Most common causes
- Clogged condensate drain line. Your AC pulls water out of the air. That water drains through a small pipe, and algae, dust, and sludge can block it so the pan overflows.
- Cracked or rusted drain pan. Older metal pans rust through, and plastic pans can crack. Water then leaks out before it reaches the drain.
- Frozen coil melting. When a frozen evaporator coil thaws, it can release more water than the pan handles at once.
- Failed condensate pump. Systems in a closet, garage, or attic without a downhill drain use a small pump. If the pump fails, water backs up.
- Disconnected drain fitting. A drain line that slipped off the pan fitting, often after other work in the attic, sends water straight onto the floor or ceiling.
- Dirty filter. A dirty filter can cause the coil to freeze and then overflow the pan as it melts.
What you can safely check
- Turn the system off at the thermostat to stop more water from forming.
- Look at the drain pan under the indoor unit. If it is full, soak up the water with a wet vacuum or towels.
- Find the float switch (a small safety switch on the pan or drain line). If the pan is full, it may have shut the system off on purpose, which is normal.
- Find where the drain line ends outside, often near the foundation, and check that it is not blocked by dirt or plants.
- If there is a cleanout cap on the drain line near the unit, pour a cup of plain white vinegar into it to help clear algae.
- Replace the air filter if it is dirty.
Call a technician if
- Water is coming through a ceiling or wall
- The drain pan is rusted, cracked, or full again soon after emptying
- The condensate pump is not running or is making noise without moving water
- You see ice on the coil or lines
- The system will not start after you cleared the drain (float switch still tripped)
What affects the repair cost
Clearing a clogged drain line is usually a simple service call. Replacing a condensate pump or float switch costs more, and replacing a drain pan depends on how hard it is to reach, since some pans sit under the coil. If ceiling or drywall damage happened, that repair is separate from the HVAC work.
