Why Is My AC Leaking Water Inside the House?

Water on the basement floor in July usually traces back to the air conditioner, even though the part making the water sits indoors. The evaporator coil that cools your house rides in a box on top of the furnace, and it pulls moisture out of the air every minute it runs. On a humid day near the lake that adds up to a steady trickle. As long as the drain path works you never see it. When the floor is wet, something in that path has failed, and most of the causes are straightforward air conditioning repair.

Where does the water from a central air conditioner come from?

The water comes from the humidity in your house air condensing on the cold indoor coil. It is the same thing that happens to a cold glass on a summer day. The water runs off the fins into a drain pan under the coil, then out through a condensate line to a floor drain, a laundry sink or a small condensate pump that lifts it to a drain. Nothing about that water is a leak in the refrigerant sense. It is the system dehumidifying the house as designed.

That also explains the timing. A leaking air conditioner leaks while cooling runs and on the days it works hardest, which in the Twin Ports means the muggy stretches of midsummer. If the puddle appears only then and dries out when the system is off, the drain path is your suspect.

What should you do first when your AC is leaking water?

Turn the air conditioner off first, then protect the floor. Every minute cooling runs it adds more water to a drain that is not keeping up, and the water is landing on or beside a furnace cabinet full of wiring.

What causes an AC to leak water inside?

A clogged condensate drain line causes most indoor AC leaks, followed by a damaged drain pan, a failed condensate pump and a line that has come apart. The table sorts the common causes by where you will see the water.

AC leaking water inside: cause, where it shows and who fixes it

Likely cause	Where the water shows up	Homeowner or technician
Drain line clogged with algae or slime	Around the coil box and furnace base, AC may shut off	Technician clears and flushes it
Cracked or rusted drain pan	Drips inside or under the cabinet even with a clear line	Technician
Failed condensate pump	Pump reservoir full and overflowing	Homeowner can check, technician replaces
Disconnected or kinked drain line	At a joint, a fitting or along the line's route	Homeowner can spot it, technician reconnects
Frozen coil melting	Sudden large puddle after the system shuts down	Homeowner thaws, technician finds the cause
Missing or plugged secondary pan or float switch	Water on a ceiling or floor below an attic or closet unit	Technician

The clogged drain line is the classic. The condensate line stays wet and dark all summer, which is perfect ground for algae and slime to grow. Over a season or two it narrows the line until the pan fills faster than it drains. A drain pan fails more slowly: older metal pans rust through, and plastic pans can crack from age and heat. Either way the water finds the low point, which is usually the furnace base.

What can a homeowner safely check on a leaking AC?

A homeowner can safely check the float switch, the drain line outlet, the condensate pump reservoir and the line itself, all with the power off. None of them require opening the furnace or the coil box.

• The float switch. Many systems have a small float switch on the drain line or pan that shuts cooling off when water backs up. If the AC stopped and you find standing water, the switch worked. Leave it in place; it is telling you where the backup is.
• The drain line outlet. Follow the condensate line from the coil box to where it empties, usually a floor drain or a laundry sink. If nothing drips while cooling runs on a humid day, or the end is buried in debris, the line is likely plugged upstream.
• The condensate pump reservoir. If the line runs to a small pump box, look at the reservoir. Water to the brim with the pump silent points to a failed pump, a stuck float or a blocked discharge tube.
• The line's route. Look along the exposed tubing for a joint that has pulled apart, a kink, or a section sagging so water pools instead of flowing.

What stops the homeowner list is anything inside a cabinet: the pan itself, the primary drain connection at the coil, the trap and the pump's wiring. Those sit beside live wiring and delicate coil fins, and they are quick work for a technician with the system powered down.

Can a frozen AC coil cause water on the floor?

Yes, a frozen evaporator coil can dump a large amount of water all at once when the ice melts. When airflow is restricted or the refrigerant charge is low from a leak, the coil ices over. Once the system shuts off, or the ice finally lets go, the melt overwhelms the drain pan and spills over. The tell is a sudden puddle after a period of weak cooling, sometimes with frost visible on the copper line outside. The causes and the thaw procedure are in why an air conditioner freezes up, and the fix is finding what froze it, not just mopping.

Is the water from the AC, the furnace or a mini-split?

The season and the equipment tell you which system is leaking. Water around the furnace in summer with cooling running is almost always the air conditioner's drain path. Water in winter comes from somewhere else, because the air conditioner is not running.

A high-efficiency condensing furnace makes its own condensate all heating season and drains it through a separate trap and line, so a winter puddle points at the furnace, as covered in why water shows up around a furnace. A ductless system is different again: each wall-mounted head has its own drain pan and line, and a drip down the wall below a head is its own problem, explained in why a mini-split leaks water inside.

When does a leaking AC need a technician?

A leaking AC needs a technician when the drain outlet is dry, the pump is full, the float switch has tripped, or the water is coming from inside the cabinet. Those all mean a blocked line, a failed pump or a damaged pan, and each involves working inside equipment beside live wiring. Water that has reached the furnace's control board or blower motor also needs a look before the system goes back on.

A service call is $89, credited toward the repair when you approve the work. The technician clears and flushes the drain, checks the pan, tests the pump and the float switch, and checks the coil for icing that would point to an airflow or refrigerant problem. Most drain problems are small repairs, and a leak caught early spares the furnace and the floor. When the puddle comes back, book AC repair rather than emptying buckets all summer.