Water Coming From Furnace — Causes & Fixes

Cause 1: Clogged Condensate Drain Line (Most Common)

High-efficiency condensing furnaces produce condensation as a byproduct of combustion, which exits through a condensate drain line. When this line becomes clogged with algae, debris, or sediment, water backs up and leaks onto the floor.

1. Turn the furnace off at the thermostat and switch off the power at the breaker.
2. Locate the white PVC condensate drain line leading from the furnace to a floor drain or utility sink.
3. Disconnect the drain line at the nearest coupling or cleanout port.
4. Use a wet/dry vacuum to suction the clog from the line for 30–60 seconds.
5. Flush the line with a mixture of warm water and a small amount of white vinegar to clear residual buildup.
6. Reconnect the line, restore power, and run the furnace to confirm the leak has stopped.
7. Consider adding a condensate drain line treatment tablet monthly to prevent future clogs.
   Condensate drain line treatment tablets help prevent algae buildup in drain lines.
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Cause 2: Damaged or Blocked Condensate Trap

The condensate trap (a U-shaped fitting) prevents flue gases from escaping while allowing water to drain. If it cracks, becomes blocked, or dries out, water can overflow or back up into the furnace cabinet.

1. Turn off the furnace at the thermostat and at the breaker.
2. Locate the condensate trap — it is typically a clear or white U-shaped plastic fitting near the bottom of the furnace.
3. Inspect the trap for visible cracks, discoloration, or standing water that won’t drain.
4. Remove the trap by loosening the hose clamps or slip fittings on each side.
5. Flush the trap under running water to clear any blockage inside the U-bend.
6. If the trap is cracked or brittle, replace it with a new condensate trap of matching diameter.
   Furnace condensate trap replacements are available in common 3/4-inch and 1-inch sizes.
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7. Reinstall, restore power, and monitor for leaks during the next heating cycle.

Cause 3: Clogged or Overflowing Evaporator Coil Drain Pan

If your furnace shares an air handler with a central air conditioning system, the evaporator coil sits directly above the furnace and has its own drain pan. A clogged pan can overflow and appear as water coming from furnace components below.

1. Turn off both the furnace and the air conditioner at the thermostat and at the breaker.
2. Locate the evaporator coil access panel on the air handler — typically secured with screws.
3. Remove the panel and inspect the drain pan beneath the coil for standing water or visible algae growth.
4. Use a wet/dry vacuum to remove standing water from the pan.
5. Pour a cup of diluted bleach solution (1 part bleach to 16 parts water) into the pan to kill algae.
6. Locate the evaporator drain line (usually 3/4-inch PVC) and clear it with a wet/dry vacuum or drain line brush.
7. If the drain pan itself is cracked or rusted through, replace it.
   Evaporator coil drain pan — measure your pan dimensions before ordering a replacement.
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8. Reinstall the access panel, restore power, and run the system for 30 minutes to confirm no overflow occurs.

Cause 4: Leaking or Improperly Sloped Flue Pipe

On high-efficiency furnaces, PVC flue pipes must slope slightly downward toward the furnace so that condensation drains back in rather than pooling at joints. An improperly sloped or disconnected flue joint can drip water near the furnace.

1. Turn off the furnace completely.
2. Trace the white PVC flue pipes from the furnace to where they exit the wall or roof.
3. Use a level to check the slope of horizontal flue sections — they should pitch back toward the furnace at approximately 1/4 inch per foot.
4. Check all pipe joints for gaps, cracks, or missing cement by running your hand along each connection.
5. If a joint is separated, re-cement it with PVC primer and PVC cement rated for flue use.
   PVC primer and cement kit for repairing flue pipe joints.
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6. If slope is incorrect, loosen pipe hangers and re-support the pipe at the correct angle using adjustable pipe straps.
7. Allow cement to cure fully before restoring power and testing.

Cause 5: Faulty Whole-House Humidifier

Many furnaces have a whole-house humidifier mounted directly on the plenum or return duct. A failed solenoid valve, cracked water panel housing, or clogged drain can cause the humidifier to drip water that pools near or under the furnace.

1. Turn off the furnace and shut off the water supply valve feeding the humidifier (typically a small saddle valve on a nearby cold water pipe).
2. Remove the humidifier cover and inspect the water distribution tray, water panel (evaporator pad), and housing for cracks or mineral scale buildup.
3. Check the humidifier drain hose for kinks or clogs and clear any blockages.
4. Replace the water panel if it is heavily scaled or collapsing — this is an annual maintenance item.
   Whole-house humidifier water panel (evaporator pad) — match the model size printed on your existing panel.
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5. If the solenoid valve drips continuously even when the humidifier is off, replace the solenoid valve.
   Humidifier solenoid valve replacement.
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6. Restore the water supply and power, then observe the humidifier through a full heating cycle for leaks.

Cause 6: Cracked Secondary Heat Exchanger (Serious — Call a Pro)

In high-efficiency furnaces, the secondary (condensing) heat exchanger extracts additional heat from exhaust gases and produces significant condensation. A crack in this component can allow water to leak internally and is also a potential carbon monoxide hazard.

1. Turn off the furnace immediately at both the thermostat and the breaker.
2. Do not attempt to operate the furnace until the heat exchanger has been inspected — a cracked heat exchanger can leak carbon monoxide into living spaces.
3. Install a carbon monoxide detector on the same level as the furnace if you don’t already have one.
   Carbon monoxide detector — essential safety device for any home with a gas furnace.
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4. Contact a licensed HVAC technician to perform a heat exchanger inspection using a combustion analyzer or smoke test.
5. If the heat exchanger is confirmed cracked, discuss with your technician whether repair (heat exchanger replacement) or full furnace replacement is more cost-effective — on older units, full replacement is often recommended.

For authoritative guidance on furnace safety and efficiency, see the U.S. Department of Energy’s Furnaces and Boilers resource.

Frequently asked questions