Troubleshooting A Blocked Furnace Flue

A blocked furnace flue, or vent pipe, can lead to poor combustion in the furnace. This can reduce heating efficiency and strain the furnace itself. Further, gasses won't properly vent from your home, which can pose a health concern. Furnace flues generally run through a dedicated vent to the roof, or the flue pipe may run through your existing fireplace chimney.

Causes of Blockages

Flue blockages can occur from either external sources or internal sources.

External Sources

External sources are things that get into the flue from the outside, often as a result of a missing or damaged vent cap. Birds are a common reason for blocked flues, either because the bird has gotten into the flue or because it has built a nest atop the flue. Aside from animals and nesting materials, debris from trees or windstorms can also get into a flue and cause a blockage.

Internal Sources

Scale and soot can get into the flue from the furnace itself. Soot is a byproduct of combustion, but it typically only forms when a furnace isn't burning clean. Its presence means you have issues beyond the flue and the entire system needs to be serviced. Scale is a result of moisture condensation due to the temperature fluctuations between the furnace venting and the outside air. It can be composed of hard minerals or more likely, rust and corrosion particles from inside the flue itself.

Symptoms of Blockages

Catching a blockage early is key to ensuring that further damage doesn't occur to the furnace itself, as well as preventing the entrapment of dangerous gasses in your home. Symptoms include the following:

1. Rust formation on the outside of the flue or vent stack, often as a result of moisture condensation rising due to trapped hot air.
2. Damaged or missing vent or chimney cap. The cap may be missing or it may appear to be off-center or sitting askew.
3. Soot in and around the furnace housing, which indicates that combustion isn't happening effectively — perhaps as a result of poor ventilation from the blocked flue.
4. Increased bird activity around your furnace vent pipe or chimney or the appearance of debris sticking out from the top of the vent pipe.
5. White efflorescence marks on the masonry, for homes where the furnace vents through the chimney. Moisture trapped in the flue causes mineral salts to leach out through the bricks.

Contact a heating repair technician immediately if you suspect that there is a blocked flue.