Heating Systems
What Is a Flue (Vent)?
The flue is the pipe carrying combustion gases from a fuel-burning appliance to outside. Its material and routing depend on the appliance: conventional furnaces use metal venting into a chimney or vertical flue, while condensing furnaces vent cooler, acidic exhaust through plastic pipe.
Also called: flue · vent pipe · venting
Why it matters to you
Venting is the part of a furnace replacement most likely to change the job. Moving from an 80% furnace to a condensing one usually means new venting rather than reusing the existing flue, which is a scope question worth raising before work starts.
What to do about it
Ask what venting a quoted furnace requires and whether the existing flue can be reused. Any sign of rust, staining or disconnection at a vent connection is a safety issue, not a cosmetic one.
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