Why Is My AC Leaking Water?

Every air handler drains the water it pulls off the coil through a narrow PVC line, usually 3/4 inch. That line stays warm, wet, and dark, which is exactly what algae and biofilm like. Over a season the gunk builds into a plug, water backs up into the pan, and the pan overflows. This is the single most common AC water leak we see in the Valley, and it spikes when monsoon dew points climb into the 50s and 60s and the coil sheds far more water than it does in the dry months. Clearing the clog once is straightforward. Keeping the line clear is what most homeowners miss, because the biofilm grows right back without a flush and a treatment.

Here's the one that fools people. Low airflow, usually from a filter nobody changed, drops the coil below freezing and ice builds up. When the system cycles off, that ice melts all at once, far faster than the pan can carry it away, and it pours over the edge. You see water and assume a leak. The real problem is airflow, or sometimes low refrigerant from an actual leak in the coil. If there's ice on the copper lines or the coil, you don't have a drain problem, you have a freezing problem, and clearing the drain won't fix it.

Under the coil sits a primary drain pan. Older air handlers used galvanized steel pans that rust through after fifteen or twenty Phoenix summers. Once there's a pinhole or a crack, water drips straight down whether or not the drain line is clear. This does the most damage in two-story homes where the air handler sits in the attic over a finished ceiling, the kind of layout common in 1990s and 2000s stock around Ahwatukee, Anthem, and north Scottsdale. Those units usually have a second, larger emergency pan underneath. If you're finding water in that secondary pan, something upstream already failed, and the pan is the only thing standing between the problem and your drywall.

A condensate float switch is a small safety device that shuts the system off when water rises in the pan, before it reaches your drywall. Plenty of Phoenix homes never had one installed. If yours did, and the AC suddenly won't run while there's water in the pan, don't just flip it back on. The switch did exactly what it's for. It caught a clog before the water found your ceiling. Reset it without clearing the clog and the next cooling cycle picks up right where it left off.

A condensate drain needs a P-trap, and many air handlers that pull air across the coil (a draw-through design) need a vent just past it, depending on the manufacturer's drain spec. The trap holds a small water seal so the blower's suction doesn't pull air backward up the drain and stall the flow. We open up Valley installs all the time that never had a proper trap, or had one that dried out over a mild winter and lost its seal. Slope matters too: the line has to fall continuously toward the exit, and a long attic run that sags in the middle will pool and back up. The fix is a drain re-piped to spec, with a proper trap, a vent where the unit calls for one, and continuous slope.

Sometimes the water isn't coming from the drain at all. When indoor humidity climbs and the refrigerant lines or the air handler cabinet aren't fully insulated, they sweat the same way a cold glass does in August, and the drips collect under the unit. That is condensation on the outside of cold metal, not a drain failure, so the fix is insulation and getting the home's humidity back under control. This shows up most during monsoon, and in homes where the ductwork runs through a hot, humid attic.