Furnace Leaking Water? Where It Comes From and 3 Things to Check
A puddle at the base of the furnace looks alarming, but a furnace doesn't hold water the way a water heater does. The water is almost always condensate — and the question is just where it's coming from and why it isn't draining.
Safety first: if you smell gas, leave the house and call your gas company from outside. Water and gas problems can happen at the same time; the gas one wins.
Where the water comes from
- A high-efficiency furnace (the kind with white PVC pipes instead of a metal flue) makes water as a byproduct of burning gas efficiently. It drains out through a small hose or pipe, often into a floor drain or a small pump.
- The air conditioner's coil usually sits right on top of the furnace. In cooling season, it drips condensation into a pan that drains away. If that drain clogs, the pan overflows — straight down onto the furnace.
- A humidifier mounted on the furnace or ductwork has a water line and drain of its own, and they leak when the pad clogs or a fitting loosens.
So: water in winter with a PVC-vented furnace points at the furnace's own drain. Water in summer points at the AC drain. Water year-round near a humidifier points at the humidifier.
1. Is the drain line clogged?
Follow the small drain hose or PVC line from the furnace or the AC pan. Where does it go — a floor drain, a utility sink, a small box (a condensate pump)? If the line is kinked, disconnected, or you can see gunk at the open end, that's the clog. If it runs to a floor drain, make sure the drain itself isn't backed up.
Clearing a drain line is a homeowner job on some setups and not on others. If you're not sure, don't force it — a technician clears it in minutes.
2. The condensate pump (if you have one)
A small plastic box near the furnace with a hose going up and out — that's a condensate pump. If it's full of water and not pumping, it has either lost power or failed. Check that it's plugged in. If it's plugged in and full, the pump is done; it's an inexpensive replacement.
Many systems have a safety switch on that pump: when it fills, it shuts the furnace or AC down until the water clears. If your system stopped and there's water, this is why.
3. The humidifier
If you have a humidifier on the furnace, turn its water supply off (there's a small valve on the water line feeding it) and see if the leak stops over a day. If it does, the humidifier is the source — usually a clogged pad or loose fitting.
When it's a pro
Water plus a furnace that won't run, a pump that's dead, a drain you can't find, or any leak you can't stop by closing the humidifier valve — call. Standing water around the furnace shouldn't sit; it rusts the cabinet and can short the controls.
Before you call, run the free troubleshooter. It captures the season, the water location, and what you found, and gives you a summary to read to the dispatcher.
Related symptoms
Start the free troubleshooter → Free. No sign-up. Don't pay $150 to flip a switch.
Start the free troubleshooter →Free · no sign-up · no personal info · about ten minutes