Adding a Chimney Liner

Flue traps will prevent weather entry and backdrafts.

by Dwight Barnett
Scripps Howard News Service

Q: My gas furnace and water heater vent into a brick chimney. What effect will the heat and gas have on its condition? Should I investigate installing a liner of some kind?--D.C., Wolcottville, Ind.

A: Adding a chimney liner is a relatively new concept, introduced 10 to 12 years ago. The main reason is safety. Older, unlined chimneys used for wood-burning can allow byproducts of combustion, such as hot gases and creosote, to escape to the walls or attic. Over time, the wood exposed to these byproducts can overheat and burn.

When the unlined chimney is used as a vent for natural gas or oil-fired appliances, several issues come to mind:

• Sizing: A flue needs to be sized for the appliance it serves. The furnace and water heater in your home have a Btu (British Thermal Unit) rating usually listed in terms of thousands of Btu. It is the heat generated by the appliance that promotes venting. If you have a large, cold chimney and a small Btu appliance, it may not generate enough heat for the flue gases to rise inside the chimney. Adding a properly sized flue liner reduces the area to be heated by the appliance and therefore increases the venting capacity of the chimney.

• Vapor: One of the byproducts of burning fossil fuels such as natural gas or oil is water vapor. If the chimney is unlined and cold, the water vapor will condense inside the chimney and the chimney will become wet. A wet chimney can stain the walls and ceiling of the home and rust out the metal flue pipes.

• Gases: Other byproducts of combustion are nitrogen dioxide and carbon monoxide. When the water vapor produced by combustion combines with nitrogen dioxide, the result is nitric acid. Acid vapors can eat away at the mortar in the brick chimney.

• Efflorescence: This is a whitish mineral powder that forms on the inside or outside brick of the chimney that is exposed to the air. When water vapor soaks into the brick, the moisture migrates through the brick, usually to the exterior, where it evaporates, leaving behind minerals collected by the water.

  The minerals form a whitish powder that can usually be brushed off. In winter, the migrating water freezes inside the brick, resulting in damaged brick or a cracked chimney.

• Wildlife: Birds and small animals cannot enter a properly installed flue that has an attached flue cap with an animal screen. The flue cap also prevents weather entry and backdrafts.

(C. Dwight Barnett is a master inspector certified by the American Society of Home Inspectors. Questions may be addressed to him at P.O. Box 14091, Evansville, IN 47728, or e-mail dbarnett@evansville.net.)

Resources
American Society of Home Inspectors (ASHI)
To locate an American Society of Home Inspectors inspector near you, call or visit the website of ASHI.
American Society of Home Inspectors
Website: www.ashi.org