Why is my AC leaking water inside?

Nearly always a clogged condensate drain backing up the pan, with a frozen coil melting off as the runner-up. The drain clog is algae and slime in the line; the frozen coil has its own diagnosis.

How do I unclog an AC condensate line?

Pull the clog from the outside end with a wet/dry vacuum sealed onto the drain outlet for a couple of minutes. Flushing from the inside pushes the clog deeper; suction from the end brings it out.

Should I put vinegar or bleach in my AC drain line?

A cup of plain white vinegar down the line's access tee every month or two of cooling season suppresses the algae that forms clogs. Vinegar over bleach: gentler on the pan, the metals, and whatever the line drains into.

What is the float switch on my AC for?

An overflow shutoff: when water backs up in the pan or line, the float rises and kills the system before the pan spills. A summer no-cool complaint that turns out to be a full drain pan is the switch doing its job.

AC Condensate Drain Clogs and Overflow Shutoffs

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

Every air conditioner is also a dehumidifier, wringing gallons of water a day out of summer air, and all of it leaves through one humble tube: the condensate drain. That tube grows algae, clogs, and overflows into ceilings and closets with such regularity that a clogged AC condensate drain is among the most common summer service calls anywhere humid. This diagnostic covers the system, the clog, the cure, and the cheap switches that make overflows impossible.

Where the water comes from

House air crossing the cold evaporator coil drops below its dew point and sheds moisture, which rains off the coil fins into the primary drain pan beneath, then flows by gravity (or a small condensate pump where gravity has no route) through a PVC line to a drain, outside, or a plumbing connection, trapped and vented along the way so the blower’s pressure cannot hold the water hostage. On a humid design day the flow is genuinely gallons, which is why the failure mode is not a drip but a spill. Attic and closet air handlers add a secondary pan underneath the unit with its own line, classically routed to drip conspicuously over a window or eave, a location chosen to be noticed: water from the secondary drain is the system announcing the primary has failed.

Why it clogs

The pan and line are dark, wet, and fed a diet of dust: an algae-and-slime habitat by design. The growth builds until the line seals, the pan backs up, and the water finds the ceiling, the closet floor, or, on protected systems, the float switch. Contributing authors: lines run with sags that hold standing water, missing traps that let air instead of water occupy the line, insulation debris in attic pans, and a dirty filter economy feeding the coil the dust that feeds the slime. The impostor worth ruling out: a frozen coil melting off produces the same puddle with a different cause and its own diagnosis; ice on the lines or coil at the time of the leak points there instead.

Clearing the clog

The technique that works pulls rather than pushes. At the line’s outside termination, seal a wet/dry vacuum onto the pipe end, tape or a rag making the seal, and run it two or three minutes: the clog arrives in the canister, usually as a memorable gray slug. Then confirm flow by pouring water into the line’s access tee near the air handler (the capped stub on the drain line) and watching it exit outside. Pushing from the inside, by contrast, compacts the clog deeper, and pressurized methods belong to technicians with the fittings to do them safely. While at the air handler, empty and wipe the pan, check its slope toward the outlet, and, on pump-equipped systems, test the pump by filling its reservoir and cleaning its check valve, since condensate pumps fail as often as gravity lines clog and flood the same closets.

Keeping it clear, and the switches that guarantee the ceiling

Maintenance is a cup of white vinegar down the access tee every month or two of cooling season, suppressing the biology before it architects a clog, plus pan tablets where the pan itself grows slime, plus the filter discipline that starves the whole food chain. Insurance is a float switch: a few-dollar overflow shutoff in the primary pan, on the secondary pan, or in-line on the drain that kills cooling the moment water backs up, converting an overflow into a no-cool call, annoying, and dry. Attic units above living space deserve switches on both pans as a matter of course, and any secondary line seen dripping is a same-week service item, because it means the ceiling is one switch or one clog from the moisture-damage story. High-efficiency furnaces share the whole subject in winter, their acidic condensate adding a neutralizer cartridge to the checklist per the venting and condensing notes.

The bottom line

Gallons a day, one slimy tube: vacuum the clog out from the end, vinegar the line monthly in season, keep the filter honest, and put float switches wherever a spill would find drywall. The condensate system is the cheapest plumbing in the house to maintain and the most expensive to ignore, ceiling by ceiling.