Why Is My AC Leaking Water? Common Causes and What to Do

A puddle under the indoor air handler is alarming, but it's rarely a sign your air conditioner is ruined. Cooling naturally pulls moisture out of the air, and that water is supposed to drain away quietly. When it ends up on the floor instead, the cause is usually one of a handful of common problems — and a few of them you can address yourself before the water damages drywall, flooring, or the ceiling below.

Where the water comes from in the first place

As warm indoor air passes over the cold evaporator coil, humidity condenses into water — exactly like beads forming on a cold glass. That condensate drips into a pan beneath the coil and flows out through a drain line. A leak almost always means that normal water is escaping somewhere along this path, not that refrigerant is "leaking out." Knowing this helps you stay calm and look in the right place.

1. A clogged condensate drain line (the most common cause)

Over time, algae, dust, and slime build up inside the drain line and block it. With nowhere to go, condensate backs up and overflows the pan. This is the single most common reason for an indoor AC leak. Many systems have a float switch that shuts the unit off when the pan fills — so a clogged drain can also explain an AC that mysteriously stopped cooling. Clearing a visible clog near the drain opening is a reasonable homeowner task; a stubborn clog deeper in the line is a job for a technician.

2. A frozen evaporator coil that's thawing

If the coil ices over — usually from weak airflow or low refrigerant — it can dump far more water than the pan and drain were sized to handle once it thaws. If you see ice on the coil or refrigerant lines, turn the system OFF and run only the fan to let it thaw, then check the air filter. Persistent freezing points to a deeper problem and warrants a professional visit.

3. A dirty air filter

A clogged filter restricts airflow across the coil, which can drop the coil temperature low enough to freeze — leading right back to cause #2. Checking and replacing a dirty filter is the easiest preventive step you can take, and it costs only a few dollars.

4. A cracked or rusted drain pan

On older systems, the pan that catches condensate can rust through or crack, letting water drip straight to the floor even when the drain line is clear. A damaged pan needs replacement by a technician who can match the correct part.

5. A disconnected or poorly sloped drain line

If a drain line was installed without enough slope, or a fitting works loose, water pools and escapes. This is more common after recent service or in a system that was never installed cleanly, and it typically needs a pro to re-route or re-seal.

What you can safely do right now

• Turn the system off to stop adding water while you investigate.
• Mop up standing water and place a towel or shallow pan to protect floors and ceilings below.
• Check and replace a dirty air filter.
• Look for ice on the coil or lines; if present, run the fan only and let it thaw.
• Clear any obvious clog at the drain line opening if you can reach it safely.

When to call a professional

Reach out to a licensed technician if the leak returns after these steps, if you find a rusted or cracked pan, if the coil keeps freezing, or if water has reached drywall, insulation, or a ceiling — where lingering moisture invites mold. Anything involving refrigerant is regulated: under U.S. EPA Section 608 rules, only certified technicians may handle it. In most cases an indoor leak is a quick, affordable fix when caught early, so it's worth addressing before the water finds something expensive to ruin.

This guide is general information for educational purposes only — not professional HVAC, electrical, or safety advice. If you smell gas or suspect a carbon-monoxide leak, leave immediately and call 911 or your gas utility. Prices are general national estimates that vary by region, system, and contractor, and change over time. Confirm specifics with a licensed HVAC professional before acting.