Why is there a puddle of water around my furnace?

October is when this shows up. The furnace starts running every day, the humidifier is turned back on, and anything that dried out or clogged over summer gets tested at once. If your furnace is a high-efficiency model, it has a second heat exchanger that pulls so much heat from the exhaust that the water vapour in it condenses. That water, often litres a day on a cold day, runs through a plastic trap and tube to the floor drain or to a small pump. When any part of that path fails, the water ends up beside the furnace instead.

Is it normal for a furnace to leak water?

No. A high-efficiency furnace makes water, but none of it should reach the floor. Find out which kind you have: a high-efficiency (condensing) furnace vents through two white or grey plastic pipes that usually go out a side wall, while an older mid-efficiency furnace vents through a metal pipe into a chimney. Trane notes that standard furnaces with metal venting do not produce condensate, so water beside one of those almost always comes from something else: a humidifier, the AC coil above it, or a nearby water heater or pipe.

What's causing the water? Narrow it down

How do I check the condensate drain myself?

Switch the furnace off first, then follow the white tubes from the side of the furnace to where they end. Look for a tube that has slipped off its fitting, a split in brittle vinyl tubing, or a trap (a clear or white plastic U or box on the cabinet) full of brown sludge. The water is mildly acidic and grows slime, which is why traps clog. If a tube has just slipped off, push it back on and secure it. If the trap is clogged and your manual shows how to remove it, rinse it in a bucket and refit it exactly as it was. Do not open the burner compartment, and do not bend tubes to make them reach: a trap that is not refitted properly can let exhaust leak or keep the furnace from starting.

Why does my condensate pump keep overflowing?

Usually the pump has failed, its float is stuck, or the hose it pumps into is blocked or frozen. Pumps are fitted where there is no floor drain close enough for gravity. Pour a little water into the pump's reservoir: it should click on and empty within seconds. If it hums but nothing moves, follow the discharge hose for kinks, and check the end of it if it runs outside, because a hose that ends outdoors can freeze shut in January. Many pumps have a safety switch wired to shut the furnace off when the reservoir is full, so an overflowing pump often shows up first as no heat.

Could the humidifier be leaking?

Yes, and fall is when it happens, because the humidifier is switched back on after summer with a dried, scaled pad. A bypass or drum humidifier sits on the duct above the furnace and sends extra water to the drain through a small tube. Look for drips on the side of the duct or a wet tube. Turn off the small saddle valve on its water line, then replace the pad and check the drain tube. Set the humidistat lower when outside temperatures drop: too much humidity also shows up as wet windows.

What if the water isn't coming from the furnace?

Dry the floor completely, put down paper towels, and watch where water appears first. In most basements the furnace shares a corner with the water heater, the laundry, the main shut-off and the floor drain, so the furnace often gets blamed for another leak. Water that spreads from the water heater base points to the tank. Damp along the wall or floor joint after rain points to foundation seepage. A floor drain that bubbles or backs up is a drain problem, not a furnace one.

In Toronto and York Region homes, what makes it more likely?

Enbridge Gas supplies natural gas in Toronto, Vaughan, Richmond Hill, Markham, Newmarket and Aurora, so most houses heat with a gas furnace in the basement, and most newer ones have a condensing furnace with a drain to look after. A few local patterns come up:

• Newer homes: about half of Vaughan's homes, and about 44% of Richmond Hill's, were built in 2001 or later (Statistics Canada, 2021 Census). Most have high-efficiency furnaces, often with a condensate pump because the furnace is far from the floor drain in a finished basement.
• Older homes: about 30% of Newmarket's homes were built before 1981. A replacement high-efficiency furnace in an older house often has long drain runs and a pump added at install, which are the parts that fail.
• Hard water: Newmarket's water was the hardest of the four York Region towns Doyo lists, at 141 mg/L, and Markham's is 123 mg/L. Hard water scales humidifier pads and valves faster.
• Freeze and thaw: a condensate hose or pump line that ends outside, or runs through an unheated garage, can freeze solid in a cold snap and back water up into the basement.

What should I tell the technician?

• When the water appears: heat on, AC on, or with the furnace off
• Where it is: under the front of the cabinet, by the pump, from the duct above, or from the side of the water heater
• The vent pipes: plastic through the wall or metal to the chimney
• Whether there is a condensate pump or humidifier, and whether the furnace has been shutting off
• The furnace's age and model (on the label inside the front panel)
• A photo of the puddle, the tubes and the pump

What does a furnace check cost on Doyo?

Furnace Maintenance on Doyo is $206.50, all-in with Doyo's fee included. Mention the water when you book, so the technician looks at the trap, the drain and the pump on the visit. A leak caused by a failed part is booked as Furnace Repair, priced by written quote before any work starts. In Ontario, gas work needs a TSSA-registered contractor and a certified gas technician, and Doyo checks the technician's certificate before they are offered heating work.