Why are flue liners required?
New more efficient gas appliances require smaller flue sizes than older units due to lower flue gas temperatures. In over-sized masonry chimneys flue gases quickly cool as they expand. Natural draft depends on the buoyancy of hot flue gases, and the cooler the gases become, the more the natural draft is reduced. This causes the flue gases to linger in the chimney, increasing the risk of them leaking into the home. Water will also condense in over-sized flues and can be transferred through walls causing peeling wall paper, blistering paint, flaking plaster, ceiling stains, damp patches and/or mold. A properly sized, UL Listed, flue liner will insure proper draft by preventing the flue gases from expanding and cooling more than they should. Also, flue liners are required when installing a 90% efficient furnace and leaving only a small water heater flue in a masonry chimney.
