You open the closet and there's a puddle under the furnace. It feels wrong — a furnace burns gas, it doesn't hold water.
Usually nothing serious has failed. Modern high-efficiency (condensing) furnaces produce water on purpose: they pull so much heat from the exhaust that its moisture condenses back to liquid, and a working unit drains several quarts a day in heating season. A puddle means the drainage path for that water has stopped working. The other case is the one people misdiagnose: on an older non-condensing furnace, water on the floor usually isn't the furnace at all.
How it usually shows up:
- A puddle or damp ring on the floor under or beside the furnace.
- Water dripping from the bottom of the cabinet, or a stained, rusting cabinet base.
- A condensate pan that's full or overflowing.
- A wet spot on the ceiling below an attic furnace, or water tracking down the flue.
- The furnace shutting itself off — a float safety cutting the call when the pan fills.
Why your furnace is leaking water
- A clogged condensate drain line or trap — the number one cause. Condensate is mildly acidic, and over a season the line and P-trap collect sludge, rust flakes, and biological growth. Once the line backs up, water rises in the trap and spills out of the collector box or pan. Attic and garage installs with long, low-pitch drain runs clog fastest.
- A cracked or full condensate pan. Plastic pans crack with age and thermal cycling; metal ones rust through from acidic condensate. A pan that's simply full points back at the drain line.
- A failed condensate pump. Furnaces below the drain line rely on a small pump to lift condensate away. When its float sticks, its motor quits, or its check valve fails, the reservoir overflows within a cycle or two.
- A blocked, kinked, or disconnected drain hose. The short tubing between collector box, trap, and drain gets pinched behind the cabinet, knocked loose during a filter change, or dried out and cracked.
- It's actually the AC coil above the furnace. The misdiagnosis we correct most. On a standard-efficiency furnace with a cooling coil on top, the furnace makes no condensate — the evaporator coil does, and it has its own pan and drain. A clogged AC drain overflows onto the furnace and puddles at its base, reading as a furnace leak. If your unit is older, non-condensing, and leaking while cooling has run, start at the coil — our AC leaking water page covers that path.
- Flue or venting condensation — uncommon but real. A poorly sized or sloped flue lets exhaust cool and condense inside the vent, running back into the furnace. It shows as rust streaking rather than a clean puddle.
The usual fix
Our tech's first question is which furnace you have, because it splits the diagnosis cleanly: a condensing furnace (white PVC exhaust out a side wall) makes water, so the job is finding where drainage failed. A non-condensing furnace (metal flue through the roof) makes none, so the source is above or beside it.
From there it's a physical trace: flush the drain line and trap, verify slope and that the trap is primed, check the collector box and pan for cracks, test the condensate pump and its float, and confirm the safety switch cuts the call when the pan fills. If the coil above is the culprit, we clear that drain instead. Wherever water has been sitting we also check for rust on the heat exchanger and corrosion at electrical connections. Most leak calls end as drain and condensate work, among the least expensive repairs we do on a furnace.
DIY vs. calling a pro — the honest version
Safe to check yourself first (free):
- Turn the furnace off at the thermostat and its power switch. That stops the leak from growing and keeps water away from live electrical parts.
- Mop up and empty the pan, then run the furnace briefly and watch where water appears. Where it starts matters more than where it pools.
- Look at the condensate line. If you can reach the exterior termination, check for a clog; some traps have a clean-out cap you can pull and flush.
- Check the drain hose connections at the collector box and trap for anything kinked or popped off.
- Change the air filter. A restricted filter contributes to freezing on the coil above, which dumps water when it thaws.
Call a pro when:
- The leak comes back after you've cleared the drain — that means slope, trap design, or the pan.
- The condensate pump isn't running, hums without pumping, or its reservoir overflows.
- The water is coming from the AC coil above the furnace rather than the furnace itself.
- You see rust or corrosion on the heat exchanger, the collector box, or the cabinet base.
- Water has reached the control board or wiring, or the furnace won't start after a leak.
If the furnace has been sitting in water, have it checked before you run it again — standing water around a gas appliance with line voltage inside isn't something to work around.
What it costs in the Bay Area
Most of these calls are on the cheap end of furnace work: clearing a clogged condensate line, flushing and re-priming the trap, replacing a drain hose, or swapping a condensate pump is straightforward labor and inexpensive parts, so it sits near the bottom of our furnace pricing.
The cost only climbs if water got somewhere it shouldn't. A control board runs about $200–$600 if condensate corroded it — exactly why a slow leak is worth fixing early — and a blower motor is roughly $400–$700 in the rarer case water got into the motor. We charge a diagnostic fee that's credited toward the repair when you approve the work, always with a firm, upfront price first.
Stop the leak before it reaches the electronics
Triton repairs gas and electric furnaces across the Peninsula and East Bay as part of our full HVAC repair service — San Mateo, Burlingame, Redwood City, Alameda, San Leandro, and Hayward — where attic and garage installs with long condensate runs, and newer coils stacked on older furnaces, produce most of the water calls we see. Same-week and often same-day appointments, upfront pricing, licensed techs.
Book online or call (888) 993-5699. Tell us whether your exhaust pipe is white PVC or metal, and we'll know where to start.
Related pages
- HVAC Repair in the SF Bay Area — our full heating & cooling hub
- AC Leaking Water — when it's the coil above the furnace, not the furnace
- Furnace Not Heating / No Heat — when a leak takes the heat out with it
- Weak or No Airflow from Vents — the airflow problems behind a frozen, dripping coil
- HVAC & Furnace Repair in Redwood City — Peninsula service area
Frequently asked questions
Why is my furnace leaking water? If it's a high-efficiency condensing furnace — white PVC exhaust out a side wall — it makes condensate by design, and a leak means drainage failed: a clogged drain line or trap, a cracked or full pan, a failed condensate pump, or a disconnected hose. If it's an older furnace with a metal flue, it makes no water at all, and the source is usually the AC evaporator coil above it.
Is a leaking furnace dangerous? The water itself isn't, but where it goes matters. Condensate is mildly acidic, so it rusts the cabinet, the collector box, and eventually the heat exchanger, and it kills control boards when it reaches them. Turn the furnace off, stop the water, and have it traced before you keep running it.
Can I fix a clogged condensate drain myself? Sometimes. If you can reach the drain line's clean-out or exterior termination, flushing the clog is a reasonable homeowner job. What you can't verify from outside is whether the line has proper slope, whether the trap is primed, and whether the pan or collector box is cracked. If the clog returns, it's one of those.
My furnace is leaking but only in summer. Why? Because it probably isn't the furnace. The AC evaporator coil sits in the ductwork right above the furnace and makes condensate whenever cooling runs. When its drain clogs, the pan overflows onto the furnace below and looks like a furnace leak. Have the coil's drain cleared rather than the furnace serviced.
How much does it cost to fix a furnace leaking water in the Bay Area? Most of it is inexpensive: clearing a condensate line, flushing the trap, replacing a drain hose, or swapping a condensate pump sits near the low end of our furnace pricing. It gets costly only if water reached the electronics — a control board runs about $200–$600, and a blower motor roughly $400–$700 if water got into the motor. We credit the diagnostic fee toward the repair and quote a firm price first.