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CampWrench
ClimateModerateAbout 60 min

RV air conditioner runs but is not cooling

The rooftop unit runs but the air coming out is barely cooler than the cabin.

Applies to: Rooftop air conditioners and heat pumps on towable and motorized RVs, including Dometic Brisk, Coleman-Mach and Furrion units.

Quick answer

Measure the temperature split first: put a thermometer at the return air grille and another at the closest supply vent. A healthy rooftop unit produces an 16 to 22 degree Fahrenheit drop. A small split with strong airflow points at a refrigerant or compressor problem. A small split with weak airflow points at a dirty filter, blocked coils, or a failed capacitor letting the fan run while the compressor never starts. Low supply voltage from a long or undersized extension cord is the most common cause of a compressor that will not start at all.

What usually causes it

  • Dirty return filter or blocked evaporator coil

    Most common

    Restricted airflow across the evaporator drops capacity fast and can ice the coil, which then blocks airflow completely. The unit runs and sounds normal while doing almost nothing.

  • Low supply voltage

    Most common

    Long or undersized extension cords, marginal campground pedestals and heavily loaded circuits all sag voltage. Compressors need a large starting surge, and below roughly 105V they will hum, trip their overload, and never start.

  • Dirty condenser coil on the roof

    Common

    Cottonwood, road film and insects clog the outer coil and stop the unit rejecting heat. Cooling falls off badly in hot weather while working acceptably on mild days.

  • Failed start or run capacitor

    Common

    A capacitor that has lost capacity lets the fan spin while the compressor only buzzes. This is one of the most common hard failures on rooftop units and one of the cheapest parts.

  • Iced evaporator coil

    Common

    Running in humid conditions with restricted airflow or a very low thermostat setting freezes the coil. Airflow drops to nearly nothing and the unit blows barely-moving air.

  • Low refrigerant or failed compressor

    Less common

    Rooftop RV units are sealed systems with no service ports on most models. They are not designed to be recharged in the field, and a genuine refrigerant loss usually means replacing the unit.

Step-by-step diagnosis

Work in this order. Each step is cheaper or safer than the one after it, so stop as soon as the symptom clears.

  1. Measure the temperature split

    With the unit running on high cool for 15 minutes, hold one thermometer at the return air grille and one at the nearest supply vent. Subtract. A 16 to 22 degree Fahrenheit drop is healthy. Write the number down: it is the single most useful data point for any technician you talk to later.

  2. Clean or replace the return air filter

    Pop the interior shroud, pull the foam or mesh filter and wash or replace it. Filters are usually neglected for years. Do this before anything else, and recheck the split afterwards.

  3. Check the supply voltage at the pedestal and inside

    Measure at an interior outlet with the air conditioner running. Below about 108V the compressor is at risk. Remove extension cords and adapters from the equation, or move to a different pedestal. Running a compressor on low voltage is how they get destroyed.

  4. Look for ice on the evaporator coil

    If you see frost or ice behind the interior shroud, switch to fan only and let it thaw completely, which can take a couple of hours. Then run again with a clean filter and a less extreme thermostat setting. Ice that returns immediately means airflow is still restricted.

  5. Clean the condenser coil on the roof

    With power off, remove the outer shroud and gently clean the coil fins with a soft brush and a fin comb. Rinse with low-pressure water from the inside out if the design allows. Straighten bent fins.

    Only go on the roof if it is rated to carry your weight, and never with shore power connected. Turn the breaker off before removing the shroud.

  6. Test the capacitor

    A capacitor with a bulged top or leaking residue is failed on sight. Otherwise measure capacitance with a suitable meter and compare against the rating printed on the case. Replace anything more than about 6 percent below rating.

    Capacitors hold a dangerous charge after power is removed. Discharge safely through a resistor before handling, and if that instruction is unfamiliar, this is the step to hand over to a technician.

  7. Confirm the ducting and return path

    On ducted systems, check that the ceiling ducts are connected and that the divider between supply and return inside the roof opening is in place. A missing divider lets the unit recirculate its own cold air and the cabin never cools.

  8. Consider a soft-start module

    If the unit cools well on shore power but will not start on a generator or inverter, a soft-start module cuts the inrush current substantially. It is a common upgrade and often cheaper than a larger generator.

Safety notes

  • Turn off the breaker and disconnect shore power before removing any shroud.
  • Capacitors store lethal charge even with power disconnected.
  • Do not walk on an RV roof that is not rated for it. Many are not.

When to call a professional

Sealed refrigerant circuits, compressor replacement and any burnt smell or scorched wiring at the unit go to a technician. Most rooftop units are not field-serviceable on the refrigerant side and a failed compressor usually means replacing the whole unit.

Frequently asked questions

What temperature difference should an RV air conditioner produce?

A 16 to 22 degree Fahrenheit drop between the return air and the supply vent when running on high cool. Less than 15 degrees with good airflow indicates a refrigerant or compressor problem.

Why does my RV air conditioner freeze up?

Restricted airflow or running in high humidity with a very low thermostat setting. Clean the filter, make sure vents are open, and set the thermostat a few degrees higher. Thaw it completely before running again.

Can I run two air conditioners on a 30 amp hookup?

Not reliably. A 30 amp service provides about 3600 watts total and one air conditioner plus normal loads consumes most of it. Two units generally require a 50 amp service.

Will a soft start let me run my AC on a small generator?

Often yes. A soft-start module reduces the compressor's inrush surge, which is what usually overwhelms a small generator or inverter. It does not reduce running consumption, so the generator still has to cover that.

Also described as

  • rv ac blowing warm air
  • camper air conditioner not cold
  • dometic rooftop ac not cooling
  • rv ac fan runs compressor wont start

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