AC Leaking Water: Why It Happens and How to Stop It

First: Water Is Supposed to Be There

Before diagnosing a leak it helps to know why there is water at all, because most people assume its presence is the fault.

Cooling air also dries it. Warm humid air passes across a coil that is well below the dew point, moisture condenses out of it, and that water collects. In humid weather a mid-size unit can produce several quarts a day, which is a lot of liquid to be generating inside a wall or a window frame. So the water is never the fault by itself. The fault is always in where it goes. Every cause below is a different version of the same failure: the drainage path stopped working.

AC Leaking a Lot of Water: Is That Normal?

How much water you see depends on the weather, not on the health of the machine. On a dry day the drain barely drips, and on a muggy one it runs in a thin stream for hours. So an AC unit leaking a lot of water at the pipe outside is doing more work, not failing. The same amount on the floor indoors is a different story, because it means all of that water is missing the drain. Tip: judge a leak by where the water is, never by how much.

What to Do When Your AC Is Leaking Water

The first ten minutes matter more than the diagnosis. Do these in order, whatever kind of unit it is.

1. Switch the cooling off at the thermostat. No cooling means no new water. If you can see ice, set the fan to ON so room air melts it.
2. Catch what is already there. Towels and a bucket under a ceiling drip, and move anything electrical away from the puddle.
3. Look at the filter. A filter gray with dust is the cheapest cause on this page, and it takes ten seconds to rule out.
4. Find the drain. On a central system that is the pan under the indoor coil and the small pipe leaving it. On a window unit it is the back edge of the base. The pipe does not always end outside: in some houses it runs to a sink drain or drops under a raised floor, so follow it from the pan.

People describe this in different ways: an AC unit leaking water, a leaking air conditioner, HVAC leaking water, or just an AC leak. Anyone looking for an AC leaking fix starts with the same four steps, because they work before you know the cause.

That is how to stop an AC from leaking water for today. It stays stopped only when the cause below is fixed, and for an indoor leak the system should stay off until then. An air conditioner dripping water outside can keep running. One leaking water inside should not.

Central AC Leaking Water

An air conditioner leaking water indoors on a central system starts at the drain pan. The evaporator coil sits above that pan, which empties through a primary drain line and, on most installations, has a secondary line as backup. Algae is the usual culprit: the line stays wet and dark, growth builds, and the primary blocks.

It is the same fault whether you found an AC unit leaking water inside, an air conditioner dripping water inside a closet, or water leaking from the AC unit inside the garage. Condensation from the AC always ends up in that pan first, so an AC dripping water inside means the pan could not empty.

What matters here is what should have happened next. A correctly installed system has a float switch that senses the rising water and shuts the cooling off before anything overflows. So water on the floor tells you two things, not one: the drain is blocked, and the safety that exists to prevent exactly this either was never fitted or has failed. Clearing the drain without addressing that leaves you with no protection the next time it happens, and the next time will usually cost more than a drain clean.

A Failed Condensate Pump or a Cracked Drain Pan

Two causes look like a blocked drain and are not. Where the indoor unit sits in a basement, below the level of any drain, a small condensate pump collects the water in a tank and pushes it up and out through a thin tube. When that pump fails or its tube blocks, the tank overflows beside the furnace. With a condensate pump not working, check that it has power and that its float moves freely before assuming the drain is at fault.

The other is the pan itself. A metal pan under an older coil rusts, a plastic one cracks, and either one then leaks with a perfectly clear drain line. Shine a light along the pan while the system runs: water on the outside of the pan, with the drain flowing, means the pan. Both are parts a technician replaces: a new condensate pump runs $100 to $450 and a drain pan $200 to $600, and the AC repair cost calculator adjusts both for your region. Tip: if clearing the drain changes nothing, look at the pump if you have one, then at the pan.

Why the AC Drain Line Keeps Clogging

Clearing the pipe fixes today's leak and often nothing more. Owners describe flushing the line every month for years and still finding it blocked again, and renters describe an AC that shuts down several times a summer for the same reason. That pattern has one explanation: the blockage is being made upstream.

