Flue Adjustment on a Furnace

Flue Size

• One way to adjust a furnace flue is to change its size by extending its height with additional piping or installing new connector pipes with different diameters. A flue's diameter determines the pressure of flue gasses and is essential to upward drafting, which relies on this pressure. The height of a flue also affects drafting. If your flue is not tall enough to draft, you can lengthen it or install a fan to force furnace gasses upward. A narrower flue or connector will restrict gasses and increase pressure, while a wider pipe will decrease pressure inside the flue.

Flue Lining

• The more efficient a furnace is, the less hot air it allows to escape through the flue. This means that furnace flues are cool enough for condensation to occur. This can lead to corrosion and holes in the flue. Flues are built with several types of interior linings, including stainless steel and clay. A new liner that resists the type of damage your flue is prone to will keep the furnace gasses drafting properly.

Repairs and Replacement

• In some cases, a flue may need replacement or repair. Leaks around the flue's joints and connecting points can be taped or wrapped to keep the pressure inside the flue constant and strong enough to cause gasses to rise. If your flue won't draft properly, you may need to adjust the exterior cap that prevents wind and rain from entering the chimney. A new cap or repositioning the existing cap will allow the flue to draft without exposing it to the elements.

Inspection and Cleaning

• Even if a flue is installed properly and functions normally, it needs periodic inspections and cleaning. Professional furnace servicers use electronic diagnostic tools to measure the pressure and chemical content of gasses rising in the flue. They also use special cleaning tools to remove buildup from the inside of a flue, along with other locations in the furnace, to keep air flowing and promote furnace efficiency and safety.