Why is my furnace leaking water?

By Dana Whitlock. Updated October 2026. Repair or replace?

Older furnaces do not make water. High-efficiency condensing furnaces do, a lot of it, and it has to drain away through a trap, a line and sometimes a small pump. Bryant notes that water pooling around a furnace is not normal and should be addressed promptly. Find the source by looking, protect the floor, and keep your hands out of the cabinet.

Likely causes

A clogged condensate drain line Safe to check

Dirt, algae and debris can block the drain, so water backs up and spills. It is the usual suspect. You can look at the accessible part of the line for clogs, kinks or loose sections. Internal traps are a technician's job.

A failed condensate pump Safe to check

Furnaces in basements often pump condensate to a drain. If the pump loses power or its float sticks, the reservoir overflows. You can check that it is plugged in and listen for it running.

A frozen drain line Safe to check

Lines routed outside or through cold spaces can freeze in winter and block drainage. Bryant warns against thawing with an open flame.

A leaking humidifier Safe to check

A whole-house humidifier mounted on the furnace can leak from its housing, supply line or drain. Shut off its water supply if you can reach the valve safely.

The air conditioning coil Safe to check

Water that shows up in summer usually comes from the AC evaporator coil sitting above the furnace. That is an air-conditioning condensate problem wearing a furnace costume.

Internal problems Pro job

Cracked internal drain parts or a heat exchanger issue can also show up as water. Bryant says not to attempt to inspect or repair the heat exchanger yourself because of carbon monoxide risk.

Safe things to check first

• Turn the heat off at the thermostat and mop up standing water.
• Replace a dirty filter. It will not fix a blocked drain, but it rules out an airflow problem.
• Trace the visible drain line for kinks, disconnected joints or ice.
• Confirm the condensate pump is plugged in and its outlet is not overflowing.
• If a humidifier is attached, close its water supply valve.
• Do not open the furnace cabinet or remove internal parts.

When to stop and call

Call a licensed HVAC technician if the line looks clear but water keeps coming, if the pump has failed, or if water is reaching electrical parts. In that last case, Bryant says to cut power at the service switch or breaker if it is safe to reach. A burning smell, a carbon monoxide alarm or a gas smell means stop. For CO, get outside and call 911. For gas, leave and call the utility from outside.

Repair or replace?

Start with its age. Type the serial from the label:

Drain cleaning and condensate pumps are inexpensive repairs worth doing on nearly any furnace. Age only matters if the leak traces back to the heat exchanger or other internal damage. DOE's analysis gives about 21.4 years for an average gas furnace. Date yours with the age decoder above. A young furnace with a drain problem is a quick fix. An older one with internal leakage should be priced against the replacement range above.