Ventilation Options for a Boiler in a Finished Basement

Boiler Flue

• Typically, a boiler ventilates its combustion byproducts through a chimney or chimney-like boiler flue. Therefore, the first thing to consider when installing a boiler in your finished basement is if your home's boiler flue reaches down to the basement. If it does not, you need to add to the flue so that it does or install a new boiler flue altogether. Some new and very efficient boilers (those with a 90 percent AFUE rating or higher) actually do not need to be vented through a boiler flue at all but can be vented directly to the outside of the home.

Boiler Efficiency

• The age and efficiency of the boiler play an important role in choosing a ventilation system. Newer, higher efficiency boilers produce fewer combustion gases than older models, and those gases are produced at a lower temperature than those of their older, less efficient predecessors. Newer boilers extract more usable heat from the combustion process. Therefore, if the boiler you plan to install in your basement is new and highly efficient, you likely will need a smaller flue to reduce boiler inefficiency. Also, the flue will need to be double-walled rather than single-walled, because lower temperature combustion byproducts can produce toxic, corrosive condensation on the inside of the flue. In a finished basement, condensation is not as likely to form because the temperature differences between the inside and outside of the flue pipe will not be as extreme as those in an unfinished basement. But it is still best to err on the side of caution by installing a double-walled flue.

Flue Lining

• Any flue used for a boiler in your basement needs to be lined with a material that can resist the corrosion of combustion byproducts. According to the U.S. Department of Energy, the best types of lining for this purpose are fire clay, masonry liner or a retrofitted metal flue liner. Just as important as installing the liner, however, is periodically inspecting the lining to check if it is damaged or degraded, and repairing or replacing it if it is.

Other Considerations

• Once you have made well-informed decisions concerning how to ventilate a boiler in your finished basement, your work is not complete. Homeowners need to check the flue, flue lining and particularly the seal between the boiler and the flue at least once yearly for corrosion and damage. The combustion byproducts that a boiler produces include carbon monoxide and other potentially harmful substances, so early detection of damage that could leak these byproducts into the home is critically important for the safety of your family.