Quick answer: the most common causes
An air conditioner that leaks water is usually dealing with one of a few problems: a clogged condensate drain line, a frozen evaporator coil (often from a dirty filter or low refrigerant), a cracked or rusted drain pan, or a failed condensate pump. Less often, the unit is not level, a drain line has come loose, or very humid conditions are producing more water than the system can handle.
If water is actively leaking, turn the AC off at the thermostat first. Then work through the causes below, starting with the simplest checks. Anything involving refrigerant, electrical parts or a frozen coil that keeps coming back is a job for a licensed HVAC technician.
How an AC makes (and removes) water
Cooling your home also removes moisture from the air. Warm, humid air passes over the cold evaporator coil, and the moisture condenses into water. That water drips into a drain pan and flows out through a condensate drain line, usually a PVC pipe that ends outside the house or at a household drain. Some systems use a small pump to move the water.
A little condensation is normal. What is not normal is water appearing on the floor, ceiling or walls near the indoor unit, or any water appearing when the system is switched off. If you are seeing water in the wrong place, something in this path has failed.
Common reasons an AC leaks water
Clogged condensate drain line
This is one of the most commonly cited causes. Dust, algae, mold and debris can build up in the drain line until water backs up, fills the pan and overflows. Signs include a pan with standing water, a musty smell, and water near the indoor unit.
A wet/dry vacuum applied to the outdoor end of the line is a common homeowner fix. Be careful: if you cannot tell how the line is connected, or you are not comfortable working on the system, ask a technician. Some manufacturers and technicians warn that doing it incorrectly can damage equipment.
Dirty air filter
A clogged filter restricts airflow across the evaporator coil. The coil can then get cold enough to freeze, and when the ice melts it can overwhelm the drain pan. Checking the filter is the easiest first step. Many manufacturers suggest checking it regularly and replacing it roughly every one to three months, depending on the filter type and your home. Follow your system’s instructions.
Frozen evaporator coil
Ice on the coil or on the copper refrigerant line is a sign of a problem rather than the problem itself. Common triggers are poor airflow (a dirty filter, blocked vents, dirty coil) or low refrigerant. When the ice melts, a large amount of water can arrive at once.
If you see ice, turn the cooling off and let it thaw completely, which can take several hours. Do not chip or scrape the ice, because you can damage the coil. If the coil freezes again after you replace the filter and confirm airflow, call a professional.
Low refrigerant
Low refrigerant can contribute to coil freezing. Refrigerant is not something to top up yourself: it requires a certified technician (in the United States, EPA Section 608 certification when work could release refrigerant), and a leak has to be found and repaired, not just topped off. See whether a leaking AC is dangerous for symptoms of a refrigerant leak.
Cracked, rusted or overflowing drain pan
Older pans can corrode or crack, letting water drip through. Pans can also fill with debris and overflow. Look for rust, cracks or standing water under the indoor unit. A damaged pan generally needs repair or replacement by a technician.
Failed condensate pump
Systems that cannot drain by gravity, such as air handlers in basements or closets, may use a condensate pump. If it fails, loses power or clogs, water backs up and overflows. Some pumps have a float switch that can stick or break.
Disconnected or damaged drain line
Drain lines can crack, sag or separate at a joint, so water escapes before it reaches its destination. This can also come from installation problems.
Unit not level or poor installation
If an indoor unit or pan is not level, water may not flow toward the drain. Window units are a special case: they are normally meant to tilt slightly toward the outside. See our window AC maintenance checklist for that.
High humidity or an oversized system
During very humid weather the system removes more moisture, producing more condensate. An improperly sized system may also struggle to control humidity. This is a question for an HVAC professional.
Match the symptom to the likely cause
| What you see | Likely causes |
|---|---|
| Puddle under the indoor unit, pan full of water | Clogged drain line, failed pump, cracked pan |
| Ice on the coil or refrigerant line, then a flood | Dirty filter, blocked airflow, low refrigerant |
| Water dripping from a ceiling vent or duct | Drain overflow, frozen coil, or duct condensation (see AC duct dripping water) |
| Musty smell near the unit | Standing water, mold in the pan or drain line |
| Water only after the unit has been off a long time | Debris or insects blocking the drain line |
| Window unit dripping indoors | Unit tilted the wrong way, blocked drain hole, dirty coils |
What to do right now
- Turn the AC off at the thermostat. If water is near the unit or electrical parts, also switch off the breaker if you can reach it safely and dry.
- Do not stand in water near electrical equipment. If the area around the unit or panel is wet, stay out and call a qualified professional.
- Soak up water with towels or a wet/dry vacuum to limit damage to floors and drywall.
- Check the air filter and replace it if dirty.
- Look at the drain pan and the outdoor end of the drain line.
- If there is ice, let it thaw fully with the cooling off.
- If the cause is unclear, or the leak returns, call an HVAC technician.
What you can do yourself, and when to call a professional
Reasonable homeowner tasks: replacing the filter, mopping up water, letting ice thaw, visually checking the pan and the drain line exit, and gently clearing a clog near the line’s outlet with a wet/dry vacuum.
Call a professional for: suspected refrigerant leaks, repeated freezing, a cracked pan, a failed pump, electrical problems, water that reached a ceiling or light fixture, or any job that means opening panels you are unsure about. If a ceiling is sagging or bulging with water, keep clear of it and call for help, since it can fail suddenly.
Can I keep using a leaking AC? Is it an emergency?
Most manufacturers advise turning a leaking system off until the cause is found, because running it can add water damage and may harm the equipment. It is usually not a “drop everything” emergency if the leak is small and you have shut the system down, but it should be dealt with promptly. Treat it as urgent if water is near electrical components, reaching a ceiling, or you notice hissing, a chemical smell or other signs of a refrigerant leak.
How to prevent AC leaks
- Check and replace the filter on a regular schedule.
- Keep vents and returns unblocked.
- Have the system serviced annually, ideally before the cooling season, so drain lines and coils are checked.
- Keep the area around the indoor unit clean.
- Consider a water leak sensor or float switch near the unit so overflow is caught early.
FAQ
Should I be worried if my AC is leaking water?
It deserves attention. A small amount of dripping can be normal when the system is running, but pooling water or leaks while the unit is off point to a problem that can damage floors, ceilings and insulation and encourage mold.
How can I stop my air conditioner from leaking water?
Turn it off, then check the filter, the drain pan and the drain line, and let any ice thaw. If these do not fix it, a technician can find the cause.
Does homeowners insurance cover AC leak damage?
Policies differ. Some may cover water damage to the home from a sudden leak but not the repair of the AC itself. Check your own policy or ask your insurer.