Toronto Chimney Repair · October 4, 2026
Why it happens
Older furnaces and water heaters shared one chimney, and the hot exhaust from the furnace kept the whole flue warm. A high-efficiency furnace vents through a plastic pipe in the wall instead. The water heater on its own produces far less heat, so its exhaust can't keep a large, cold masonry flue dry. Gas appliances in Ontario must vent through a flue sized for them, which is why installers and inspectors flag this situation.
Signs it is already happening
White crust or staining on the outside of the chimney
Damp patches or peeling paint on the wall around it
Crumbling mortar or bits of tile at the base of the flue
Rust on the water heater vent connector
The fix
A stainless liner sized to the water heater runs inside the old flue, with a cap on top. It keeps the exhaust warm enough to vent properly and protects the masonry. In the GTA these usually cost $1,450 to $2,500 plus HST, depending on the height of the chimney. If nothing else uses the chimney and the water heater is replaced with a power-vented model, removing the chimney above the roofline can be the better long-term answer.
