Construction of a Furnace Flue

Flue Size

• The size of your flue depends on several factors such as the height of the chimney, the shape of the flue, the lateral length and type of the vent pipe, the number of connectors and the size of the furnace. Calculating flue size can be complicated. Most states use the International Residential Code, which includes charts to help you size your flues correctly. Your furnace’s installation manual will also include information about the flue size needed for that furnace.

Connectors

• Flue vent connectors are also known as flue pipes or stack pipes. These metal pipes connect the furnace to the chimney. Connector types vary, depending on the type of furnace you have. For instance, gas furnaces use Type B vents while oil furnaces require Type A. Connectors should also be kept as short as possible to avoid problems with condensates, soot and debris. Building codes regulate the slope of the connector between the furnace and the chimney, and limit the slope to no more than one-fourth of an inch per foot of horizontal run.

Liners

• All flues must be lined. Unlined flues let carbon dioxide and smoke leak into the living areas of your house. Smoke also enters the chimney chase area, which is not accessible for cleaning. Creosote can build up here and cause a dangerous chimney fire. Flue liners are made from different materials such as ceramic, metal or clay. Gas furnaces can use aluminum liners because the gases they produce aren’t as corrosive as those produced by oil furnaces. In addition, flue temperatures are lower. Metal liners for oil furnaces should be made out of stainless steel. The National Fire Protection Association publishes codes and standards that cover the construction, installation, and required clearances for flue liners.

Sealing

• Gaps around the furnace flue opening should be sealed with aluminum flashing and high-temperature silicon caulk, according to the United States Environmental Protection Agency. Most building codes require a 1-inch clearance between metal flues and any nearby combustibles such as insulation. Masonry chimneys need a 2-inch clearance. Build a metal dam in front of the insulation to keep it away from the pipe.