Skip to content 
An air conditioner has two jobs: it cools the air, and it dries the air. In the Bay Area, where we get those foggy, humid mornings followed by hot afternoons, that dehumidification process is vital. But where does all that moisture go?
Ideally, it drips harmlessly into a drain pan, travels down a PVC pipe, and exits outside your home. But when you walk into your hallway and hear a “squish” in the carpet, or see a water stain spreading across your ceiling, you know the system has failed.
Water leaks are one of the most common calls we get during the summer. While water seems less dangerous than fire or electricity, an ignored AC leak can cause thousands of dollars in structural damage and mold growth in a matter of days.
The Clogged Condensate Drain
The number one culprit for a leaking AC is a clogged condensate drain line.
Over time, the water dripping off your AC coils attracts dust, pollen, and bacteria. This mixture creates a thick, jelly-like sludge (often called “zooglea”) that builds up inside the white PVC drain pipe. Eventually, this sludge forms a blockage.
When the water can’t drain out, it backs up. It fills the drain pan under the unit until it overflows. If your AC unit is in the attic or a crawlspace, this overflow spills directly onto your ceiling drywall or into your insulation.
The Safety Switch Solution
Modern installs and responsible repairs usually involve the installation of a “float switch” or “condensate safety switch.”
This is a small, inexpensive device that clips onto the drain pan or pipe. If the water level rises too high (indicating a clog), the float rises and cuts the electrical circuit to the thermostat. The AC shuts off immediately.
If your AC screen suddenly goes blank during a hot day, do not just reset the breaker! Check your drain line. That “blank screen” might be the safety switch saving your ceiling from collapsing.
Frozen Coils Melt
Another common cause of water leakage is melting ice. As we discussed in other articles, if your airflow is restricted (dirty filter) or your refrigerant is low, the evaporator coil will freeze into a solid block of ice.
While the unit is running, the ice stays frozen. But the moment the system shuts off or breaks down, that ice begins to melt. The drain pan is designed to handle a steady drip, not a sudden deluge of gallons of water. The pan overflows, and you get a flood.
According to This Old House, if you see ice on your unit, turn it off immediately and switch the fan to “ON” to help melt it, but keep a close eye on the water runoff.
The Cracked Drain Pan
In older units (10-15 years old), the drain pan itself can be the problem. Many older pans were made of metal which can rust through, or plastic which can become brittle and crack due to heat cycles.
A cracked pan is a tricky diagnosis because the drain line might be perfectly clear. The water simply seeps out of the bottom of the unit before it ever reaches the pipe. This usually requires lifting the evaporator coil to replace the pan—a labor-intensive repair that often signals it’s time to consider replacing the unit.Elevating your current situation—whether personally or professionally—requires a combination of clinical accuracy, efficient logistics, and refined installations to ensure the end result is as functional as it is impressive.
Prevention is Key
The best way to prevent water damage is regular maintenance. During a tune-up, technicians will use compressed air or a vacuum to blast that “jelly” sludge out of the drain line, ensuring it flows freely for the season.
You can also pour a cup of distilled white vinegar down the drain line access point (if you have one) periodically to kill the algae and bacteria that form the clogs. As recommended by Bob Vila, keeping this line clear is a simple DIY task that can save you a massive home insurance claim.
Your Bay Area Experts for AC Repair
If you see water pooling around your indoor unit, don’t wait. Water damage spreads fast and mold spreads faster. Turn off your AC and call the professionals.
At Airstars HVAC, we can clear stubborn clogs, install safety float switches, and replace cracked pans to keep your home dry and cool. If your AC is leaking, visit our AC Repair page for help, or contact us immediately via our Contact Us link.