Why Is My Water Heater Vent Pipe Sweating?

Published: July 20, 2026 — CANRO Heating & Cooling — Greater Vancouver, BC

Why It Happens

Vent pipe sweating occurs when warm, moist exhaust gases cool below the dew point as they travel through the vent pipe. This is more common in cold weather when the vent pipe is in an unheated space. The condensate is slightly acidic and can corrode metal vents.

When It Is a Problem

Excessive condensation can: rust through metal vent pipes (creating CO leakage paths), drip water back into the water heater burner area, or cause water damage to the surrounding area. If you see rust or water stains near vent connections, the vent may need replacement.

Solutions

Solutions include: double-wall vent pipe (stays warmer than single-wall), insulating the vent pipe in cold areas, reducing the water heater temperature (less condensation at lower exhaust temperatures), or switching to a power vent model that uses PVC venting (unaffected by condensation).

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Safety Reminder

If you smell gas, suspect carbon monoxide or believe there is an immediate danger, leave the property and contact emergency services or the appropriate gas emergency authority. Do not remain inside — exit the building immediately and call for help from outside.