AC leaking water inside the house: causes and fixes

Where the water comes from

An air conditioner dries the air as it cools it. Warm, humid house air passes over the cold evaporator coil, moisture condenses on the coil the way it does on a glass of iced tea, and it drips into a pan under the coil. A drain line carries it outside or to a plumbing drain. On a muggy Triangle afternoon, that adds up to gallons of water a day, all of it flowing through a plastic pipe less than an inch across. When that pipe stops draining, the water has to go somewhere.

Why the AC overflows

• A clogged condensate line. Algae and sludge grow in the warm, wet line and block it, by far the most common cause.
• A rusted or cracked drain pan, common in older air handlers and furnaces.
• A disconnected or poorly sloped drain line that leaks at a joint or holds water.
• A frozen coil. A dirty filter, a weak blower or low refrigerant lets the coil ice over; when the ice melts, more water arrives than the pan can handle.
• A failed condensate pump on systems where the water has to be pumped up and out.

Attic air handlers and the backup systems

Many two-story houses in the Triangle have an air handler in the attic, directly above bedrooms. Because an overflow there damages finished ceilings, code requires a backup where a leak would cause damage. That is usually a secondary pan under the unit with its own drain, often ending above a window so someone sees it dripping, or a float switch that shuts the system off when water rises in the pan. Some systems have both. When the backup works, the warning is a dripping pipe outside or a system that shuts itself off on a hot day. When it does not, the first warning is a stain on the ceiling.

What an overflow damages

• Ceiling drywall under an attic unit, often in a bedroom or hallway.
• Attic insulation, which soaks up the water, packs down and stays wet against the drywall.
• Closet floors and walls around an indoor unit or furnace on the main floor.
• Garage drywall and stored items below a unit in a garage.
• The air handler itself, where standing water and wet insulation inside the cabinet grow mold.

What to do right now

1. Turn the system off at the thermostat.
2. Check the pan under the indoor unit, if you can reach it safely, and soak up standing water.
3. Put a bucket under any drip, and deal with a sagging ceiling as described on the ceiling leak page.
4. Photograph the unit, the pan and the damage.
5. Call an HVAC technician to clear the drain and find the cause, and a restoration crew if water reached ceilings, walls or floors.

Drying, repairs and mold

Wet attic insulation is usually removed so the ceiling drywall below can dry, and the drywall is dried in place or replaced depending on how long it was wet. Ceilings that have been getting a little wet all summer often have mold on the back of the drywall or in the insulation, and that is removed before anything is closed. Mold inside the air handler cabinet or on the coil needs attention too; mold in air ducts and HVAC explains how. Ceiling and wall repairs follow once everything reads dry.

Keeping the drain clear

• Have the condensate line flushed and the pan checked during spring maintenance, before the humid season.
• Change filters on schedule so the coil does not ice over.
• Confirm the attic unit has a working float switch or secondary pan, and test the switch.
• If the secondary drain over a window ever drips, schedule service the same week.
• Make sure the drain line is insulated where it runs through living space, so it does not sweat on its own.