Why Is My AC Leaking Water? Causes and What to Do First

Water coming from an air conditioner alarms people, and it should, because the damage adds up fast. The good news is that the most common cause is also the cheapest to fix and largely preventable.

Do this first#

Switch the system off at the thermostat.

That stops the coil from producing more condensate, which stops more water reaching your drywall. Everything else can wait a few minutes.

Then put a container under the drip and pull anything valuable out of the way. If water is coming through a ceiling, the drywall above is already saturated, and it may come down.

Why your AC produces water at all#

This part is normal and worth understanding.

Warm humid air passes across the cold evaporator coil, and moisture condenses out of it exactly the way it beads on a cold glass. That condensate collects in a pan under the coil and drains away through a PVC line, usually to the outside of the house.

In the Rio Grande Valley, where relative humidity regularly sits above 70 percent, a residential system can pull several gallons of water out of the air on a summer day. That volume is why drain problems are so much more common here than in dry climates.

Water is supposed to be produced. It is only a problem when it does not leave.

Cause 1: clogged condensate drain line#

This is the cause in the large majority of cases.

The drain line stays damp all summer, which is ideal for algae and biological growth. Combined with dust that gets past the filter, it eventually forms a plug. Once the line blocks, the pan fills and overflows.

Signs: water directly under or around the indoor air handler, standing water in the drain pan, or a system that shuts itself off (that is the float switch doing its job).

Fix it yourself: Find the drain line where it exits your house, usually a PVC pipe near the outdoor unit or on an exterior wall. Seal a wet/dry vacuum hose over the end and run it for two or three minutes to pull the blockage out. Then find the access point near the indoor unit, usually a capped tee, and pour in about a cup of distilled white vinegar.

Prevent it: Repeat the vinegar step every two months during cooling season. This is the single most valuable five minute maintenance task a Valley homeowner can do.

Cause 2: the float switch did its job#

Many systems have a safety float switch in the drain pan or in the drain line. When water backs up, it cuts power to the system.

If your air conditioner stopped working and you find standing water in the pan, this is what happened. The switch prevented a ceiling leak.

What to do: Clear the drain as described above. Once the water drains, the switch resets and the system runs again. If it trips again quickly, the line is still blocked.

Some homes do not have a float switch. If yours does not, adding one is inexpensive and it is the difference between a shutdown and a ceiling repair.

Cause 3: frozen evaporator coil, now thawing#

If the coil froze into a block of ice, that ice eventually melts, and it can produce far more water at once than the pan and drain were designed to handle. The overflow looks like a leak.

Signs: you noticed poor cooling before the water appeared, or you can see ice on the copper refrigerant line.

What to do: Turn cooling off but set the fan to ON so the coil thaws, which usually takes two to four hours. Replace the filter and open all supply registers.

Then deal with why it froze. Freezing means restricted airflow or low refrigerant, and if it is refrigerant, there is a leak that needs finding.

Cause 4: cracked, rusted, or misaligned drain pan#

Drain pans sit in constant moisture for years. Older metal pans rust through, and plastic pans crack.

Signs: water is present even though the drain line flows freely.

What to do: This one needs a technician. The pan usually sits directly under the coil, and replacing it often requires partially removing the coil.

Cause 5: disconnected or improperly sloped drain line#

Drain lines rely on gravity. If a line has come apart at a joint, or if it was installed without consistent fall, water pools in the line or exits somewhere it should not.

Signs: water in the attic near the ductwork, or a wet spot on the ceiling away from the air handler.

What to do: Technician. Re-sloping a line frequently means re-running part of it.

Cause 6: failed condensate pump#

Where gravity drainage is not possible, such as an air handler in a closet below the drain point, a small pump moves the water. Pumps fail, and when they do, the reservoir overflows.

Signs: a small box near the indoor unit with water around it, sometimes with a humming noise or no noise at all.

What to do: Pumps are replaceable and not expensive. Most have a safety switch that can also be wired to shut the system down before an overflow, which is worth asking about.

Cause 7: dirty air filter#

Worth a mention because it is free to check.

A badly clogged filter restricts airflow enough to freeze the coil, which then thaws and overflows. This is the same failure path as cause 3, but with an obvious and free first step.

Cause 8: poor duct insulation#

Not technically a leak, but it looks like one.

When ductwork in a 130 degree attic is not properly insulated, or the insulation has torn away, moisture condenses on the outside of the cold duct and drips. You get water on a ceiling with nothing wrong with the drain system at all.

Signs: wet spots on the ceiling away from the air handler, worst on the most humid days.

What to do: The duct insulation needs repairing or replacing. This is common in older Valley homes where insulation has degraded.

Decision guide#

When to call rather than troubleshoot#

• Water is dripping through a ceiling or running down a wall
• The drain line will not clear after vacuuming
• You see ice and it returns after thawing
• The pan is visibly cracked or rusted through
• There is a musty smell alongside the water, which suggests moisture has been present for a while

That last point matters in this climate. Wet building materials in Valley humidity grow mold quickly, so a slow leak that has been going on for weeks is a bigger problem than the water you can see.

The short version#

Turn the system off, then check the drain line. Clogged condensate lines cause most AC water leaks in the Rio Grande Valley, and a wet/dry vacuum plus a cup of vinegar clears most of them. Doing the vinegar step every two months means you will rarely have this problem again.