Why Is My AC Leaking Water?

Why is my air conditioner leaking water?

Your AC leaks water because the condensate drainage path is blocked or broken, not because the refrigerant is leaking. A central air conditioner can pull 5 to 20 gallons of water out of your indoor air on a humid day. That water condenses on the cold evaporator coil the same way a glass of ice sweats, drips into a drain pan underneath the coil, and flows out through a PVC drain line, either to a floor drain, a condensate pump, or a spot near the outdoor unit. This is normal and constant while the AC runs. You only see a leak when something interrupts that path.

So a water leak is a drainage problem, and it is almost always cheap and often a do-it-yourself fix. That is very different from an AC that is low on refrigerant, which is a sealed-system leak that needs a licensed technician. The one place the two overlap is a frozen coil: when a coil ices over and then melts, it dumps water faster than the pan can handle, and the underlying cause of the freeze can be low refrigerant. Our guide on why an AC freezes up covers that path, and the evaporator coil explainer shows where the coil and pan sit in the system.

What are the most common reasons an AC leaks water?

The number one cause is a clogged condensate drain line. Algae, mold, and dust build into a slimy sludge inside the PVC line, the water backs up behind the clog, fills the pan, and spills over. A clogged drain line is behind the large majority of indoor AC water leaks. Close behind it is a rusted-through or cracked drain pan, common on older systems where the metal pan under the coil has corroded, so water drips straight through instead of draining. A drain pan that is simply overflowing points right back to a clog downstream.

After the drain, the next causes sit around the coil and the equipment. A frozen evaporator coil melts and overwhelms the pan, usually triggered by a dirty air filter or low refrigerant; a filter you can no longer see through is the classic starting point, which is why the filter schedule matters. A failed condensate pump, used when the air handler sits in a basement or attic below the drain, lets water pool once the pump quits. And on newer installs, an improperly sloped or disconnected drain line or a clogged pan float switch can leak from day one. Low refrigerant belongs on this list only through the frozen-coil path, and handling refrigerant is a certified job.

How do I unclog the AC condensate drain line myself?

Clearing the drain line is the fix for most indoor water leaks, and it is a safe do-it-yourself job. Turn the system off at the thermostat and the breaker, find the drain pan and the white PVC drain line at the indoor air handler, and locate the other end of that line where it exits near the outdoor unit or a floor drain. Pull any standing water out of the pan with a wet/dry vacuum or towels so you are not fighting a full pan while you work. Look for the small capped access tee on the drain line near the air handler; that cap is your cleanout port.

With the pan emptied, clear the clog from both ends. Hold a wet/dry vacuum tightly over the outdoor end of the drain line for two to three minutes to suck the sludge clog out, then pour a cup of distilled white vinegar (or a 50/50 vinegar and warm water mix) into the cleanout port to flush the line and kill the algae. Run water through afterward and watch it drain freely at the outside end. To keep it clear, pour a quarter cup of vinegar into the cleanout port once a month during cooling season. Do not use bleach on a regular basis, since it can corrode the metal pan and fittings. If the line still will not drain after a vacuum and flush, the clog is deeper and it is a pro job.

Why is water leaking around my furnace or indoor unit?

Water around the furnace in summer is almost always the air conditioner's condensate, not the furnace itself. On most homes the AC evaporator coil sits directly on top of the furnace or inside the air handler cabinet, so its drain pan and drain line share that space; a clog or a cracked pan there drips down onto and around the furnace. If you see water pooling at the base of the furnace during cooling season, treat it as an AC condensate leak: check the drain line and pan first, exactly as above. A cracked secondary or emergency drain pan under the coil is a common culprit on attic and closet installs.

In heating season the story changes only for one furnace type. A high-efficiency condensing furnace (90 percent AFUE or higher) makes its own condensate and drains it through a plastic line, so a clog or a bad condensate trap on that furnace leaks water in winter too. A standard (80 percent) furnace does not produce condensate, so winter water around it means a humidifier, a plumbing line, or a roof leak instead. Either way, standing water near electrical and gas equipment is worth clearing quickly. If the leak comes with no heat or a burning smell, stop and read the furnace maintenance checklist for what is owner-safe and what is a pro call.

Why is there condensation on my AC vents and ducts?

Water beading on the outside of your vents or ducts is sweating, not a drain leak, and it comes from humid air hitting cold metal. When warm, humid air in an attic, crawl space, or unconditioned room touches a cold supply vent or an uninsulated duct, the moisture condenses on the surface exactly like dew on a cold can. The three drivers are high indoor humidity, poor or missing duct insulation, and low airflow that lets the metal get colder than it should. It shows up worst in humid climates and in rooms where the register is metal and the air outside the duct is muggy.

The fixes target humidity and insulation rather than the drain. Wrap or replace insulation on ducts running through hot, humid spaces, keep indoor humidity in the 40 to 50 percent range with the AC or a dehumidifier, and make sure airflow is not so weak that the coil and ducts overcool. Sealing attic and crawl space air leaks helps too, since that is where the muggy air comes from. A weak-airflow problem often ties back to a dirty filter or closed vents, the same airflow issues that freeze a coil, so start there. If sweating ducts are a recurring problem, controlling the room's moisture is the durable answer.

Can I fix an AC water leak myself, or do I need a pro?

You can handle the common causes yourself, and they are the cheap ones. The do-it-yourself fixes are clearing the condensate drain line, emptying and cleaning the drain pan, replacing a dirty filter, and resetting a tripped float switch. A wet/dry vacuum and a bottle of vinegar cost little and solve most indoor leaks. If your air handler is in a basement or attic and the water is pooling despite a clear drain, check whether a condensate pump has failed, since a stuck pump backs the whole system up; a new pump is an inexpensive part but often worth having installed.

The pro fixes are the ones involving the pan, the pump wiring, or refrigerant. A replacement drain pan runs about $200 to $600 installed, a new condensate pump is roughly $150 to $450 installed, and clearing a deep or professionally snaked drain clog is about $75 to $250. If the leak traces back to a frozen coil that keeps icing over after you have cleaned the filter, the cause is low refrigerant or a failing blower, which is a licensed technician's job; under EPA Section 608 it is illegal to handle most refrigerants without certification. For a full breakdown of what HVAC repairs cost by part, see our guide on AC repair costs.

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