Finding your AC leaking water is alarming, especially when the stain is on a ceiling below the attic. The good news is that the cooling side of the system is often fine. Air conditioners pull a lot of moisture out of humid Houston air, and all of it has to drain away. When you see water where it should not be, a drain, pan, pump or switch along that path has failed, and catching it fast keeps a small repair from becoming a drywall and insulation job.
This guide is written by Houston Air & Heating, a free phone line rather than an HVAC contractor. If you need help, the line transfers your call to an independent local heating and air company, which comes out, quotes and does the work, and which you pay directly.
AC leaking water: what to do in the first ten minutes
- Turn the system off at the thermostat. Set it to Off, not just a higher temperature. A cooling coil keeps producing water as long as it runs.
- Stay clear of wet electrical items. If water is dripping near a light fixture, ceiling fan or outlet, keep people away from it. If you can reach the breaker panel without standing in water, switch that room's circuit off.
- Catch and soak up what you can. Buckets, towels and a wet vacuum on the floor. Move furniture, rugs, electronics and boxes out of the drip zone.
- Watch the ceiling. A sagging or bulging spot can hold a lot of water. Do not stand under it.
- Take photos. Pictures of the stain, the indoor unit and any standing water help the technician and may matter later for an insurance claim.
- Call for service. Tell the company where the indoor unit is (attic, closet or garage), whether the leak started suddenly, and whether the system had stopped cooling first.
Do not take apart the indoor unit, and do not run the AC to see if the leak comes back. If the air handler is in the attic and you choose to look, walk only on the decking or joists, use a flashlight, and look without touching anything electrical. Water sitting in a metal pan under the unit, or a small switch on the drain line, tells you a lot and is worth describing on the call.
Why your air conditioner makes so much water
Inside the indoor unit is the evaporator coil, a set of cold metal fins with refrigerant running through them. As the blower pushes warm, damp house air across it, the air cools and its moisture condenses on the fins, the same way a glass of iced tea sweats on a Houston porch. That water drips into a pan below the coil and runs out through a drain line, by gravity or with a small pump.
Drying the air is half of what makes a home comfortable here. On the Gulf Coast the outdoor air holds a great deal of moisture for most of the year, so a system in a Houston attic can send a steady stream of water down its drain on a summer afternoon. The more humid the day, the more water there is, which is why leaks so often show up in the stickiest weeks. According to the Department of Energy, a plugged drain also hurts humidity control, and the excess water it leaves behind may stain walls or carpet.
The parts that keep the water where it belongs
- Primary drain panSits right under the coil and collects the condensate. In older units it can rust through.
- Primary drain lineUsually a white plastic pipe that carries water outside or into a plumbing drain. It needs a steady downhill slope, and it is the part that clogs most often.
- TrapA U-shaped bend in the line on many systems that keeps air from being pulled through the drain. Sludge likes to settle there.
- Secondary (auxiliary) panA wider, shallow pan under the whole unit, common in attics, that catches overflow before it reaches the ceiling.
- Secondary drain lineOften ends somewhere you will see it, such as above a window or under an eave, so dripping there warns you the main drain has backed up.
- Float (overflow) switchA small water sensor in the pan or drain line that shuts the system off when water rises too high. It is the reason a backed-up drain often shows up as an AC that stopped working.
- Condensate pumpUsed where gravity cannot carry the water away, such as a unit in an interior closet far from an outside wall. If the pump fails, the pan fills.
What the residential code asks for
The International Residential Code, the model code for one and two family homes, deals with this directly in its section on condensate disposal, M1411. Where an overflow from the main drain could damage the building, which describes almost any attic unit above a ceiling, the code calls for a backup. The options include a second pan beneath the equipment that drains on its own line, a separate overflow drain connected to the equipment's pan, or a water level detection device that shuts the equipment off before water spills.
Texas law adopts the IRC, as it stood in 2012, as the residential building code for its cities, and each city can adopt later editions and its own local amendments. Houston and the suburbs around it each set which edition applies. You do not need to read the code yourself. Ask the company whether your setup has the backup protection the local code requires, and if a new system or air handler is going in, confirm it is part of the permitted job.
