Air conditioner leaking water inside the house
An air conditioner is supposed to make water. Several gallons of it on a humid afternoon. The problem is never that the water exists. It is that it stopped going where it was meant to go.
Why does an air conditioner produce water at all?
Warm indoor air passes over the evaporator coil, the coil sits below the dew point of that air. And moisture condenses onto the fins exactly the way it beads on a cold glass. On a humid day a residential system can pull several gallons of water out of the house in an afternoon, and that is the system working correctly. Dehumidifying is half of what cooling does.
The water is supposed to run off the coil into a sloped pan underneath it, out through a condensate drain line. And away to a floor drain, a standpipe, the outdoors, or a small condensate pump when the equipment sits below its drain. Water on the floor means one of those handoffs failed. Every cause below is a failure of drainage or a failure of airflow, not a machine that suddenly started leaking.
Where the water shows up tells you most of what you need
| Where you find it | What it usually points to | Whose job it is |
|---|---|---|
| A steady drip or puddle right at the indoor air handler or furnace | The condensate drain line is blocked, or the pan has rusted through or cracked | Clearing a blockage is often reasonable at home; a failed pan is a technician's |
| A sheet of water that appears after the system has been off for a while | A coil that iced over and is now thawing faster than the pan can carry away | Technician, once you know why it iced |
| A stain or drip on a ceiling below an attic unit | The primary drain is blocked and the secondary pan is overflowing, or has no drain of its own | Technician |
| An overflowing box with a small pump beside the unit | A condensate pump that has failed, come unplugged, or has a blocked discharge tube | Power and tube are checkable at home; replacement is a technician's |
| Water on the ground under the outdoor unit in cold weather | The defrost cycle of a heat pump melting frost off the outdoor coil | Nobody. This one is expected |
The condensate drain line is blocked
This is the most common answer by a wide margin. The drain carries water that has just washed across a cold metal surface in a dark cabinet. And over a season that line accumulates a slime of algae, dust and biological growth that eventually closes it off. The pan then fills and overflows at its lowest edge, which is rarely the edge you would expect.
Most systems installed in the last couple of decades have a float switch in the pan or the drain. It cuts the cooling call when the water rises. That is why a related complaint (the AC stopped cooling and there is no water anywhere) is very often the same blocked drain, caught before it flooded anything. The line, the switch and what you can reasonably clear yourself are covered in the piece on a clogged condensate drain line.
The coil froze, and what you are seeing is the thaw
A pan and drain are sized for condensate that arrives steadily, drip by drip. They cannot handle a block of ice releasing all at once. So a coil that iced over and then thawed (because you turned the system off, or because it finally cycled) routinely sends water past the pan and onto the floor, even when the drain itself is perfectly clear. The tell is timing: a puddle that appears while the system is off, not while it is running.
Icing has two usual causes, restricted airflow and a low refrigerant charge, and both are explained in why an air conditioner freezes up. The important part here is that mopping the floor does not address either one. If the coil iced once it will ice again, and the second flood will be the same size as the first.
The pan, the slope or the pump
When the drain is clear and nothing is frozen, the remaining causes are physical. Drain pans corrode, particularly the older metal ones, and a pinhole low in a pan produces a small persistent drip with no blockage anywhere. A drain fitting can work loose at the pan connection.
A line that was run with too little fall, or that has sagged between supports, holds standing water and backs up at the first bit of debris. And where the equipment sits below its drain (a basement furnace with the coil on top, most commonly) a condensate pump does the lifting. And a pump that has failed, lost power or had its discharge tube kinked or blocked will fill its reservoir and spill.
Attic installations get one extra layer: a secondary pan under the whole unit. It catches exactly this and routes the water somewhere visible, often a pipe that drips over a window. Water staining a ceiling below an attic unit usually means the primary drain failed and the secondary path failed or was never connected.
Is water under the outdoor unit a problem?
Usually not. In cooling, condensate forms indoors, so the outdoor unit should be dry. In heating, a heat pump runs a defrost cycle that melts accumulated frost off the outdoor coil. And the meltwater runs out onto the pad: normal, and a sign the defrost control is doing its job. What is worth attention is a mound of ice that never clears between cycles, or meltwater pooling somewhere it will refreeze across a walkway.
What not to do
- Do not bypass, tape or hold up a float switch to get the cooling running again. It shut the system down to keep water out of the house, and defeating it is how a small drain problem becomes a ceiling repair.
- Do not open the electrical compartment of either unit to trace where water is coming from. Capacitors inside can hold a dangerous charge even after the power is off. That compartment is a licensed technician's.
- Do not add refrigerant, and be wary of an offer to simply top it off. Only a technician certified under EPA Section 608 may legally purchase or handle refrigerant, and a system that is low is a system with a leak to find.
- Do not chip or scrape at ice on the coil or the lines, and do not keep running the system while any ice is visible.
- Do not shrug off water landing on a gas furnace. Many indoor coils sit directly on top of one, and condensate reaching the control board or the burner assembly is a repair to report to a technician, not a puddle to wipe.
What to do now, in order
Turn the cooling off at the thermostat first. That stops more water being made while you work out where it is coming from. Then get the standing water up and put something absorbent under the unit. The secondary damage is almost always to the floor, the drywall or whatever is stored nearby, not to the equipment.
Next, look for ice before anything else, since ice changes the answer completely. Check the filter, then look at the drain line and the pump. If the water returns after the drain is clear and nothing is frozen, call a licensed HVAC technician. Do not keep cycling the system and mopping.
Frequently asked questions
My AC stopped cooling but there is no water anywhere. Can it still be the drain?
Very often, yes. Most systems have a float switch that cuts the cooling call when water rises in the pan or the drain, so a blocked drain caught early shows up as a system that simply stops cooling with nothing visible on the floor. That is the protection working as designed, and defeating the switch to keep cooling is what turns it into a flood.
Is it normal for the drain to run constantly on a humid day?
Yes. Removing moisture is half of what cooling does, and a system working hard in humid weather can produce several gallons a day. A steady trickle at the drain termination is a sign of normal operation; the concern is water anywhere other than the drain.
Can a dirty filter really cause a puddle?
Indirectly, and it is one of the most common chains of events in the whole subject. A clogged filter restricts airflow across the evaporator coil, the coil drops below freezing, condensate freezes onto it instead of draining, and when that ice eventually thaws it arrives far faster than the pan and drain were sized for.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Energy Savings at Home: Heating & Cooling
- US EPA. A Brief Guide to Mold, Moisture and Your Home