Furnace Leaking Water When AC Is On: Fix It Fast

Quick symptom-to-cause diagnosis

Cause 1: Clogged condensate drain line (most common)

When your AC runs, the evaporator coil removes humidity from the air. That condensation drains through a small PVC line — and if algae or debris block it, water backs up and spills out of the furnace cabinet.

1. Turn off the furnace and AC at the thermostat and the breaker.
2. Locate the white PVC condensate drain line, usually a ¾-inch pipe running from the air handler to a floor drain or utility sink.
3. Find the access port or T-fitting on the drain line and remove the cap.
4. Pour ¼ cup of plain white vinegar into the access port to dissolve algae buildup. Let it sit for 30 minutes.
5. Attach a wet/dry vacuum to the end of the drain line (use tape or a rubber coupling to seal it) and run the vacuum for 1–2 minutes to pull out the clog.
6. Pour a cup of water into the access port and watch for it to flow freely out the discharge end.
7. Replace the cap, restore power, and monitor for leaks over the next hour.

Cause 2: Cracked or corroded condensate drain pan

The drain pan sits beneath the evaporator coil to catch condensation before it reaches the drain line. Age, rust, or physical damage can crack the pan, causing water to leak directly onto the floor.

1. Turn off the furnace and AC at the thermostat and breaker.
2. Open the furnace access panel to locate the drain pan — it sits directly under the evaporator coil.
3. Use a flashlight to inspect the entire pan for rust spots, hairline cracks, or visible holes.
4. Soak up standing water with old towels or a wet/dry vacuum.
5. As a temporary fix, apply HVAC-rated waterproof sealant over small cracks. Let it cure fully per the product instructions before restoring power.
6. If the pan is heavily rusted or cracked in multiple places, measure its dimensions and replace it entirely — pans are model-specific by size but sold generically by dimension.

Cause 3: Failed or clogged condensate pump

If your furnace or air handler is installed below the drainage level of your home (e.g., a basement), a condensate pump lifts the water up and out. When this pump fails or its reservoir fills with slime, water overflows onto the floor.

1. Turn off the furnace and AC at the breaker.
2. Locate the condensate pump — a small white or grey plastic box near the base of the air handler, with tubing running into it.
3. Unplug the pump from its outlet.
4. Remove the reservoir lid and check for slime, algae, or debris blocking the float or impeller. Clean with warm water and a small brush.
5. Plug the pump back in. Pour water slowly into the reservoir. The float should trigger the pump to activate within a few seconds.
6. If the pump does not activate or makes a grinding noise, the motor has failed and the unit needs to be replaced.
7. Install the new pump, reconnect the tubing, restore power, and run the AC to confirm proper drainage.

Cause 4: Dirty or frozen evaporator coil

A dirty evaporator coil restricts airflow, causing the coil temperature to drop below freezing. When the AC cycles off, the ice melts rapidly and can overwhelm the drain pan — producing a sudden burst of leaking water.

1. Turn off the AC at the thermostat and let the system run on “fan only” for 1–2 hours to thaw any ice completely. Do not skip this step.
2. Turn off power at the breaker.
3. Check your air filter first — a severely clogged filter is the single most common cause of coil freezing. Replace it if it appears grey or matted with debris.
4. Open the furnace access panel and inspect the evaporator coil with a flashlight. Look for dust buildup, ice residue, or bent fins.
5. If the coil is coated in dust, apply a no-rinse evaporator coil cleaner foam. Let it drip into the drain pan as directed.
6. Restore power, run the AC, and check that condensation is draining normally without overflow.
7. If the coil freezes again within days despite a clean filter, suspect a refrigerant issue and call a certified HVAC technician.

Cause 5: Disconnected or leaking condensate drain fittings

PVC fittings and rubber couplings on the condensate drain line can loosen over time from vibration or improper installation, causing water to drip at the joint rather than flow to the drain.

1. Turn off the furnace and AC at the breaker.
2. Trace the entire condensate drain line from the drain pan outlet to its discharge point.
3. Run your hand along each joint, fitting, and rubber coupling, feeling for moisture or drips.
4. Tighten any loose hose clamps with a screwdriver. Hand-tight is not sufficient — use a tool.
5. For cracked PVC joints, cut out the damaged section with a PVC pipe cutter and replace it using a slip coupler and PVC primer and cement.
6. Wrap threaded fittings with fresh PTFE (Teflon) tape before reassembly.
7. Restore power and run the AC for 30 minutes, then inspect all joints for drips.

Repair cost reference guide

For deeper guidance on condensate drainage systems and HVAC maintenance, see the U.S. Department of Energy’s central air conditioning maintenance guide.

Frequently asked questions