Water dripping from your indoor AC unit is alarming — and if left unaddressed, it can cause significant water damage to your ceiling, walls, and floors.

The good news is that most AC water leaks have straightforward causes. Here is how to diagnose the problem and what to do about it.

Why Your AC Produces Water in the First Place

As warm, humid air passes over your AC's cold evaporator coil, moisture condenses on the coil — the same way a cold glass sweats on a humid day. This condensate drips into a drain pan and flows out through a condensate drain line. On a humid Long Island summer day, your AC can remove several gallons of water from the air.

When this drainage system fails or is overwhelmed, water overflows and leaks into your home.

Cause 1: Clogged Condensate Drain Line

This is the most common cause. Algae, mold, and debris accumulate in the drain line over time, eventually blocking it completely. Water backs up into the drain pan and overflows.

What to do: Locate the condensate drain line (usually a white PVC pipe near the air handler) and check for blockage. You can try clearing it with a wet/dry vacuum at the drain outlet or by carefully pouring a diluted bleach solution into the drain pan. If the line is severely clogged, call for service.

Cause 2: Frozen Evaporator Coil Thawing

A frozen evaporator coil — caused by restricted airflow (dirty filter, blocked vents) or low refrigerant — produces a large amount of water when it thaws. If you notice the leak started after the system was running for a while and then stopped, a frozen coil is likely.

What to do: Turn the system off and let it thaw completely (2–3 hours). Check and replace the air filter. If the coil freezes again after restarting, call for service — you likely have a refrigerant or airflow problem.

Cause 3: Disconnected or Cracked Drain Pan

The drain pan under the evaporator coil can crack over time, especially in older systems. The secondary drain pan (a backup pan below the primary) may also fill up if the primary pan overflows repeatedly.

What to do: Inspect the drain pan for cracks or overflow. A cracked pan needs to be replaced by a technician.

Cause 4: Low Refrigerant

Low refrigerant causes the evaporator coil to get too cold, leading to ice formation and eventual thawing that overwhelms the drain system. Low refrigerant always indicates a leak that needs to be found and repaired.

What to do: Call for service. Refrigerant leaks do not fix themselves.

Cause 5: Improper Installation or Slope

The air handler must be level (or slightly sloped toward the drain) for condensate to flow properly. If the unit was installed incorrectly or has shifted over time, water may pool in the pan and overflow rather than draining.

Preventing AC Water Leaks

• Change your air filter monthly during summer
• Pour a cup of diluted bleach into the drain pan monthly to prevent algae buildup
• Schedule annual AC maintenance that includes drain line inspection and cleaning

Call A1 for AC Water Leak Repair

A1 Heating & Air Conditioning diagnoses and repairs AC water leaks throughout Long Island. Call 631-331-2102 or schedule service online.