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AC Leaking Water or Not Cooling: How to Find the Cause

Three faults account for most air conditioner complaints: water where it should not be, air that is not cold, and a coil covered in ice. Each has a small number of real causes, and several of them cost nothing to check. This page works through them in the order that saves you the most money.

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 litres 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.

Window AC Leaking Water Inside

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 spirit 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 behaviours can be correct. Water outside the unit or no visible water at all is normal. Water inside the room is not.

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 louvres during operation, points instead at a frozen coil that is now thawing, which sends you to the airflow section below.

Central AC Leaking Water

The evaporator coil sits above a drain 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.

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.

AC Running But Not Cooling

Work through the free checks before anyone gets paid. In order:

  • Thermostat mode and fan setting. Set to cool, and the fan on AUTO rather than ON. This one catches people constantly: with the fan set to ON it runs continuously and pushes uncooled air through the vents between compressor cycles, which feels exactly like a system that has stopped cooling.
  • The air filter. The most common single cause, and the cheapest.
  • Breakers. Indoor and outdoor units are usually on separate circuits, so the inside can run while the outside is dead.
  • Is the outdoor unit actually running? Fan turning, compressor audible. A humming unit with a still fan is often a failed capacitor.
  • Is the indoor coil iced? If yes, stop and go to the next section.

If all of that is clear, the remaining causes cost money: a capacitor, a contactor, a refrigerant leak, or a compressor. A capacitor is commonly a $150 to $400 job.

And the one to be firm about: refrigerant is not consumed. It circulates in a sealed loop and does not deplete with use. As one technician put it, it is neither created nor destroyed inside the system. So a unit that is low on refrigerant has a leak, full stop. Topping it up without finding that leak is refilling a bucket with a hole in it, and a contractor who offers to do that every spring is selling you the same repair annually rather than fixing anything.

AC Freezing Up

Ice on the coil has two causes and they present identically to the owner, which is why so much money gets spent on the wrong one.

Not enough air over the coil. A dirty filter, closed or furniture-blocked vents, a starved return, a failing blower, or ductwork too small for the system. With too little warm air passing over it, the coil temperature falls below freezing and condensate turns to ice instead of draining. If undersized ducts are the suspect, our duct size calculator shows what the system actually needs.

Not enough refrigerant. Lower pressure means a lower coil temperature, with the same result. And per the section above, that means a leak.

How to tell them apart without paying anyone: switch cooling off and run the fan alone until the ice is completely gone, which takes a few hours rather than minutes. Fit a clean filter, open every vent, and run it again. If it stays clear, it was airflow and you have fixed it. If it ices up again with a clean filter and open vents, it is refrigerant, and that needs a professional with recovery equipment.

Do not run cooling while the coil is iced. A flooded coil sends liquid refrigerant back to a compressor built to handle gas, and that is how a dirty filter becomes a compressor replacement.

When It Is Sizing, Not a Fault

Some complaints that look like faults are the system doing exactly what it was installed to do. A unit that cools quickly then leaves the room cold and clammy is usually oversized rather than broken, because it satisfies the thermostat before it has removed much moisture. One that runs constantly on the hottest days without ever reaching setpoint may simply be too small. Neither is diagnosed by opening anything up. If the behaviour has been there since installation rather than appearing suddenly, check the sizing with the AC tonnage calculator or, for one room, the window AC size calculator before spending anything on repairs.

Frequently Asked Questions

Why is my window AC leaking water inside?

Almost always because the water has nowhere else to go. A window unit must sit with a slight downward slope toward the outside, roughly a quarter inch, so condensate runs out the back. Mounted level or tipped inward it runs into the room instead. If the tilt is right, the next suspects are a blocked weep hole at the rear, a clogged drain channel, or a frozen coil that melts faster than the drain can clear. Check the tilt first because it costs nothing and it is the most common cause.

Is it normal for an air conditioner to produce water?

Completely normal, and a sign it is working. Cooling air also dries it: warm humid air passes over a cold coil, moisture condenses out, and in humid weather a mid-size unit can pull several litres a day. Water is never the fault by itself. The fault is only ever in where that water ends up.

Why is my mini split leaking water from the indoor head?

A ductless head has only a shallow tray behind the cover, and the water leaves through a gravity drain running inside the wall sleeve. Anything that interrupts that path shows up as water on the wall below the unit. The usual causes are a blocked drain line, a sleeve drilled level or sloping the wrong way, or a failed condensate pump if one is fitted. Pumps have limited lift, often around two feet, so an install that asks for more than that strains them.

Why is water leaking from my central AC?

The evaporator coil sits over a drain pan with a primary drain line and usually a secondary one. When the primary blocks, typically with algae, the pan overflows. On a correctly installed system a float switch should sense that and shut the system off before there is water on the floor. If you have water on the floor, either no float switch was fitted or it has failed, and that is worth fixing regardless of the drain itself.

My AC is running but not cooling. What should I check first?

Work through the free checks in order. Confirm the thermostat is set to cool and the fan is on AUTO rather than ON, because fan ON circulates unconditioned air between cycles and feels exactly like a system that has stopped cooling. Then the air filter, which is the single most common cause. Then the breakers, remembering indoor and outdoor units are often on separate circuits. Then look at the outdoor unit and confirm it is actually running. Then check whether the indoor coil has iced over.

Does an air conditioner need refrigerant topped up?

No, and this is the most expensive misunderstanding in home cooling. Refrigerant circulates in a sealed loop and is not consumed. It does not deplete with use the way fuel does. If a system is low, refrigerant has escaped through a leak, and adding more without finding that leak is refilling a bucket with a hole in it. Any contractor who offers to top it up annually without ever looking for the leak is selling you the same repair every year.

Why does my AC keep freezing up?

Two causes, and they look identical from the outside: not enough air moving over the coil, or not enough refrigerant in it. Low airflow comes from a dirty filter, closed or blocked vents, a starved return, or undersized ductwork. Low refrigerant means a leak. You can tell them apart cheaply: thaw the coil fully with the system off and the fan running, fit a clean filter, and run it again. If it stays clear it was airflow. If it ices up again, it is refrigerant and it needs a professional.

Can I run my AC while it is frozen?

No. Running a system with an iced coil pushes liquid refrigerant back toward the compressor, which is built to compress gas rather than liquid, and that is how a cheap airflow problem turns into a compressor replacement. Switch cooling off, leave the fan running to melt the ice, and expect it to take a few hours. Only then look for the cause.

When should I stop troubleshooting and call someone?

Once the free checks are done and the problem is still there. Thermostat, filter, breakers, outdoor unit running, coil thawed: if all of that is clear and it still will not cool, the remaining causes are a failed capacitor, a failed contactor, a refrigerant leak, or a compressor, and all of those need tools and refrigerant handling that are legally restricted. A capacitor is often a $150 to $400 job, so calling early is frequently cheaper than a week of guessing.