Why is my air conditioner leaking water inside the house?

Your air conditioner pulls water out of the house every hour it runs. How much depends on how humid it is indoors and how long the system runs. When that water shows up on the floor, one part of a short path has failed: coil, pan, drain line, then outside or into a pump. Find where the water stopped moving and you've usually found the repair.

Trane and Carrier both list the same short set of causes. A clogged condensate drain line is the most common, then a damaged drain pan, a frozen evaporator coil, and, on systems that can't drain by gravity, a condensate pump that has quit or come unplugged.

First, turn it off at the thermostat

Trane's advice for a dripping air handler is to shut the system off at the thermostat or breaker before anything else, because running it keeps making water. Do that now. Then get the standing water up with towels or a wet/dry vac.

The clock matters more than the repair. EPA's mold guidance says wet materials dried within 24 to 48 hours usually won't grow mold. If water has soaked into drywall, carpet pad, or a ceiling below an attic unit, drying it within that window matters more than finding the cause the same afternoon. EPA also puts the limit on DIY mold cleanup at about 10 square feet. Beyond that, or after a lot of water damage, it's a job for a remediation pro.

Read the leak by where the water is

The drain-clearing and pan figures above are market ranges, not manufacturer prices. Carrier's own national range for common HVAC repairs is $150 to $1,200. Where your job lands in that range depends on access: a pan in a basement is a short job, but one above an attic furnace takes much longer.

The drain line: what's yours and what isn't

The condensate line is usually white 3/4-inch PVC. It leaves the indoor unit, runs through a trap, and ends outside near the foundation or in a floor drain. Over a cooling season, algae, mold, dust, and debris build up inside it until the flow stops. Carrier lists the warning signs: a musty or swampy smell from the vents, standing water near the indoor unit, and a system that shuts off on its own.

Two maintenance steps are safe for a homeowner, and both are on Carrier's list. You can pull the clog out with a wet/dry vac at the outdoor end of the line, following the equipment and vacuum manufacturer's instructions. Keep powered cleanup equipment away from wet electrical components. You can also pour a cup of distilled vinegar into the indoor access port, usually a capped tee near the coil. Trane suggests a cup of hot water down the line about once a month as a preventive habit. ENERGY STAR puts checking the condensate drain on its standard maintenance checklist, because a plugged drain causes both water damage and higher indoor humidity.

Stop there. If the vac doesn't pull the clog, or the line is glued into the unit with no cleanout, it needs a technician with a snake or pressure tools and the panel open. That service is also the time to fix how the line was installed. IRC Section M1411.3 requires the drain to slope at least 1/8 inch per foot toward its outlet. A line that sags, runs flat, or has no cleanout will clog again next summer.

The float switch that turned your AC off

If the system stopped cooling and there's water in the pan, a float switch probably shut it off on purpose. IRC M1411.3.1 requires overflow protection wherever a backup could damage the building. That usually means attic and closet installs. The code allows three setups:

• A secondary pan under the unit with its own drain line to a conspicuous spot, often over a window.
• A secondary pan with no drain, fitted with a UL 508 water-level switch that shuts the unit off before the pan overflows.
• A water-level switch on the primary drain that shuts the unit off when the drain blocks.

A common version is the RectorSeal Safe-T-Switch SS2. Its specs list a 24-volt, 3/4-inch float switch that fits onto the pan's auxiliary drain outlet and cuts the thermostat circuit when water rises.

Here's the rule: never bypass a float switch to get the cooling back on. The switch is the only thing standing between a clogged drain and a ceiling collapse. Clear the drain and the switch resets itself.

The same logic applies to a secondary drain that drips over a window. That stream means your primary drain is clogged. Clearing the primary drain fixes it.

Pans and pumps: when it's hardware

Drain pans corrode and crack with age, according to both Carrier and Trane. A cracked secondary pan is a fairly simple swap. A cracked primary pan is harder, because it sits inside or directly under the evaporator coil. Replacing it can mean pulling the coil, which is why that line in the table runs to $1,200.

If your quote for a primary pan comes in near that top figure and the system is old enough that you're already weighing replacement, get a written replacement price before you approve the pan. Our AC replacement cost guide covers what that number should include.

A condensate pump is only there when gravity can't carry the water to a drain, usually because the unit sits in a basement below the nearest outlet. Many furnace-and-coil setups drain by gravity and have no pump at all, so look for a small box with a tube before assuming you have one. Trane notes the pump may simply be unplugged, or its line kinked or blocked. Check the plug and the tubing. If the pump hums but doesn't move water, or doesn't run at all while its reservoir fills, it needs replacing.

If the coil is iced

A frozen evaporator coil leaks in a different way. The ice melts after the system stops, and more water arrives at once than the pan and drain were built for. The cause is restricted airflow, usually a clogged filter, or low refrigerant. Carrier recommends replacing filters every one to three months.

Let the coil thaw with the system off. Change the filter using a replacement that matches the equipment's specified size and rating. If it ices again within a day or two, it's probably a refrigerant problem, and that's a certified technician's job, not yours. The full diagnosis for that case is in our AC blowing warm air guide.

What to tell the technician

Before you call, note three things: where the water was, whether the system shut itself off, and whether there was ice on the copper lines. A technician who knows it's a tripped float switch and a clogged drain arrives with a vac and a snake, not a refrigerant cart.

Ask them to confirm the drain's slope and trap and to add a cleanout tee if there isn't one. A drain that clogs every year is usually an installation problem, not bad luck.

Research sources

• Trane: AC Leaking Water? Find Out Why and What to Do
• Trane: AC Condensate Drain Line Clogged?
• Trane: Air Handler Dripping Water: Causes and Solutions
• Trane: Furnace Leaking Water?
• Carrier: Why Is My AC Leaking Water?
• Carrier: How to Clean AC Drain Line
• ICC Digital Codes: 2021 IRC Section M1411.3, Condensate Disposal
• RectorSeal: Safe-T-Switch condensate overflow protection
• ENERGY STAR: Maintenance Checklist
• EPA: Mold Cleanup in Your Home
• EPA: A Brief Guide to Mold, Moisture and Your Home