By Mario Vaghei, General Manager, Aire One Peel Heating & Cooling · Published November 2, 2026 · Facts verified October 30, 2026
Short answer
Most water around a furnace in Ontario comes from a high-efficiency furnace. These furnaces make water (condensate) as they run and drain it away through a small plastic line. If the drain line is clogged or blocked, the water backs up and leaks from the furnace. Other causes include a broken condensate pump, a leaking humidifier and, more seriously, a cracked or corroded heat exchanger. Wipe up the water, keep it away from wiring, and book a service call.
On this page
Why a high-efficiency furnace makes water · Common causes · What to do right now · When water is a safety warning · How to prevent it · When to call us · Frequently asked questions
Why a high-efficiency furnace makes water
A high-efficiency gas furnace pulls so much heat out of its exhaust that water vapour in the exhaust condenses into liquid. That water, called condensate, runs through a trap and a small drain line to a floor drain or to a condensate pump. Carrier notes that high-efficiency units produce more condensate, so a drain problem shows up faster on them. A little water in the drain line is normal. Water on the floor is not.
Common causes
| Cause | What you might see | Who fixes it |
|---|---|---|
| Clogged or blocked condensate drain | Water pooling under or beside the furnace; the furnace may shut off | A technician clears and flushes the drain and trap |
| Broken or loose condensate pump | Water around the small pump box; the pump does not run, or a hose has come loose | Check the hose is in place; a technician replaces a failed pump |
| Faulty or leaking humidifier | Water on the side of the ductwork where the humidifier is mounted | Shut the humidifier water valve; book a service visit |
| Condensate not draining properly | Drips from the vent or drain fittings, a sagging or disconnected line | A technician re-slopes or reconnects the line |
| Cracked or corroded heat exchanger | Water or rust inside the cabinet, soot, a yellow flame, or a carbon monoxide alarm | Switch it off and book an inspection right away |
Table: causes as listed by Carrier Canada ("Furnace is leaking water", read October 30, 2026), with what our technicians see on service calls in Peel Region.
In summer, the air conditioner coil that sits on top of the furnace also makes water. A blocked air conditioner drain can look like a furnace leak. Our evaporator coil guide covers that side.
What to do right now
- Keep water away from electricity. If water is near the furnace's wiring, switch the furnace off at the thermostat and at the service switch on or near the furnace.
- Wipe up the water so it does not soak into flooring or drywall. Standing moisture leads to mould.
- Look at the drain line and pump. Check that the small plastic line is still connected and that the pump's hose has not slipped out of the floor drain.
- Shut the humidifier's water valve if the water is coming from the humidifier.
- Book a service call. Carrier advises calling a professional instead of attempting repairs yourself.
Do not open the burner compartment or bypass a safety switch to keep the furnace running.
When water is a safety warning
A cracked heat exchanger can let combustion gases, including carbon monoxide, into the air you breathe. If a carbon monoxide alarm sounds, or anyone has headaches, nausea or dizziness that ease outside, get everyone out and call 911 from outside. Our guides to a cracked heat exchanger and furnace carbon monoxide in Ontario explain the signs and the rules.
How to prevent it
- Have the condensate drain and trap checked and cleaned as part of the yearly furnace maintenance.
- Change the filter on schedule.
- Have the humidifier pad and valve checked each fall.
When to call us
If you find water around your furnace, book furnace repair in Mississauga. Our TSSA certified technicians serve Mississauga, Brampton, Etobicoke, Milton, Georgetown, Caledon and Bolton. Call 905-564-8545.
Frequently asked questions
Why is my furnace leaking water?
On a high-efficiency furnace the most common cause is a clogged or blocked condensate drain, which makes the water back up and leak. A broken condensate pump, a leaking humidifier or a cracked heat exchanger can also cause it.
Is it normal for a furnace to have water?
A high-efficiency furnace makes water and drains it through a small line, so water in the drain line is normal. Water on the floor around the furnace is not.
Is a furnace leaking water dangerous?
It can be. Water near wiring is an electrical hazard, moisture can lead to mould, and if the cause is a cracked heat exchanger there is a risk of carbon monoxide. Switch the furnace off if water is near wiring and book a service call.
Can I still run my furnace if it is leaking water?
If the water is near electrical parts, or a carbon monoxide alarm has sounded, switch it off. Otherwise, book a service call as soon as you can rather than letting it keep leaking.
How do I stop a furnace from leaking water?
Have the condensate drain and trap cleaned during yearly maintenance, check that the drain line and pump hose are connected, and have the humidifier checked each fall.
Sources and references
- Carrier Canada, "Furnace is leaking water", read October 30, 2026.