Water Leaks Around a Furnace Flue

Condensation

• High-efficiency furnaces cause low temperatures in the flue because they extract more heat from burning fuel, instead of sending it up the chimney. If the flue temperature is too low, water vapor condenses inside the chimney instead of being vented out. The water vapor, which is a byproduct of burning fuel, contains acidic pollutants that eat away at the liner and mortar, causing them to crumble and causing flue blockages. Installing a correctly sized, insulated flue liner can solve this problem.

Cap

• Chimney caps keep water and debris out of the flue. However, many masonry chimneys don’t have a cap because local code regulations may not require them. When the cap is missing from your chimney, water can enter through the flue and cause the chimney to corrode. Prefabricated chimneys come with a cap, but sometimes they’re blown off or damaged by rust, allowing water to soak the liner and causing it to deteriorate.

Crown

• Chimney crowns are designed to remove water from the top of the chimney, but cracked chimney crowns are one of the main reasons why masonry chimneys leak water. That’s because many masons use simple brick mortar instead of cement or concrete mortar when they’re constructing the crown. Brick mortar cracks when the air freezes and thaws, and the cracks fill up with water that finds its way into your house when it rains.

Flashing

• Flashing, which is usually made out of sheet metal, is installed to keep water from entering the space between the chimney and the roof. However, damaged flashing allows water to leak into the chimney. Sometimes the sealant used on the flashing will deteriorate, allowing the flashing to rust and leak.

Symptoms

• Look for powdery deposits on the outside of your masonry chimney. Other symptoms of water damage include a cracked chimney cap, cracks in the ceramic flue liner, or eroded mortar joints. There may also be rust on metal flue pipes, or the cleanout door at the base of the flue may be corroded. In addition, you may see peeling wallpaper, blistered paint or damp spots on the walls.