Flues

Flues are passages that carry combustion exhaust from appliances such as fireplaces, furnaces, or boilers to the outdoors. They may be built as part of a masonry chimney or as metal venting systems, and are designed to create a safe, controlled path for hot gases and smoke to exit a building.

Construction and types: In traditional buildings, flues are masonry chimneys lined with clay tiles to protect the brick from heat and corrosive gases. Modern installations often use metal flues, typically stainless steel or coated galvanized steel. Some flues are insulated or double-walled to reduce heat loss and condensation. Flue liners or inner liners improve safety and performance by providing a smooth, corrosion-resistant channel.

Operation and performance: A natural draft relies on buoyancy—hot gases rise and create a draft that pulls air into the appliance. Draft is influenced by the flue height, cross-sectional area, interior smoothness, bends, and temperature difference between the flue and outside air. When requirements are not met, backdraft, condensation, or leakage may occur. Dampers or fans can regulate flow.

Safety and maintenance: Flues must be installed per building codes and appliance instructions. Regular inspection and cleaning remove creosote buildup that can cause chimney fires. Cracks, blockages, or improper clearances pose fire and carbon monoxide risks. A rain cap or storm collar helps prevent water ingress.

Sizing, codes, and terms: The flue size must match the appliance outlet and the venting configuration; common residential flues range from about 3 to 8 inches in diameter, with larger chimneys for fireplaces and boilers. Building codes require proper lining and vent termination to the outdoors. The term "flue gas" refers to the combustion products transported by the flue.