Why is my furnace leaking water?
A furnace leaks water when a drain fails. On a 90%+ furnace, that's usually a clogged condensate trap or line, a dead condensate pump or a frozen drain line. A humidifier can leak in any season, and in summer so can the AC coil above the furnace. An 80% furnace makes no condensate.
Key points
- Find the source before you fix anything: the condensate trap and hose, the pump, the humidifier, the AC coil or plumbing nearby.
- Only 90%+ furnaces make condensate, and the longer the burners run, the more water they make.
- If the drip is coming from the humidifier, closing its small water valve usually stops it.
- Water on the control board, inside the burner area or dripping from a metal vent pipe means turn the furnace off and call.
- HomeGuide's typical national ranges: $100–$450 for a condensate pump and $75–$250 to clear a drain line.
A furnace leaks water when one of the drains around it stops doing its job. On a high-efficiency furnace (90% AFUE and up), that's usually a clogged condensate trap or drain hose, a condensate pump that quit, or a drain line frozen where it leaves heated space; a whole-home humidifier can leak in any season, and in summer the air conditioner's coil on top of the furnace is the usual source. A standard 80% furnace makes no condensate at all (Lennox's leak guide says as much), so a puddle around one of those is coming from something next to it.
Where the water shows up, and when, tells you most of what you need to know.
Start with where and when you see the water
| Where the water is | When | Most likely source | How urgent |
|---|---|---|---|
| Front or base of a 90%+ furnace, near the small hoses | Heating season | Plugged condensate trap or drain hose | Today; the furnace may lock out |
| Around a small pump box next to the furnace | Any season | Condensate pump full, float stuck or pump worn out | Today |
| Running down the ductwork above the furnace | Heating season | Humidifier drain plugged, or its water valve stuck open | Close the humidifier's water valve now |
| Dripping from the coil box on top of the furnace | Cooling season | Clogged AC drain, or a frozen coil melting | Turn the cooling off |
| Dripping from the metal vent pipe of an 80% furnace | Cold weather | Exhaust condensing in the vent or chimney | Book a venting check this week |
| Inside the burner compartment, or rust on the burners | Any season | A leak inside the furnace | Turn the furnace off and call |
| A puddle with nothing dripping above it | Any season | Water heater, floor drain or a plumbing line | Check those first |
How does a high-efficiency furnace drain water?
A condensing furnace pulls so much heat out of its exhaust that the water vapor in it turns to liquid in a second heat exchanger, which Trane's guide to condensing furnaces walks through. That water collects in a box on the furnace, passes through a trap, and leaves through a small hose to a floor drain or a condensate pump. The trap holds a little water as a seal so exhaust can't escape down the drain.
The longer the burners run, the more water the furnace makes, so a slow drain that kept up in October can overflow in January. Our AFUE explainer covers how condensing and 80% furnaces differ.
The usual causes, in the order we check them
A plugged trap or drain hose
Sediment and slime collect in the trap and the first few inches of hose. When water can't get out, it backs up into the furnace, and on many models it reaches the pressure switch: the inducer runs, the burners never light, and the furnace locks out. You may hear gurgling at startup or see water at the front of the cabinet. A kinked hose, a hose end that slipped out of the drain, or a sag that holds water does the same thing.
A condensate pump that's full or failed
The pump is a small box with a reservoir and a float that switches it on as the water rises. Slime can hold the float down, the check valve can plug, and pumps wear out. Many pumps also have an overflow switch wired to shut the system down before the reservoir spills, which is why a dead pump can look like a dead furnace.
A drain line frozen in a cold spot
A line that runs through a garage or crawlspace, or out through the rim joist to the yard, can freeze in a cold snap and back water up into the furnace. Our guide to a frozen furnace condensate line covers thawing it safely and the fixes that stop it from happening every January.
The humidifier
A bypass humidifier hangs on the ductwork above or beside the furnace. When it runs, a valve sends water over a pad, and the water that doesn't evaporate goes down a small drain line, often to the same pump. A plugged drain line, a cracked housing or a valve stuck open sends that water down the outside of the duct and into the furnace cabinet. Our whole-home humidifier guide shows how the pieces fit.
In summer: the AC coil
In most houses with central air, the cooling coil sits on top of the furnace and makes water all summer. A clogged drain overflows the pan, and a coil that iced over from low airflow drips as it thaws. Our AC condensate drain guide walks through clearing the line. Filter trouble, clogged or missing, is a common reason a coil ices; see our post on running a furnace without a filter.
Which furnace leaks need a technician?
Water inside the furnace. If water is coming from the burner area or the cabinet itself, the leak is inside the furnace: a cracked collector box, a loose internal hose, or a leaking secondary heat exchanger, which Lennox lists among the causes of furnace leaks. Water there rusts burners and can ruin a control board. Turn the furnace off and call.
