Any appliance that you have that burns fuel needs to be vented to the outside of your home. This can be done through a chimney liner, which is the material on the inside of your chimney’s flue that contains the combustion products from your appliance until they are vented out the top of the chimney. Fuel burning appliances that need to be vented include wood stoves, wood burning fireplace inserts, fireplaces, gas fired boilers and furnaces, and gas fired water heaters. If you use gas in your kitchen for cooking, you should also have a fan driven vent to collect kitchen fumes and combustion products from your stove’s burners to be exhausted outside your home.

Venting Other Appliances Through Your Chimney Liner

It is common to have gas fired appliances vented directly into a metal pipe leading to a roof vent, but they can also be vented by piping the exhaust gases into a flue that runs up your chimney. Exhaust gases from fuel burning appliances are corrosive due to the presence of water and carbon dioxide, along with other impurities, in the combustion products, so it is critical that your chimney or vent piping be made of material that can withstand this acidic environment.

Older homes with masonry or brick chimneys often do not have liners inside the flues, but rely upon the integrity of the bricks and parging (mortar) to keep noxious gases and corrosive liquids from penetrating the chimney walls and leaking into the attached structure. Such chimneys should be inspected regularly. If damage due to corrosive gases, water, structural settling, chimney fires, or other causes is found, you should have your chimney repaired and line your chimney flues with an impervious material to prevent further damage.

There are several materials that are commonly used to build chimney liners. These include special mortar mixtures, clay tile, and metals including stainless steel and aluminum.

The type of chimney liner that you should install depends upon the type of appliance that you will be venting into the flue:

  • Clay tile liners or poured mortar liners are adequate for most fireplace applications.
  • For natural gas or oil burning appliances, a metal liner is best.
  • Flexible aluminum liners are inexpensive, but should only be used with low efficiency gas appliances. The exhaust gases from these appliances are hot enough to keep the combustion products evaporated all the way out of the house, so their corrosive properties do not damage the aluminum liner quickly.
  • For a high efficiency gas burning furnace or other appliance, however, the chimney liner or vent piping should be constructed of a special stainless steel alloy that is highly resistant to corrosion. The high efficiency of these appliances means that most of the heat generated by them is delivered into the house, rather up the chimney, so the combustion gases are much colder going up the venting system and are therefore likely to condense on the surfaces of the liner before they are all exhausted to the outside. This constant corrosive exposure will eat away at aluminum or masonry liners and shorten their useful lives considerably. Many woodstoves and wood burning fireplace inserts are designed for use with stainless steel liners of the diameter specified by the manufacturer.