Common causes of an AC leak
| Cause | What you might notice | What usually fixes it |
|---|---|---|
| Clogged primary drain line | Pan full, water spilling over, or the float switch has shut the system off | Clearing and flushing the line, checking the slope and trap |
| Rusted or cracked drain pan | Water under the unit while the pan looks only partly full; common on older systems | Pan repair or replacement |
| Disconnected or sagging drain line | Water near a joint, often after other work in the attic or closet | Reconnecting and resupporting the line |
| Frozen coil thawing | A sudden flood after the system stopped cooling; ice on the copper line | Fixing the airflow or refrigerant problem that caused the freeze |
| Failed condensate pump | Pump not running, its reservoir overflowing | Pump repair or replacement |
| Dirty filter or coil | Weak airflow, ice, or water blowing off the coil | New filter, coil cleaning |
A furnace can leak water too. High-efficiency gas furnaces make their own condensate and drain it through the same kind of line, so a puddle near the furnace in winter is worth the same attention.
One thing a water leak is not: a refrigerant leak. Refrigerant escapes as a gas and does not puddle. It shows up instead as weak cooling, long run times and sometimes ice on the lines. If the leak began after the system iced over, both problems may be present, and the AC not cooling guide covers the ice side.
Water, mold and how fast to act
The EPA's guidance on mold is simple: control the moisture and dry water-damaged materials and items within 24 to 48 hours to prevent mold growth. Mold needs moisture to grow, so stopping the leak is the first step and drying is the second. The EPA also suggests keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, which a properly working air conditioner helps with.
Attic insulation and the paper face of drywall soak up water and dry slowly. If a ceiling stayed wet for days, or you can smell mustiness, ask whether damaged materials should be opened up or replaced. For a large moldy area, the EPA recommends following its guidance for bigger cleanups rather than a quick wipe.
What a technician does on a leak call
A careful technician confirms where the water is coming from before fixing anything. That usually means checking the primary pan and line, finding the clog and clearing it, testing that water flows freely to its outlet, inspecting the secondary pan and line, and testing the float switch so it really shuts the system off. If the coil froze, the visit should also cover why: the filter, the blower and the refrigerant charge. You should leave with a clear note of what failed and what was done.
Ask the company for its price in writing before the work starts, and for its Texas license number. The AC repair cost guide shows published ranges for drain line cleaning and drain pan replacement so you can sanity check a quote.
Verify the license before work begins. Texas requires anyone contracting to do air conditioning, refrigeration or heating work to hold an Air Conditioning and Refrigeration (ACR) Contractor license from the Texas Department of Licensing and Regulation (TDLR). Ask the company for its ACR license number, then confirm it on the TDLR license search (opens in a new tab). The line does not check this for you.
How to prevent the next leak
- Change the filter on time. Dust that gets past a clogged filter coats the coil and feeds the sludge that blocks the drain.
- Glance at the outside drain on hot days. While the AC runs in summer, the condensate line should drip. If it goes dry during a humid stretch, the water may be backing up inside.
- Watch the overflow pipe. Water from the pipe above a window or eave means call soon, before the backup gives out.
- Keep the area around an attic unit clear so a technician can reach the pan and drain easily.
- Schedule a yearly cooling checkup. ENERGY STAR's maintenance checklist includes inspecting the condensate drain, since a plugged drain can cause water damage. A tune-up is a good time to have the drain flushed and the float switch tested.
Water on the ceiling or around the indoor unit? Turn the system off and call the line. You will be put through to a local HVAC company that can find the source and give you its price before it starts.
- Dial the numberAn automated assistant picks up (832) 662-4107 at any hour and asks what you need help with.
- Say what is wrongTell it whether the trouble is cooling or heating, where in the Houston area you are, and what the system is doing.
- Reach a local companyYour call is transferred to an independent, locally owned HVAC company, which gets a short summary of your problem before it answers.
- Work it out with themThat company inspects, quotes, repairs and bills you directly. If nobody can pick up, your name, number and problem are passed on for a call back.
About this line: Houston Air & Heating is a free phone line, not an HVAC contractor. It answers your call and connects you with an independent local heating and air conditioning company, which inspects, quotes and does the work, and which you pay directly. Companies may pay a referral fee. You never pay the line. Read how the line works.