of the week, which is a compressor working against a condenser that cannot reject heat.
• It has been getting slowly worse for two seasons rather than failing overnight.

That last one is the tell. Components fail; dirt accumulates.

Thirty-one minutes

What packs a coil in Central Texas

Coils here fill up faster than the manuals assume, for reasons that are specific to where we are:

• Cedar and cottonwood. Anything parked under trees in the Hill Country collects a season’s worth of fine debris that mats straight into the fins.
• Caliche dust. Unpaved drives, storage yards and park roads throw a fine, sticky dust that behaves nothing like ordinary dirt.
• Mud daubers and wasps. A rooftop shroud is an excellent place to build, and a nest between the fins blocks airflow completely in one small area.
• Nobody looks. This is the real reason. The coil is on the roof, under a shroud, behind screws. In nine years of doing this the most common answer to “when was it last cleaned” is that it never has been.

Checking your own

This is genuinely worth doing yourself once a year, before the heat arrives.

• Kill the power first — both kinds. Shore power off at the breaker and the battery disconnect. A rooftop unit can start on its own while you have your hands in it.
• Get the shroud off. Usually four to six screws. Note where they came from; they are not all the same length on every unit.
• Look at the coil face. You should be able to see daylight through the fins. If you cannot, that is your problem.
• Check the return air filter inside while you are at it. It is usually a foam pad in the ceiling assembly, and it is the other half of the airflow story.
• Look at the ceiling assembly gasket — the foam ring between the rooftop unit and the interior. If it has flattened, you are cooling the roof cavity as well as the coach.

If you are not comfortable on a roof, that is a completely reasonable place to draw the line. Walking an RV roof that you are not certain is sound is a genuine hazard, and it is one of the things we check as a matter of course.

When it is not the coil

Sometimes the coil is clean and the unit still will not cool. In rough order of how often we see it:

• Capacitor. Classic symptom is a unit that hums but will not start the compressor, or that starts sometimes. Cheap part, quick job.
• Condenser fan motor. Fan not turning means heat is not being rejected, which looks exactly like a low charge from inside.
• Control board or thermostat. Usually intermittent, usually maddening.
• A genuinely failed sealed system. Real, but far rarer than it is diagnosed — and the honest answer there is replacement, which is what the Chill Cube job was.

The measurement that separates these is the temperature split between the return air and the supply air, taken with the unit settled. That is a two-minute check with a thermometer and it is the first thing we do.