Furnace Condensate Drain Clogged — Emergency Fix Guide
Published: July 23, 2026 — BC Wide Home Services Ltd, doing business as BC Wide Heating & Air Conditioning — Greater Vancouver, BC
Why It Happens
The condensate drain on high-efficiency furnaces carries acidic water from the heat exchanger. Algae, mould, and debris can clog the drain line, causing the safety float switch to trip and shut down the furnace.
Emergency Steps
Turn off power to the furnace. Locate the condensate drain line (usually a white PVC pipe). Use a wet/dry vacuum at the exterior exit to suck out the clog. Pour a cup of white vinegar down the drain line to dissolve algae. Reset the furnace by cycling power at the breaker.
When to Call a Professional
If the drain remains clogged after clearing, or if the clog is inside the furnace cabinet (requires disassembly), call for professional service. Consider installing a condensate drain safety switch cover that makes future clearing easier.