Why Is My AC Leaking Water? (And When to Worry)

Every July we get a run of the same call: there's water around the furnace, and the homeowner is worried the water heater finally let go. Most of the time the water heater is innocent. The puddle is coming from the air conditioner, or more precisely, from a drain line the air conditioner has been quietly using for years without anyone knowing it existed.

The good news is that an AC leaking water is rarely an expensive problem. The bad news is that the water can do expensive damage to whatever it lands on. Here's where the water comes from, why it ends up on your floor, and what to do about it.

Where the Water Comes From

Your AC doesn't just cool the air. It dries it. As warm household air passes over the cold evaporator coil, moisture condenses on the coil the same way it beads up on a glass of iced tea in August. On a humid West Michigan day, a central air system can wring several gallons of water out of your house.

All of that water drips off the coil into a drain pan, and from the pan it flows out through a PVC pipe called the condensate line. Usually the line runs to a floor drain, or to a small pump that pushes the water to wherever it can drain. When everything works, you never see a drop. When something in that path fails, the water finds its own way out, and it always picks the least convenient route.

The Usual Suspect: a Clogged Drain Line

Nine times out of ten, the culprit is the condensate line. It's a dark, damp pipe that sits unused for eight months of the year, which makes it a perfect home for algae and sludge. Once the buildup chokes off the line, the drain pan fills, then overflows, and the water heads for your furnace cabinet or the floor.

There's a clue that points to this cause: the leak shows up on the muggiest days. A partially clogged line might keep up when the AC is only pulling a little moisture, then fall behind when humidity spikes and the condensation doubles. If your floor is dry all June and wet during the first sticky week of July, think drain line.

Some homes have a condensate pump instead of a gravity drain, a small plastic box near the furnace with a tube running out of it. Pumps wear out. If yours hums but doesn't move water, or does nothing at all, the pan it feeds will overflow just the same.

When Ice Is the Real Problem

Sometimes the water isn't a drain problem at all. If the evaporator coil freezes over and then thaws, all that melting ice can overwhelm the pan at once, and you get a flood instead of a drip. In that case the puddle is a symptom of the freeze, and the freeze has its own causes, usually a dirty filter, blocked airflow, or low refrigerant.

The tell here is ice on the copper lines or the indoor unit, or an AC that runs constantly without cooling much. If that sounds familiar, the leak is the least of it. Our frozen coil guide walks through how to thaw it safely and what's behind it. Whatever you do, don't keep running a frozen system. That's how compressors die, and the compressor is the most expensive part in the whole setup.

The Drain Pan Itself

On older systems, the pan under the coil can rust through or crack. When that happens, water skips the drain entirely and drips straight down through the cabinet. This one is more common on systems 15 years and up, and it usually shows up as a slow, steady leak that doesn't care how humid it is outside.

A cracked pan isn't a patch-and-forget repair. Primary pans are part of the coil assembly on most units, so the fix ranges from a replacement pan to a bigger conversation about the coil. On a system that age, it's fair to weigh the repair against where the system is in its life.

The Float Switch: When the AC Shuts Itself Off

Many systems have a small safety called a float switch sitting in or above the drain pan. When the pan fills too high, the switch kills the system before the water goes over the edge. It's a five-dollar part that has saved a lot of ceilings, especially where the air handler lives in an attic.

So if your AC quit on a hot day and won't restart, and there's water in the pan, you may not have a broken air conditioner at all. You have a clogged drain and a float switch doing its job. Clear the drain, and the system comes back.

What You Can Do Yourself

A few things are safely on the homeowner side of the line:

1. Shut the cooling off at the thermostat and mop up the water so it stops spreading.
2. Check the filter. A dirty filter causes freezing, and freezing causes floods.
3. Look for the PVC drain line near the indoor unit. If there's a capped vent tee, pour a cup of plain white vinegar down it to discourage algae. Skip the bleach, it's hard on some pans and pumps.
4. If you own a wet/dry vacuum, seal it against the outdoor end of the drain line and run it for a minute or two. That clears a lot of routine clogs.
5. If there's a condensate pump, make sure it has power and its outlet tube isn't kinked.

If the line clears and the pan drains, run the system and keep an eye on it for a day. A cup of vinegar down the vent every couple of months during cooling season keeps most lines clean. It's also one of the items we handle on every AC maintenance visit, along with flushing the pan, so tune-up customers rarely make this particular call.

When to Call

Call a pro when the clog won't clear, when the pan or pump looks like the problem, when there's ice involved, or when the leak keeps returning after you've cleaned the line. Also call promptly if the unit sits above finished space. An attic air handler with a plugged drain is a ceiling repair waiting to happen, and the service call costs a lot less than the drywall.

We've been chasing summer water leaks around Jenison, Hudsonville, and the rest of the Grand Rapids area since 1987. Most of these are quick visits: clear the line, flush the pan, test the switch, done. When it's something bigger, we'll tell you exactly what we found and what it costs before we touch it. That's how an AC repair call should work.