Water around the indoor unit while the AC runs comes from one place: the indoor coil, which pulls moisture out of the air and is supposed to drain it outside. When you see water on the floor, in the closet, or dripping from the ceiling below an attic unit, the drain path has failed, the pan has failed, or the coil froze and is now melting. Those three cover nearly every call we get for this in Pasadena, CA and across the San Gabriel and San Fernando valleys, and the first one is by far the most common.
Where the water comes from
Your evaporator coil runs cold. Air blowing across it drops its moisture on the coil surface, the way a cold glass sweats. That water runs down into a drain pan under the coil, then out a PVC drain line to the outside of the house, often to a small pipe you have seen dripping near the foundation or at the edge of the roof.
Our climate is dry, so the system makes less water than it would somewhere humid. But on a muggy monsoon afternoon in July or September, or a long run on a 100°F day, it still makes gallons. Enough to soak a hallway carpet or stain a ceiling.
Cause one: the drain line is clogged
The drain line is a small pipe with a slow trickle of water in it, sitting in a warm attic or closet for months. Algae, dust from the coil, and rust flakes build up until the trickle stops. Water backs up in the pan, the pan overflows, and you see it wherever the unit lives.
- Closet or garage air handler: water on the floor around the unit.
- Attic unit, common in 1950s and 60s ranch homes: a water stain on the ceiling below, or water dripping from a ceiling register.
- Rooftop package unit on a flat roof: water pooling on the roof, sometimes finding its way inside through the duct penetration.
Many systems have a secondary drain line, which comes out of the pan higher up and terminates somewhere you will notice it, often above a window or doorway. If you see water dripping from a pipe above a window, that is not a leak. That is the secondary drain telling you the primary is clogged. Fix it before the secondary clogs too.
Some systems have a float switch in the pan or on the drain line that shuts the AC off when water rises. If your AC quit on a hot day for no obvious reason and you find water in the pan, the float switch did its job.
Cause two: the pan is rusted or cracked
Older coils sit in a metal primary pan. Years of water and a bit of rust, and the pan develops a pinhole. Water drips out below the pan instead of going to the drain. Plastic pans crack, especially after a decade of attic heat. A pan leak looks like a drain clog but keeps happening after the line is cleared, and the water shows up at a spot that is not the drain outlet.
The secondary pan, the wide shallow tray under an attic air handler, also rusts. If that one fails, there is nothing left between the water and your ceiling.
Cause three: the coil froze and is melting
A coil that ices over for a few hours holds a lot of water. When the system shuts off or the ice finally lets go, it melts faster than the pan and drain can handle and overflows. You will usually see the ice, or the frost on the copper line at the unit, before the puddle. The freezing itself traces to airflow or refrigerant: a clogged filter, a dirty coil, a weak blower, or a low charge from a leak. Clearing the water does not fix any of those.
Less common causes
- Drain line with no slope, or sagging. A line that runs flat or dips in the attic holds water and clogs faster. Common on installs where the line was just laid across the joists.
- Disconnected drain. A PVC joint that was never glued and finally came apart. Attic heat and vibration do this.
- Dirty coil. A coil coated in dust sheds water in the wrong places, dripping off the edges instead of into the pan. A coil cleaning is the fix.
What you can check yourself
- Turn the AC off at the thermostat. Running it keeps making water.
- Look at the unit. Is there water standing in the pan? Is there ice on the coil or copper lines? Ice means leave it off with the fan on "on" until it thaws.
- Replace the filter if it is dirty.
- Find where the primary drain exits outside. If nothing drips from it while the AC runs on a warm day, it is clogged.
- If the drain line has a cleanout, usually a PVC tee with a cap near the air handler, remove the cap and pour a cup of plain white vinegar in. Wait half an hour. This clears mild algae buildup.
- At the outside end of the drain line, a wet/dry vacuum held tight over the pipe for a minute or two often pulls the clog out. Sealing the connection with a rag or your hand makes it work.
- Check for water dripping from a secondary drain outlet above a window or door. If it is dripping, the primary is clogged.
Do not pour bleach into a drain line that runs through a metal pan, do not remove the coil access panel to get at the pan, and do not run the system with water standing in the secondary pan.
When to call a licensed tech
- The vinegar and the vacuum did not clear it, or it clogged again within weeks.
- Water shows up somewhere other than the drain outlet after the line is clear. That is a pan.
- The coil has frozen. The freezing has a cause, and it needs measuring.
- Water is coming through a ceiling. The secondary pan or its drain has failed and the next overflow goes straight into the drywall.
- The float switch has shut the system off and you cannot find why.
- You want the drain line flushed, the pan checked, and the coil looked at before summer instead of during it. That is a normal part of a maintenance visit. If it is already leaking, an AC repair visit starts with the drain and works back from there.
Common questions
Why is my AC leaking water only on hot days?
Because it makes more water on hot days. A partial clog passes a trickle on a mild afternoon and backs up when the coil runs hard. It is on its way to a full clog.
Is it safe to keep running the AC if it is leaking?
No. Every hour it runs adds water to a pan that is already full, and that water goes into the floor, the ceiling, or the furnace cabinet below. Turn it off and get the drain cleared. On a heat wave day that is a hard call, but a wet ceiling is worse.
How often should the drain line be cleaned?
Once a year before summer is enough for most homes. A cup of vinegar in the cleanout every couple of months during the cooling season keeps algae from getting a start. Systems in attics with a flat drain run may need it more often.
Our diagnostic visit is $69, quoted before booking and credited toward the work. Call Boost Services Heating and Cooling at 818-277-5056 or contact us here.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.