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Chimney Flue Oversizing

Oversize Chimney Diagram


Flue oversizing is a common code violation in wood stove installations, because the flue size requirement for today's high-efficiency woodburners is much smaller than the flue size needed to vent the extreme volume of exhaust created by the "smoke dragons" of the past.

As exhaust from the fire passes through the stovepipe into the chimney, it must expand to fill the oversize flue. This slows the rate of draft and allows the wood exhaust to cool in the flue, leading to draft problems and excessive creosote formation.

Gas exhaust is relatively cool and very wet, so it travels up the chimney flue quite slowly. When it is forced to expand to fill an oversized chimney, its flow rate can be reduced to the point where the poisonous exhaust gases seek another outlet - into the room.


A UL listed, stainless steel flue liner, the same diameter as the exhaust opening on the insert, is lowered down the chimney flue and connected to a stainless steel tee, which is mortared in place at the thimble. Flexible stainless pipe is used if there are offsets in the chimney. A sealer plate at the top of the chimney creates dead-air insulation, keeping flue gases warmer and improving updraft, which reduces creosote formation. Ceramic blanket insulation may be added if warmer flue temperatures are needed. A tight-sealing top plate and rain cap complete the installation at the top.

For gas appliances, UL listed aluminum alloy pipe is used in place of stainless steel, due to its resistance to both water damage and nitric acid corrosion. A properly sized liner is the first step to insure against gas exhaust backdrafting.


For more info about installing a stainless liner, click here.

Note: Each Code Authority chooses the standards that regulate appliance and chimney installation and usage in their jurisdiction, and may modify code specs as desired. The above-referenced specification is from the Washington Association of Building Officials Woodstove Installation Code Book, and may not reflect the code requirements in your area. Contact your Code Authority for local regulations.

Manufacturers who submit an appliance to a recognized laboratory for safety testing may receive a specific listing for that appliance which may be accepted by your Code Authority in lieu of the standard code requirements. These listings are unique to each model, and can be found in the appliance's installation manual.

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