Start by identifying which pipe is leaking
A furnace can have several pipes and lines, and the right fix depends on which one is wet. Turn the thermostat to off first, and cut power at the breaker if you can reach it safely.
- Condensate drain line or trap (condensing furnaces): usually PVC or clear/flexible tubing leading to a floor drain or condensate pump.
- Exhaust or vent pipe: larger PVC (or metal flue on older furnaces) that carries combustion gases outside.
- Intake pipe: brings combustion air in on sealed-combustion units.
- Humidifier supply or drain line: a small tube, often with a saddle valve, connected to the plumbing.
- Air conditioner drain line: if you have central air, the coil drain often runs near the furnace.
If you are unsure, dry the area with a towel, then watch for where fresh water appears.
Is it normal to have water coming out of an exhaust pipe?
For a high-efficiency furnace, some moisture in the exhaust is part of how it works, and a little water or vapor at the outdoor termination can be normal in cold weather. Water leaking from joints inside the house, or running down the vent pipe indoors, is not normal and can indicate a bad seal, poor pipe slope or a blockage. Vent problems relate to combustion gases, so treat them as a job for a professional.
Common causes
Clogged condensate drain or trap
Dust, debris and slime can clog the trap or line so water backs up and overflows. This is one of the most common causes on condensing furnaces.
Failed condensate pump
If a pump lifts condensate to a drain, a stuck float, a clogged pump or a lost power connection can cause overflow.
Cracked, loose or disconnected tubing
Old tubing can crack, and fittings can come loose, particularly after maintenance or vibration.
Humidifier leak
Mineral buildup, a stuck valve or a blocked humidifier drain can leak water that looks like a furnace problem.
Air conditioner drain or pan
When the air conditioner is running, a clogged coil drain can overflow near the furnace. See why a furnace leaks water in summer.
Internal component issues
Cracked inducer housings, bad seals or heat exchanger problems may cause water inside the cabinet. These require a technician.
Safe first steps
- Turn the furnace off at the thermostat, and at the breaker if safely reachable.
- Do not wade through standing water to reach the electrical panel. Call an electrician or HVAC company instead.
- Mop up water and set a fan to dry the area.
- Check that tubing is not kinked, disconnected or visibly cracked.
- Check whether a condensate pump is plugged in and its reservoir is full.
- Note the furnace brand and model for any service call.
How to clear a condensate drain
Follow your furnace manual because layouts vary. In general, with the power off:
- Locate the drain line and trap near the base of the furnace.
- Place a pan or bucket underneath to catch water.
- Disconnect the line or trap carefully, noting how it fits back together.
- Rinse out debris with water, and use a small brush if needed. Some people use a wet/dry vacuum on the outlet end of the line to pull out a clog.
- Reassemble snugly and make sure the line slopes toward the drain without dips where water can pool.
- Restore power and run the furnace while watching for drips.
Avoid using strong chemicals in the line unless the manufacturer says it is acceptable, since some can damage fittings or components. Do not remove or bypass safety switches such as a condensate overflow switch.
When to call a professional
- the water appears to come from the vent pipe, inducer or inside the cabinet
- you smell gas, see soot or have any carbon monoxide concern (leave the home and call emergency services and your gas utility)
- the leak returns after you clear the drain
- water has reached wiring or the control board
- you are not comfortable working near a gas appliance
For a broader look at risk, see is water leaking from a furnace dangerous.
Preventing repeat leaks
- Have the condensate drain, trap and pump checked at annual maintenance.
- Keep filters changed as the manufacturer recommends.
- Make sure drain tubing is supported and has no sags.
- Consider a float or overflow switch on pumps and AC pans, installed by a professional.
FAQ
Are furnaces supposed to leak water? Condensing furnaces produce water, but it should drain away through the condensate line. Puddles are a sign of a problem.
How long do gas furnaces last? It varies by model and maintenance, and many run well over a decade. A technician can assess your unit’s condition if you are weighing repair against replacement.
Can I use a pipe sealant on a cracked drain pipe? Replacing a cracked section or fitting is usually more reliable than a sealant. Use the correct parts and follow the manual.
Does a leaking pipe also apply to boilers? Boiler leaks involve hot pressurized water and need different precautions. See is a boiler leaking water dangerous.