Water from the vent of an 80% furnace. An 80% furnace sends hot exhaust up a metal vent. If that exhaust cools enough to condense on the way out, usually in a cold exterior chimney or a long run through unheated space, water drips back down the pipe. That's a venting problem, and EPA warns that blocked, leaking or damaged chimneys and flues can release combustion gases, including carbon monoxide. Have it checked, and see our post on whether a furnace can leak carbon monoxide.
What you can safely check today
- Protect the electrical parts. Put towels down. If water is on or near the control board or wiring, turn the furnace off at its service switch and stop here.
- Note the timing. Water only with the heat on points to the condensate or humidifier, only with the AC on to the coil drain, and with neither to the water heater or plumbing.
- Check the pump. Make sure it's plugged in. Unplug it before you lift the lid, rinse the float if it's slimy, plug it back in and pour in a cup of water. It should kick on and empty.
- Trace the drain hose from the furnace to the drain. Look for a hose end that slipped out, a kink, a sag full of water, or ice at the outside end.
- Shut off the humidifier if the water is coming from it: close the small valve on its water line and turn its control down.
- Reset once. If the furnace locked out, turn the service switch off for about 30 seconds, then back on, and watch a full cycle.
Leave the trap and the small rubber hoses on the furnace alone. The trap's water seal keeps exhaust out of the drain, and a hose put back wrong can leak or trip the pressure switch. Never bypass a pump's overflow switch to get the heat back.
Stop and call if:
- Water reached the control board, the wiring or the burner area.
- The water is coming from inside the cabinet or from a metal vent pipe.
- The furnace locks out again after one reset.
- The leak comes back within a day or two.
Why do Minnesota basements see more furnace leaks?
A Minnesota condensate system works most of the year. The furnace drains water from fall into spring, the humidifier adds more all winter, and the air conditioner takes over in June. In many basements all three empty into one pump, so a pump that fails can stop the furnace in January and flood the floor in July.
House age matters too. Older Minneapolis and Saint Paul basements often have a single floor drain across the room from the furnace, so the hose runs a long way along the floor, or the pump sits behind a finished wall where nobody sees it fill. Newer houses without a nearby floor drain sometimes send the pump's discharge out through the rim joist, where it freezes. We see both on furnace repair calls in Minneapolis and in Saint Paul.
What does it cost to fix a leaking furnace?
HomeGuide's typical national ranges are $100–$450 to replace a condensate pump and $75–$250 to clear a drain line. Our Minneapolis cost page explains why those pumps are so common in city basements. If water has already ruined the control board, HomeGuide's typical national range for a new board is $300–$650, plus $90–$200 if rewiring is needed, which is the best argument for turning the furnace off early.
With us, a service call is a $79 trip charge, waived when you approve the repair, and you get a written, flat-rate price before any work starts. A furnace tune-up is $129, or $219 with the air conditioner, and on a high-efficiency furnace it includes flushing the condensate trap and checking the drain or pump before the season starts.
When to call us
If the water keeps coming back, it's coming from inside the furnace, or the furnace won't stay lit, call 612-429-3465. We're open 7 days, 7am–9pm, and no-heat calls go to the front of the schedule. Our furnace repair page covers what every visit includes.
Questions
Is it safe to keep running a furnace that's leaking water?
It depends on where the water is. A slow drip from the drain hose or pump onto the floor can usually wait a day with towels down, and many furnaces shut themselves off when condensate backs up. Water on the control board, the wiring or inside the burner area can't wait: turn the furnace off at its service switch and call. Neither can water dripping from a metal vent pipe.
Why does my furnace leak water only when the heat is on?
On a high-efficiency furnace, the burners make condensate every cycle, so a plugged trap, a slipped drain hose, a failing pump or a frozen line only shows up while the heat runs. A whole-home humidifier also runs only with the heat, so check it next. The air conditioner's coil doesn't make water in winter, which rules it out. On an 80% furnace, which makes no condensate, look at the humidifier first.
Why is there water around my furnace in the summer?
In summer the furnace is usually innocent. The air conditioner's coil sits on top of it and pulls water out of humid air, and that water drains through a trap and hose, often to the same pump the furnace uses in winter. A clogged drain, a full pump or a coil that iced over and is now melting are the common causes. Turn the cooling off and check the drain first.
What does it cost to fix a furnace that's leaking water?
Often not much, if you catch it early. HomeGuide's typical national ranges are $75–$250 to clear a condensate drain and $100–$450 to replace a condensate pump. Water that reached the control board costs more: HomeGuide puts a new board at $300–$650. With us, a service call is a $79 trip charge, waived when you approve the repair, and the price is written down before any work starts.
Sources
- HomeGuide cost guides (2026 editions) homeguide.com
- U.S. Environmental Protection Agency epa.gov