Why Your RV Air Conditioner Drips Inside (and How to Fix It Before the Ceiling Stains)

Water drips from the ceiling trim around the air conditioner, or runs out of the vents, or shows up as a brown ring on the ceiling panel. It happens when the AC is running on a hot, sticky afternoon, or it happens during a storm, and the difference between those two matters more than anything else in this guide.

Rooftop units (Dometic Brisk II, Penguin II, Coleman-Mach 8 and 15, and the Advent and Furrion equivalents) make a lot of water on purpose. The evaporator coil pulls humidity out of the air, it collects in a pan under the coil, and it drains out holes in the base of the unit onto the roof. In order of likelihood when that water ends up inside: clogged drain holes, a rig parked nose-down or side-low so the pan drains toward the opening, a gasket that has lost compression, an evaporator that froze and then dumped, condensation on cold duct surfaces, and a cracked pan. Rain leaks are a separate problem that gets blamed on the AC because it is the biggest hole in the roof.

You need a ladder and a roof you can safely stand on, a socket driver for the shroud screws, a flashlight, a hose, and about an hour.

Where the water comes from

Warm, humid air is pulled through the return filter into the roof unit and across the evaporator coil, which runs around 40 to 50°F. Water condenses on the fins and runs down into the base pan of the unit, which sits on the roof. The pan has drain holes or slots at its edges or corners, and the water runs out onto the roof and off the side. On a humid day a unit can make a quart or more per hour; a wet roof or a drip line down the side of the rig is normal.

The unit sits over a 14 in by 14 in opening cut in the roof. A foam gasket about an inch thick sits between the roof unit’s base and the roof, around the opening. The ceiling assembly bolts up from inside through the roof to the unit, and tightening the four bolts compresses the gasket. That gasket is what keeps both condensate and rain out of the opening. It is the only seal; there is no sealant on top of the roof unit, and putting some there does not fix a bad gasket.

So water gets inside three ways: the pan fills faster than it drains and overflows toward the opening, the gasket lets water from the roof or the pan into the opening, or water forms on cold surfaces below the roof in the ceiling plenum and ducts.

Step 1Rain or condensate

Do this before anything else. It decides which half of the guide you use.

Run the AC on a humid day and watch the trim ring and the vents. Drips appear only after the unit has run a while, and stop within an hour of shutting it off: condensate.

Wait for rain, or run a garden hose gently on the roof around the AC for 15 minutes with the AC off and someone inside watching the trim ring with a flashlight. Water appears: rain leak. Go to the gasket section and then the roof sealant guide.

Both: often a gasket problem. A gasket with no compression lets condensate from the pan run into the opening and lets rain in too.

Step 2Clear the drain holes

Shut off the AC at the thermostat and the breaker. Climb up, remove the shroud (four to eight Phillips or 1/4 in hex screws, usually with plastic washers; keep them), and set it aside somewhere it will not blow off the roof.

Look at the base pan below the evaporator (the coil on the side toward the front of the rig on most units; the condenser is toward the rear with the compressor). The pan has drain holes or weep slots at its low edges or rear corners. On a Brisk II they are small round holes in the pan floor near the corners; on Coleman-Mach units they are notches along the pan lip. Cottonwood fluff, pine needles, dirt, and dead insects pack into them, the pan fills, and water finds its way to the 14x14 opening instead.

Clear each hole with a stiff wire or pipe cleaner and vacuum out the pan. Pour a cup of water into the pan and watch it drain out onto the roof. If the pan holds water with the holes clear, the rig is out of level or the pan is damaged.

While the shroud is off, check the evaporator fins for debris and the condenser fins for bent-over sections. A fin comb straightens them.

Step 3Level the rig

The drain holes are at the low points of the pan when the rig is level. Park nose-down on a sloped site and the pan’s low point moves forward, toward the evaporator and the opening, and the holes at the rear are high and dry. Same with a hard side lean.

Put a level on the floor inside. Get within a bubble’s width of level both ways. If the drip stops, that was it, and it will come back on the next sloped site. Some owners add a second set of drain holes at the forward corners of the pan to cover nose-down parking; a 1/4 in drill through the pan floor into open air outside the gasket line does it, but drill only through the pan, with a stop on the bit, and check from below for wiring first.

Step 4Check gasket compression

The gasket does its job only when compressed. The Dometic Brisk II installation manual calls for tightening the four ceiling assembly bolts until the gasket compresses to about 1/2 in from its 1 in uncompressed thickness, and Coleman-Mach specifies a similar range with the gasket compressed to roughly half; check your manual for the exact number. Over the years the foam takes a set, the roof flexes, and the bolts end up loose. Water runs under the unit and in.

