Quick answer
A furnace leaking water is almost always a high-efficiency (condensing) furnace with a blocked, cracked or frozen condensate drain — the unit makes several litres of water a day and it has nowhere to go. Other causes: a dead condensate pump, a leaking furnace-mounted humidifier, or in summer a frozen AC coil thawing above the furnace. Shut it off, mop up, and expect a $150–$450 fix.
A puddle at the base of the furnace usually has a mundane cause. A high-efficiency furnace pulls so much heat out of the exhaust that water vapour condenses inside the secondary heat exchanger and the vent pipe, and a furnace running hard in a GTA January produces several litres of acidic condensate a day. That water leaves through a small trap and drain line, and when any part of that path fails it ends up on the basement floor instead. If your furnace is a mid-efficiency model venting into a chimney, water is a different and more serious story, covered below.
Most likely causes of a furnace leaking water, in order
- Clogged condensate drain line — algae, dust and combustion residue block the small PVC or vinyl line and water backs up into the cabinet. This is the cause on the majority of calls.
- Failed condensate pump — where the furnace drains uphill to a laundry sink or floor drain, the pump beside the unit dies or its float sticks, and the reservoir overflows.
- Cracked or loose trap and fittings — the trap under the furnace is plastic; vibration loosens hose clamps over the years and the trap body itself can split.
- Frozen drain line — a drain routed through an unheated garage or out through the wall ices up in a cold snap, and the furnace shuts down on the float or pressure switch soon after; it is one of the winter causes in why a furnace will not turn on.
- Whole-home humidifier leak — a furnace-mounted humidifier has its own water feed and drain; a stuck solenoid, a scaled pad frame or a kinked drain hose drips down onto the furnace cabinet.
- Frozen evaporator coil (summer) — if the leak appears while cooling, the AC coil above the furnace has iced over and is melting; the AC freezing up guide explains why.
- Blocked or wrongly sloped vent — rare, but an intake or exhaust pipe pitched the wrong way traps condensate where it should not sit, and usually trips a pressure-switch code too.
Is it the furnace or the air conditioner?
Timing answers this. A leak during heating season is the furnace or the humidifier. A leak in July or August with the AC running is the evaporator coil: its drain pan sits in the plenum above the furnace and often shares the same drain, so a clogged coil drain, a rusted pan or an iced coil all send water down the outside of the cabinet. In shoulder seasons, when both run in the same week, check the humidifier first — close its damper and water valve and see whether the drip stops.
What to do right now
- Turn the furnace off at the thermostat and at the switch beside the unit, so it stops making condensate.
- Mop up standing water and keep it away from the control board at the bottom of the cabinet and from any floor-level receptacle.
- Trace the drain line from the trap to where it ends: look for a kink, a disconnected hose, a full pump reservoir or a section frozen solid.
- Photograph the wet spot and the drain route — it tells the technician where to start.
- Check the light in the furnace door for a flash code; a pressure-switch or float-switch fault confirms the drain is the problem.
Is a leaking furnace dangerous?
Water itself will not make a furnace unsafe, but it corrodes what it reaches: the burners, the cabinet base and the control board. A furnace that has run wet for weeks turns a $200 drain cleaning into a $1,000+ board and inducer job. Condensate is also mildly acidic — about the pH of orange juice — so a drain that lets it pool on concrete leaves a white crust and eventually pits the floor. If water has reached the board, or the furnace is cycling on and off with a fault code, stop running it and book furnace repair.
A mid-efficiency furnace should never leak
An 80% furnace venting into a masonry chimney is not supposed to produce condensate at all. If one is dripping at the vent connector or leaving rust streaks down the chimney breast, the flue gases are condensing inside the chimney — usually because an oversized or unlined chimney, or one that now serves only a small water heater, runs too cold. That condensate eats the mortar and the vent pipe and can end in spillage. It is a venting problem, not a drain problem, and it needs a licensed gas technician rather than a mop.
What a furnace water leak costs to fix
Condensate drain cleaning or trap replacement runs $150–$350 in the GTA, and a new condensate pump installed is $250–$450. Humidifier repairs vary with the part but are usually under $400. Re-routing a drain that freezes every winter — inside the heated envelope, or with a heat-traced section — is a small job done once. Every annual maintenance visit includes flushing the trap and drain line, which is why furnaces on a plan almost never leak.
If the water keeps coming back after you have cleared the line, or the furnace has shut itself down, ZK Mechanical can trace the whole condensate path in one visit anywhere across the GTA and Hamilton. Book a repair online or call (647) 801-1252.