Furnace Condensate Drain Clogged in Greater Vancouver

High-efficiency condensing furnaces (90%+ AFUE) produce condensate — acidic water that forms when combustion gases cool. When the condensate drain system clogs, the furnace will trip a safety sensor and shut down.

What This Symptom Means

Condensate must flow continuously from the heat exchanger and inducer assembly to a drain. If the condensate tubing, trap, or pump becomes blocked, condensate backs up, triggers a float switch, and the furnace goes into lockout. The furnace will not restart until the drain is cleared.

Common Causes

• Algae or biofilm growth inside condensate tubing — common in warm months
• Condensate trap blocked with debris or biological growth
• Drain line pitched incorrectly — water pools rather than draining by gravity
• Condensate pump failure if pump is used instead of gravity drain
• Cold temperatures causing condensate to freeze in external drain line

What Homeowners Can Check Safely

• Check the condensate drain line — a clear plastic tube leading from the furnace to a floor drain or condensate pump — for visible kinks or blockages
• If a condensate pump is present, check it has power and is not making unusual noises

When to Call a Licensed Technician

Some furnace issues require a licensed HVAC technician. Do not attempt to open gas lines, replace internal components, or reset persistent lockouts without professional diagnosis.

• Condensate drain cleaning — flushing with diluted bleach or drain cleaner
• Condensate trap cleaning and reinstallation
• Condensate pump replacement if pump motor has failed
• Drain line re-routing or re-pitching if slope is inadequate
• Freeze protection measures for external drain runs in cold areas

Frequently Asked Questions

How do I prevent my furnace condensate drain from clogging?

Pouring a cup of diluted bleach (1 part bleach to 10 parts water) down the condensate drain inlet annually inhibits algae and biological growth. Your technician can do this during an annual tune-up. Replacing the condensate trap annually is also a good preventive measure.

Why does only my high-efficiency furnace have a condensate drain?

High-efficiency condensing furnaces extract so much heat from combustion gases that the gases cool below their dew point inside the heat exchanger, condensing water vapour into liquid. Lower-efficiency furnaces exhaust gases at higher temperatures — the gases leave as vapour and do not condense. The condensation is a byproduct of high efficiency.

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