A high-efficiency furnace (90%+ AFUE) pulls so much heat out of the exhaust that water vapour condenses into liquid, roughly a few litres a day in cold weather. That condensate drains away through a small plastic line and trap. When the line clogs with algae or sediment, freezes at an exterior termination, or the trap backs up, water pools and a safety switch shuts the furnace down. It is one of the most common no-heat calls we get, and often a fix a homeowner can handle.
Signs of a Condensate Drain Problem
- Water pooling on the floor around the base of the furnace.
- The furnace starts, runs briefly, then shuts off in a repeating cycle.
- A gurgling sound from the drain line or trap.
- A pressure-switch fault code on the control board (the number of flashes varies by brand, so check the chart on the furnace door).
- A full or overflowing condensate pump reservoir (if your furnace uses a pump).
What Causes a Furnace Drain to Clog or Fail
Algae and sediment clogs
The most common cause. Condensate is slightly acidic and grows a slimy biofilm that builds up in the trap and line over a season or two, blocking flow.
A frozen drain line
If the line runs through an unheated space or terminates outside, an Ontario cold snap can freeze the water solid and block drainage. This is a winter-specific cause.
A blocked or dry trap
The P-trap can clog with debris or, less often, dry out and let flue gases interfere with the pressure switch.
A failed condensate pump
Furnaces in basements below the drain line use a small pump to lift condensate to a drain. A stuck float or failed pump backs everything up.
How to Clear a Furnace Condensate Drain
- Turn the furnace off at the thermostat and the furnace power switch before you start.
- Find the clear or white plastic drain line running from the furnace to a floor drain or condensate pump.
- Disconnect the line at the trap and check for a slimy clog. Flush it with a mix of warm water and a little white vinegar (avoid bleach on some components; vinegar is safer).
- Clean the trap itself, then reconnect everything snugly.
- If the line runs outside and is frozen, thaw it with warm water or a hair dryer on low; never use an open flame on the plastic.
- If you have a condensate pump, check that the float moves freely and the pump runs when you pour water in.
- Restore power and run a heat cycle to confirm the water drains and the furnace stays on.
When to Call a Technician
Call EcoFrost if the furnace still short-cycles after you have cleared the line, if you see a cracked or leaking drain pan, if the condensate pump has failed, or if the line freezes repeatedly (it may need rerouting). A pressure-switch code that persists after the drain is clear points to a separate fault that needs diagnosis.
Condensate faults are common on condensing furnaces 15 to 25 years old, which describes many original furnaces in 2000s Mississauga neighbourhoods such as Churchill Meadows and Lisgar. Our Mississauga furnace repair page lists what those repairs cost; for other cities see furnace repair across the GTA.
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