Remove the inside ceiling grille and filter cover. The four bolts are in the corners of the ceiling plenum, sometimes behind the air divider. With a socket, snug each bolt a quarter turn at a time in a cross pattern and look at the gasket from the roof (shroud off) or feel the gap at the base. Stop when you reach the spec. Do not crank them. Overtightened bolts crush the gasket flat so it no longer springs back, bow the roof, and crack the plastic base pan, which is a worse leak than you started with.

If the gasket is flattened to a quarter inch, cracked, or pulled apart, it needs replacing, which means lifting the roof unit. A 14x14 gasket is inexpensive (Dometic and Coleman-Mach sell them; a generic one fits either) but the unit weighs 80 to 100 lb, and two people and a plywood sheet to slide it on make the job safe.

Step 5Evaporator freezing and the thaw dump

If the drip comes in a rush after the unit has run for hours, and the airflow from the vents has been dropping, the evaporator froze. Ice builds on the fins, blocks airflow, and then when the compressor cycles off or you shut the unit down, the ice melts all at once, faster than the drain holes can take it, and the pan overflows.

Causes, in order:

1. Dirty return filter. Less air across the coil means the coil runs colder. Wash or replace the foam filter monthly in dusty areas.
2. Fan on low with high humidity. Low fan plus a humid day is the classic freeze-up. Run the fan on high when it is sticky out, and let the thermostat cycle the compressor.
3. Low outdoor temperature. Running the AC below about 70°F outside pushes the coil toward freezing. Use the fan only, or the furnace.
4. Blocked ducting or closed vents. Same effect as a dirty filter.
5. Low refrigerant. RV rooftop units are sealed systems with no service ports; low charge is a leak and the unit is replaced, not recharged.

Many units have a freeze sensor, a thermistor clipped to the evaporator fins that tells the control board to shut the compressor off when the coil approaches 32°F and restart when it warms. Coleman-Mach calls it a freeze control; Dometic Brisk II units with electronic controls have one. If the sensor has fallen off the fins or failed, the compressor runs through a freeze-up. With the shroud off, find the small sensor on the evaporator and make sure it is clipped firmly to the fins, not hanging in the air. A sensor that reads open or shorted at the board is a replacement part; check your manual for its resistance at room temperature.

Step 6Condensation on ducts and the plenum

With a ducted system, cold air passes through the ceiling plenum and ducts in a hot attic space. On very humid days the outside of the duct, the metal plenum box, and even the ceiling grille sweat, and that water drips from the vents or runs down the trim ring. It looks like a leak and is not one.

Check for air leaks in the plenum where cold air escapes into the ceiling cavity; foil tape over gaps helps. Make sure the divider between return and supply in the ceiling assembly is sealed, since cold supply air leaking into the return side chills the whole box. Run the fan on high to raise the duct surface temperature. A dehumidifier inside the rig lowers the dew point enough to stop grille sweat in most cases.

Step 7Cracked drain pan

A pan cracked by a hard freeze with water in it, a dropped tool, or overtightened bolts leaks straight down into the opening. With the shroud off and the pan clean, pour water in and watch for it to appear below the pan rather than at the drain holes. A hairline crack in a plastic pan can be sealed with a two-part epoxy rated for plastic as a stopgap; a cracked metal pan can be repaired with a brushable seam sealer. Both are stopgaps. A badly cracked pan means a new unit on most models, since the pan is the base casting.

Symptom → likely cause

Parts and tools mentioned

• 14x14 rooftop AC gasket (Dometic or Coleman-Mach branded, or universal; verify thickness against your manual)
• Replacement return air filter for your unit (foam, cut-to-fit works)
• Freeze sensor / freeze control thermistor for your model (verify)
• Fin comb, stiff wire or pipe cleaner, foil HVAC tape
• Two-part plastic epoxy or seam sealer for a hairline pan crack (stopgap)
• Socket driver, flashlight, level, garden hose, ladder

Questions people ask next

Water drips off the side of the rig when the AC runs. Is that a leak? No. That is condensate draining from the pan onto the roof and off the edge, and it means the drain holes are working. The problem is only when water comes inside.

Can I put sealant around the AC on the roof? Not as a fix. The seal is the gasket under the unit, and sealant on top traps water against the base and hides a gasket problem. Sealant belongs on the roof seams and the screws around the opening, not on the AC base.

How tight should the four bolts be? Tight enough to compress the gasket to your manual’s spec, about half its original thickness on a Brisk II. There is no torque figure in most manuals; it is a visual check of the gasket. If the bolts feel loose, snug a quarter turn at a time and look.

Why does it only drip at night? Night is when humidity peaks, the fan is on low for quiet, and the compressor runs long cycles. That is the freeze-up recipe. Run the fan on high, or on auto if your thermostat has it.

The drip started after I replaced the unit. Gasket compression, almost every time. A new gasket needs the bolts rechecked after the first week of use and after the first few hundred miles, because the foam settles and the roof flexes.