How to Use a Flex Duct on a Wood Furnace

Wood furnaces often serve to heat homes in regions where firewood is plentiful and inexpensive, such as in rural areas and on farms. Wood furnaces may be a home's primary heat source, or they may supplement another furnace. Some wood "furnaces" are actually fireplace inserts that use blowers to distribute heat but are not connected to ductwork in a house. True wood furnaces are installed outside the house, vented through chimneys and connected to ducts that distribute heat throughout the house. Dual systems sometimes share interior ducts, but ducting for wood furnaces involves special considerations.

Flexduct rated for 250 degrees

Instructions

- 1
  Use flexible ductwork, called flex duct, flex vent or a variation thereof, only for interior heat distribution with a wood furnace; it is not acceptable for exterior installation, such as between the plenum or output point on the furnace and a house. Avoid installing flex duct where it will be exposed to ultraviolet rays from the sun. Connect a wood furnace to house ducting with galvanized pipe, with external heat-resistant insulation to improve heat flow.
- 2
  Install flex duct for the supply or output side of a wood furnace that is rated at 250 degrees -- 50 degrees higher than standard ducting. Use conventional flex duct for return ducts, which collect air from inside the house and return it to the furnace, but only if it is rated for external use and insulated to avoid damage from the sun.
- 3
  Connect a wood furnace to existing ductwork only if those ducts are rated for wood furnace use. Install separate ducting for a wood furnace, if possible. Tie ducting for a dual system to a controller that will shut down vents for a furnace when the wood furnace is active. Failing to close off ducts from an oil or gas furnace will simply drain heat away from the wood output into the other furnace.

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