The slime that plugs the pipe grows in the drain pan and on the wet coil above it, then washes down in pieces. Vacuum the pipe and you remove the plug, not the source. So when a drain line clogs more than once in a season, the pan and the coil need cleaning, and the pipe needs checking for a sag where water sits between cycles. Tip: one clog is maintenance. A clog that keeps coming back is a dirty pan or a pipe with a low spot, and flushing alone will not end it.

Water Dripping From a Ceiling Vent or the Air Handler

A roof leak is rarely the reason for water dripping from an AC vent in the ceiling. When the air handler sits in the attic, an overflowing drain pan runs along the duct and comes out at the nearest register. An air handler leaking water in a closet or basement is the same fault at floor level. In both cases the drain line is blocked, and the first move is to switch the cooling off so the pan stops filling.

That covers water dripping from AC vent covers, an AC vent leaking water, and AC vents dripping water in one room only. When water is dripping from AC vents in several rooms at once, suspect the pan before the ducts.

The other cause is a duct that sweats. A cold metal duct or boot with torn or missing insulation collects moisture from humid attic air, and it drips at the register even though the drain is clear. If the pan is dry and the drip only appears on humid days, look at the insulation around that duct.

Tip: an AC leaking water inside the house is a drain or insulation problem, and an AC dripping water outside is usually normal. The small pipe that drips beside the outdoor wall is the drain doing its job. No drip at all on a humid day is the one to worry about, because it means the water is going somewhere else.

AC Unit in Attic Leaking Water and an AC Drain Pan Full of Water

An attic unit usually sits inside a second, larger tray. That outer tray is the emergency pan, and it is meant to be dry. If you climb up and find the AC drain pan full of water, the pan under the coil has already overflowed into it, and the only thing between that water and your ceiling is how fast the emergency pan can drain.

So a drip pan full of water is not the leak. It is the warning that the main drain line has blocked. Switch the cooling off, empty the pan with a wet/dry vacuum, and clear the main line before you run the system again. One owner with a sensor in that pan still got a wet ceiling, because a corner of the metal pan had rusted through below the sensor. So look at the pan itself while you are up there. If the attic pan has no drain of its own and no float switch in it, ask for a switch to be added, because that pan is the last thing protecting the ceiling.

AC Drain Line Dripping Outside: Which Pipe Is It?

Many houses have two small pipes for this, and they mean opposite things. The main drain ends low, near the ground or beside the outdoor unit, and a steady drip there on a humid day is exactly right.

The second pipe ends somewhere you cannot miss it, often above a window or at the edge of the roof. Installers put the backup drain there on purpose. Water from that pipe means the main drain has blocked and the system is now running on its spare. A drip from the low pipe is normal. A drip from the high pipe is the last warning before the ceiling gets wet.

Furnace Leaking Water When AC Is On: Where the Water Comes From

The furnace is not making that water. In most homes the cooling coil sits in a metal box directly on top of the furnace, and the two share one cabinet and one blower. When the coil's pan or drain blocks, the water has only one way to go, which is down through the furnace and out at its base. In winter, with the heat on, water at a furnace has other causes, covered in furnace troubleshooting.

That is why a furnace leaking water when the air conditioner is on is the same blocked drain described above, seen from below. It deserves more urgency than a puddle on a slab, though, because the water passes the furnace's control board and burners on its way down. Switch the cooling off, and have the drain cleared before the heating season, not during it.

Window AC Leaking Water Inside

A window AC unit leaking water inside is the most common leak of all, and the easiest to fix. Whether it is a window AC dripping water inside onto the sill or a window air conditioner leaking water inside down the wall, check these in order. The first is free and fixes most cases.

• The tilt. A window unit must sit sloping slightly down toward the outside, around a quarter of an inch. Mounted dead level, or tipped back into the room by a sagging sill, condensate runs indoors instead of out. A level and a shim solve it.
• The weep hole. There is a small drain opening at the rear of the base pan. Leaves, insects and grime block it, and once it is blocked the pan fills until it spills over the front lip.
• The filter. A clogged filter starves the coil of air, the coil drops below freezing, and the ice that forms melts faster than the drain can carry away. The leak is a symptom; the filter is the cause.
• A cracked or rusted pan. Common on older units and not economically repairable.

