AC Leaking Water Inside the House: Causes and What to Do

An AC leaks water inside when the water its indoor coil pulls out of the air doesn't make it down the drain. The usual causes are a clogged trap or drain line, a condensate pump that quit, a frozen coil overflowing its pan as it thaws, or a pan that's cracked or rusted through, so turn the cooling off, keep the water away from the furnace wiring, find where it starts, and then clear the drain or call.

Some water is supposed to be there, though, so start by sorting normal from not.

Is it normal for a central AC to drip water?

Every central AC makes water. Trane puts it plainly: as house air passes through the evaporator coil, the coil absorbs heat and water from it, and the condensed water leaves through a separate drain. On a humid Minnesota afternoon that drain carries a steady trickle.

Normal:

• A trickle at the end of the drain line, at a floor drain, a laundry tub, a condensate pump or a pipe through the wall
• A condensate pump that kicks on for a few seconds every so often
• Condensation dripping off the cold copper line and its valves at the outdoor unit on a muggy day

Not normal:

• Water on the floor around the furnace or under the coil box
• Water inside the furnace cabinet, or rust streaks down its sides
• A pump that's full and silent, or one that never stops
• A stain on a ceiling below an attic or closet unit, or water down the wall under a ductless head

ENERGY STAR's maintenance checklist warns that a plugged condensate drain can cause water damage and affect indoor humidity, so chase a leak the same day.

What to do right now

1. Turn cooling off at the thermostat. Leave the fan off for now so the coil stops making water.
2. Protect the furnace. If water is on or under the furnace, switch it off at its service switch, but only if you can reach the switch without standing in water. If water has reached an outlet, a cord or the electrical panel, stay out of it and call.
3. Soak it up. Towels, a mop or a wet/dry vacuum, and move stored boxes off the floor. EPA advises drying water-damaged areas and items within 24–48 hours to prevent mold.
4. Find where it starts. Follow the water uphill: coil box, trap, drain line, pump. The table below matches what you find to the usual cause.
5. Clear the drain. Most leaks are a slimed-up trap or line. Our guide to clearing a clogged condensate drain walks through flushing the cleanout and vacuuming the line from its far end.
6. If you find ice, let it melt. Cooling off, fan set to On, towels around the furnace. It can take several hours.
7. Restart and watch. Run cooling for 20 minutes. You want a steady trickle at the end of the line and nothing collecting under the coil.

If the AC went dead along with the leak, a float switch in the pan, on the drain line or in the pump probably shut it off on purpose, before the floor filled. It usually resets once the water drains. Never bypass one to get the AC running. If your system kept running while water spilled, it has no switch or a failed one, and it's worth adding one. An attic or upstairs-closet air handler should always have a secondary pan with its own switch or drain, because water up there has nowhere to go but through a ceiling.

Where the water is, and what it usually means

When it isn't the drain

A frozen coil

A Department of Energy field guide notes that a system low on refrigerant can form ice on the evaporator coil, and that a coil fouled with dirt cuts airflow until it freezes over. The ice can build out past the edges of the pan, and when the system shuts off it can melt faster than the drain can carry it. Replace the filter and make sure the blower is moving air. If the coil refreezes within a day, the cause is refrigerant or airflow that needs a technician; our guides to an AC running but not cooling and a leaking evaporator coil cover both. Refrigerant leaves no puddle, so a water leak and a refrigerant leak are separate problems.

A cracked or rusted pan

Many older coils sat in steel pans that can rust through, and plastic pans can crack with age. Water then drips inside the coil box instead of reaching the drain, and on a typical basement furnace it lands on the furnace below. Rust inside the furnace under the coil means it has been going on a while. Some pans can be replaced by themselves; on other coils the pan is part of the assembly and the fix is a coil.

Sweating metal

Cold surfaces in a humid basement pull water out of the air: a copper line with split insulation, the coil box, the supply plenum, bare cold water pipes. It looks like a leak but isn't. EPA's advice is to insulate cold surfaces such as cold water pipes and keep indoor humidity below 60 percent, ideally 30 to 50. A basement that stays damp all summer may need a whole-house dehumidifier.

The humidifier, the furnace or a ductless head

Many Minnesota furnaces carry a whole-home humidifier, and a stuck water valve or plugged drain can dribble water down the furnace in July. Aprilaire's summer shutdown for its bypass models is to close the bypass damper, with a manual control turned off, and closing the small water valve that feeds it stops the flow until it's fixed. A high-efficiency furnace makes its own water all winter, often through the same pump; for heating-season leaks, see why a furnace leaks water. A ductless mini-split head drips when its drain clogs, kinks or loses its slope.

Where do AC leaks usually start in Minnesota houses?

• The floor drain is across the room. Many older basements have one floor drain, so the coil drains into a condensate pump. With a high-efficiency furnace on the same pump, it works all winter and all summer, and a tired pump can quit in the first muggy week of June.
• Attic air handlers. In story-and-a-half and radiator-heated houses in Minneapolis and Saint Paul, central air can come from an air handler in the attic, where a clog drips through plaster. Our guide to high-velocity systems covers that setup and why the secondary pan matters.

How much does it cost to fix an AC water leak?

National cost guides give a sense of scale. HomeGuide's typical national ranges are $75–$250 to clear a condensate drain line and $100–$450 to replace a condensate pump. If a rusted or cracked pan means a new coil, HomeGuide puts evaporator coil replacement at $2,500–$4,500 out of warranty and $1,000–$2,500 with the coil under warranty. Those are national figures, not our prices, and our Minneapolis HVAC cost page covers the condensate pumps common in low basements far from a floor drain.

With us, a repair visit carries a $79 trip charge, waived when you approve the repair, and you get a written flat-rate price before any work starts. Completed repairs carry our written 1-year parts-and-labor warranty. A spring AC tune-up is $129, or $219 with the furnace, and includes clearing the condensate drain line and checking the pan under the coil, the easiest way to catch a slow drain before it overflows.

When should you call a technician about an AC leak?

• Water inside the furnace cabinet or on the control board
• A stain or drip on a ceiling below an attic or closet unit
• Ice that comes back within a day of thawing
• A pump that won't run after you've cleaned the float
• A leak that returns within days of clearing the drain
• A musty smell with visible growth in the pan or coil box. EPA advises against running an HVAC system you suspect is contaminated with mold, since it can spread mold through the house.

Call 612-429-3465, open 7 days, 7am–9pm. The price is the same from AC repair in Minneapolis to AC repair in Saint Paul.

Questions

Sources

• EPA advises epa.gov
• HomeGuide cost guides (2026 editions) homeguide.com
• ENERGY STAR energystar.gov
• U.S. Department of Energy energy.gov