AC Leaking Water Inside: Clear the Condensate Drain and Find the Cause

An air conditioner removes moisture from indoor air as well as heat. That water collects on the cold indoor coil, drips into a drain pan underneath, and leaves the house through a condensate drain line, sometimes with help from a small pump. On a humid day this can be gallons of water. When that path is blocked, the water ends up on the floor, in the ceiling below an attic unit, or soaking into a basement wall.

Find where the water comes from

What you see	Likely cause
Water pooling around the indoor unit on a humid day	Clogged condensate drain line
Drain pan full of standing water	Clogged drain or failed condensate pump
Water dripping from the bottom of the pan itself	Cracked or rusted drain pan
Water after the system has been off for a while, ice earlier	Frozen coil melting
Water stain on a ceiling below an attic air handler	Overflowing pan; check the secondary drain or float switch
System suddenly shuts off with water in the pan	Float switch cut power to prevent overflow

Step 1: Turn the system off

Turn the thermostat to OFF and switch off the air handler at its service switch or breaker before working near it. Soak up standing water so you can see where it is coming from.

Step 2: Clear the condensate drain

ENERGY STAR’s maintenance checklist includes inspecting the condensate drain in a central air conditioner, furnace or heat pump, and warns that a plugged drain can cause water damage and affect indoor humidity. Algae, dust and slime are the usual culprits.

1. Find where the drain line exits the house, often a PVC pipe near the outdoor unit or a floor drain.
2. Attach a wet/dry vacuum hose to the end of the pipe, sealing it with your hand or a rag, and run the vacuum for a minute or two. Check the vacuum for the clog.
3. Many drain lines have a capped T fitting near the indoor unit. Remove the cap and pour about a cup of distilled white vinegar into the line to discourage algae growth, then flush with water.
4. Check that the water now drains freely from the pan.

Step 3: Check the condensate pump

Units in basements often pump water up to a drain. The pump has a small reservoir and a float. Pour water into the reservoir: the pump should switch on and empty it. If it does not, check that it is plugged in and its outlet works. Clean any sludge from the reservoir. A failed pump must be replaced.

Step 4: Inspect the drain pan

With a flashlight, look at the pan under the coil. Rust holes in metal pans or cracks in plastic ones let water drip through even when the drain is clear. Replacing a primary pan usually means removing the coil, which is technician work. Attic units should also have a secondary pan under the whole unit with its own drain or float switch.

Step 5: Rule out a frozen coil

If you saw frost on the refrigerant lines or airflow was weak before the leak, the coil may have frozen and the melt overwhelmed the drain. Replace the filter and read our guide to an AC freezing up.

Why act quickly

The EPA’s mold course lists condensate drain pans inside air handling units, the back side of drywall, and the underside of carpets and pads among places where mold can grow out of sight, and warns that undiscovered or ignored moisture problems can lead to mold. A slow condensate leak into drywall, ceilings or flooring creates exactly those conditions. Dry the area thoroughly and watch for staining or musty odors.

Common mistakes

• Disabling the float switch so the system keeps running.
• Pouring bleach repeatedly into the line; it can damage some components and is unnecessary with regular flushing.
• Ignoring a ceiling stain below an attic unit.
• Running the AC with a cracked pan.

When to call a technician

Call an HVAC technician if the drain clogs again quickly, if the pan is cracked or rusted through, if the condensate pump needs replacement and you are not comfortable doing it, if the coil keeps freezing, or if water has reached ceilings or walls. ENERGY STAR recommends a spring cooling check-up each year; ask the technician to clean the drain and pan as part of it.