One thing worth knowing before you assume a fault: many modern window units are designed to hold water on purpose. A slinger ring on the condenser fan picks condensate up off the base and throws it against the hot coil, which improves efficiency and evaporates the water. Owners report units that hold water without dripping, and units that throw it out of the back, and both behaviors can be correct. Water outside the unit or no visible water at all is normal. Water inside the room is not.

Window AC Leaking Water When It Rains

The outdoor half of a window unit is built to get wet. Rain falls through the top grille, lands in the same base pan as the condensate and leaves by the same route. So if a window AC unit is leaking water inside only when it rains, the rain is not getting past a seal. It is filling a pan that cannot drain. Check the tilt and the weep hole exactly as above, then look at the gaps beside the unit, where wind can push rain around the side panels. If it leaks in rain and in dry weather, it is the drain. If it leaks only in driving rain, it is the gap around the unit.

Mini Split Leaking Water From the Indoor Head

A ductless head has no large pan to buffer anything. Condensate collects in a shallow tray behind the cover and leaves through a drain hose bundled inside the wall sleeve with the refrigerant lines. That path is short, hidden, and entirely dependent on gravity.

• Blocked drain line. The most common cause by far. Biological growth builds up inside a hose that stays permanently damp, and once it blocks, the tray overflows down the wall.
• Sleeve drilled the wrong way. The wall penetration must slope downward toward the outside. Drilled level, or worse sloping inward, water tracks back along the lines and appears as a stain below the unit.
• Failed condensate pump. Fitted where gravity drainage is not possible. These have limited lift, commonly around two feet, so an installation that asks for more than the pump can manage will fail intermittently and then permanently.

The tell: a stain on the wall directly under the head almost always means the drain path, not the unit. Water appearing anywhere else, or dripping from the front louvers during operation, points instead at a frozen coil that is now thawing, which sends you to the section on an AC freezing up.

Portable AC Leaking Water

A portable unit has no outdoor half to drip from, so its water stays inside the cabinet. Most models blow some of it out through the exhaust hose as vapor and collect the rest in a tank at the bottom. In dry weather the tank may never fill. In humid weather it fills in hours, and the unit stops with a full-tank light.

A portable air conditioner leaking water from the bottom is nearly always that tank: a drain plug that was not pushed all the way in after the last emptying, a cap that has cracked, or a unit standing on a slope so the tank spills before the sensor trips. Unplug it, empty it through the drain port into a shallow tray, reseat the plug and stand it level. If you are emptying it every day, fit a drain hose to the same port and run it to a floor drain or a bucket, because the weather will not change to suit the tank.

AC Leaking Water and Not Cooling

These two together are one fault, not two. An AC leaking water and not cooling has almost always iced over: the ice blocks the air, so the rooms warm up, and then it melts, so the water appears. The second possibility is a float switch that has tripped on a full pan and switched the cooling off, which is that switch doing its job, and it is one of the six checks for an AC not turning on. Look for ice on the coil or the copper pipe first. If you find it, the cause and the fix are in the section on an AC not blowing cold air. Ice means airflow or refrigerant. No ice and a full pan means the drain.

How to Keep an AC From Leaking Water Again

• Change the filter on a schedule. A clean filter prevents the ice that causes half the leaks on this page.
• Watch the drain on humid days. A steady drip at the low pipe outside means the path is clear. No drip is your early warning.
• Know where your float switch is, or whether you have one. It is the difference between an AC that shuts off and a ceiling that comes down.
• Check a window unit's tilt each spring. Sills settle and brackets sag over a winter.

None of these costs anything, and each takes a minute. A leak that reaches the floor is almost never the first sign. It is the first one anybody looked at.

When a Leak Needs a Technician

Most water leaks on this page end with a cleared drain, a shim or a clean filter, and none of those needs a license. Three things do. A float switch that is missing or dead is a wiring job. A coil that freezes a second time after the filter has been changed is low on refrigerant, which means finding the leak and recharging the system, a $200 to $1,500 job. And an evaporator coil that has rusted through its own pan is a $1,000 to $2,500 part, which on an older system is the point to compare the repair with the HVAC replacement cost calculator before agreeing to it. Clear the drain yourself first. Call when the water comes back with the drain clear.