Why Your Ceiling Is Leaking When It Hasn't Rained: Attic AC Condensate

Published by Water Damage Restoration Specialists of San Antonio · Last updated 2026-07-25

It is 100 degrees, it has not rained in three weeks, and there is a spreading brown stain on your upstairs ceiling. This is one of the most common calls we take between May and September, particularly in the two-story housing across Stone Oak, Shavano Park, Alamo Ranch and the north-side subdivisions — and it is almost always the air conditioning.

Why it happens

Your air handler does not just cool air, it dehumidifies it. In a San Antonio August that means pulling several gallons of water out of the air every day. That condensate collects in a drain pan and runs out through a small-diameter PVC line. When the line blocks — and it blocks with algae, dust and general sludge, which thrive in a warm dark pipe full of water — the pan overflows.

If the air handler is in the attic, as it is in most San Antonio two-story homes, everything below it is in the path: ceiling drywall, insulation, wall cavities, flooring and sometimes the story below that.

The warning sign most people miss

Look for a small PVC pipe protruding from an exterior wall, usually high up and often above a window. That is the secondary drain line. It exists to catch overflow when the primary line fails. If you ever see it dripping, that is not a quirk — it is your system explicitly telling you the primary drain has blocked and the emergency backup is now doing the job. Most people notice it, think nothing of it, and get a ceiling stain a few weeks later.

What to do right now

1. Turn the system off at the thermostatEvery minute it runs adds more water. This is the single most useful thing you can do.
2. Do not poke the ceiling from belowUnless it is bulging and holding water, in which case keep everyone clear and relieve it from the side into a bucket. Never stand underneath it.
3. Check the attic if it is safeYou may find a full drain pan or standing water on the insulation. Do not walk between joists.
4. Photograph everythingBefore anything is moved or dried, for the claim.
5. Call an HVAC technician and a restoration contractorThe HVAC technician clears the line. We deal with what has gone into the structure.

Why the ceiling stain is the small part

The visible stain is where water finally came through. Above it, the insulation is saturated, the top of the ceiling drywall has been wet for as long as the leak has run, and moisture has usually tracked along joists well beyond the stained area. Attic condensate leaks are slow and quiet, which means by the time you see them they have frequently been running for weeks — long past the 24-to-48-hour window in which mold begins establishing. That is why we treat these as potential mold jobs rather than simple dry-outs until the readings say otherwise.

Preventing it

• Have the condensate line cleared at every HVAC service — it takes minutes
• Ask your technician to fit a float switch that shuts the system down if the pan fills
• Check that the drain pan under an attic unit is intact and actually plumbed to a drain
• Walk outside occasionally in summer and glance at the secondary drain line
• If your attic also holds the water heater, check that too while you are up there

A float switch costs very little and turns a five-figure ceiling loss into an inconvenient afternoon without air conditioning. It is the best value preventive measure in a San Antonio home.

Related guides

• Burst Pipe in Your Home: What To Do in the First Ten Minutes
• How to Tell If You Have a Slab Leak
• How Long Does Mold Take to Grow After Water Damage?

Need help with this?

We provide water damage restoration, mold remediation and fire damage restoration across all 58 San Antonio ZIP codes, 24 hours a day. Call (844) 476-1017.