Short answer: A running air conditioner pulls gallons of water out of the air every day, and it leaves through a small condensate drain line. When that line clogs with algae, the water backs up, fills the drain pan and overflows onto whatever the air handler sits on. In an attic unit that is your ceiling. Clearing the line is a fifteen-minute job and often DIY; the platform, insulation and ceiling it soaked are a separate repair.
What is actually going on
The evaporator coil inside the air handler runs cold and wet, and the condensation drips into a pan that drains through a PVC line, usually 3/4", to the outside or to a plumbing drain. In a Lowcountry summer that line carries water continuously, warm, in the dark, which is exactly what algae wants. It grows a slime plug, the water backs up, and the primary pan overflows.
Newer installations have a secondary pan under the whole unit with its own drain (the pipe that drips over a window or door: that drip is a warning, not a leak) and often a float switch that shuts the system off before the secondary pan overflows. Older installs, and plenty of newer ones in Summerville, Mount Pleasant and Fort Mill subdivisions, have a secondary pan with no float switch, or a float switch that was never wired. Then the first sign is a stain on the ceiling under the attic.
Two other causes look the same: a frozen coil (low refrigerant or a filthy filter) that thaws and dumps water all at once, and a cracked primary pan, common on units over about fifteen years old.
How to check it
- Turn the system off at the thermostat. Every minute it runs is more water.
- Go to the air handler (attic, closet, garage). Water in the secondary pan, or on the platform or floor, confirms it.
- Find the drain line. A white PVC pipe leaving the unit, usually with a vertical tee and a cap near the unit. Take the cap off. Water standing in the tee means the line is blocked downstream.
- Check the filter and the coil. A solid block of ice on the coil means it froze; that is a service call, not a drain problem.
- Look at what the water reached. The wood platform under the unit, the insulation around it, the drywall below. Press the platform; if it is soft, it has been wet before.
The honest DIY version
Clearing the line: with the system off, put a wet/dry vacuum on the outside end of the drain line (where it exits the house, often near the condenser) and run it for a minute; you will hear the plug come out. Then pour a cup of white vinegar into the tee at the unit. Every month or two in summer, another cup of vinegar keeps it clear. That is genuinely all most of these need, and it is worth learning.
What not to do: bleach in the line (it attacks the pan and some coils), running the system with the pan full, or ignoring the drip from the secondary drain over the window. That drip means the primary is already blocked.
When it is serious
- The ceiling below is sagging or dripping. See water leak in the ceiling: relieve it, kill the power to that room.
- The coil is frozen. Turning it off to thaw is fine; running it frozen damages the compressor. Then get it serviced.
- The platform is soft or the unit has shifted. A 150-lb air handler on a rotted platform in an attic is a real hazard.
- It has happened more than once: install a float switch. It costs less than any ceiling repair.
What is left after the HVAC tech leaves — and who does it
The HVAC company clears the line, maybe replaces the pan, maybe wires a float switch. They will not rebuild the platform, replace the soaked attic insulation, or repair and repaint the ceiling. That is general-contractor work: drywall and ceiling repair, attic insulation replacement, a new platform built to hold the unit level with a proper secondary pan under it, and where it has run long enough, water damage restoration with mold treatment. We do all of it, and we coordinate the HVAC company if the unit needs to come up to rebuild the platform.
Charleston and York County notes
Charleston: condensate volume is enormous here (a 3-ton system can produce 10–20 gallons a day in August) and the algae season runs April to October. Attic air handlers are the norm in newer Mount Pleasant, Daniel Island and Summerville houses, so the overflow lands on a bedroom ceiling. The vinegar routine is not optional in this climate. York County: same failure, shorter season, and the more common variant we see in Rock Hill and Fort Mill is the unit in a garage or closet, where the overflow goes into the wall cavity behind it and shows up as musty drywall rather than a ceiling stain.
Sources
- US Department of Energy — maintaining your air conditioner: energy.gov/energysaver/maintaining-your-ai…
- International Residential Code M1411.3 — condensate disposal and auxiliary drain pans: codes.iccsafe.org/content/IRC2021P1/chapte…
General guidance for homeowners in Charleston and York County, South Carolina, written from what we see on jobs. Conditions vary house to house; anything structural, gas or electrical is licensed work in South Carolina and should be looked at in person.
Frequently Asked Questions
Why is my AC leaking water inside?
Almost always a clogged condensate drain line. Algae blocks the line, the drain pan fills and overflows. Less often, a frozen coil that has thawed, or a cracked pan on an old unit.
Can I unclog the AC drain line myself?
Yes. System off, wet/dry vacuum on the outside end of the line for a minute, then a cup of white vinegar into the tee at the air handler. Repeat the vinegar every month or two through summer.
Why is water dripping from a pipe over my window?
That is the secondary (emergency) drain from the pan under the air handler. It only drips when the primary drain is blocked, so it is a warning: clear the primary line now, before the secondary pan overflows too.
Who repairs the ceiling after the AC overflowed?
The HVAC company clears the drain and fixes the unit. The ceiling, the wet insulation and the platform are general-contractor work, which is what Southeastern Renovation does in Charleston and York County.
Is AC condensate damage covered by insurance?
Often not, because insurers treat a clogged drain as maintenance rather than sudden damage. Some policies cover the resulting ceiling damage from a single overflow event. Check the wording and photograph everything.