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Sizing Chimney Liners for Woodburners
When lining a chimney for a woodstove, wood furnace or wood insert, use the same size stainless steel liner as the exhaust opening on the
appliance. If venting into a chimney with a masonry liner, the cross-sectional area (CSA) of the liner must be at least as large as the CSA of the
exhaust collar on the appliance, and, preferably, no larger. To ensure proper updraft and minimize creosote formation in the flue, care must be
taken not to vent into a masonry liner that is too large.
The above chart shows recommended liner sizes below the most common woodstove exhaust collar sizes. The first liner size in each column is
closest to the CSA of the exhaust collar size shown in the circle above it, and is the best choice. Each liner size shown below has a progressively
larger CSA, and is progressively less desirable. In no case may a woodburning appliance be vented into a chimney liner with more than three times
the CSA as the exhaust collar on the appliance.
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Q: Hello,
I need your help. I think I got some bad advice from my local dealer. I did my own install of a Hearthstone Homestead stove, and my local dealer
advised me to go with 5 inch solid pipe with insulating wrap because the recommended 6 inch pipe with insulation would not fit into the flue tiles
lining my chimney. They assured me the difference between a 5 inch and 6 inch pipe was insignificant. The chimney is not too tall; only about 13 feet
from smoke shelf to top, and I lined the whole flue and installed a cap. The stove works fine, but it is almost impossible to open the door to add more
wood without some smoke wafting into the house. Would I have been better off with a solid 6 inch non-insulated pipe? Can I improve things by
extending the length of pipe sticking out of the chimney (lengthening the chimney)? Would a VacuStack help? I would appreciate your opinion.
Thanks,
Vince Roscigno
The minimum vent size for a given woodstove model is not arrived at arbitrarily: it is determined during meticulous laboratory testing. This
is why Hearthstone states in their brochure, on their website and in the Homestead owners/installation manual that 6" connector pipe and chimney
is the smallest size that may be used.
A 6" round liner has a cross-sectional area of 28.28 sq.inches. Your 5" liner has a cross-sectional area of only 19.64 sq. inches, which is a flue size
reduction of 30 percent! Further, 13 feet is the absolute minimum chimney height specified in the manual. Minimum means all other factors must be
nearly perfect to ensure adequate updraft in a 13 ft. chimney. The combination of flue undersizing and barely-adequate chimney height reads like a
recipe for inefficient burn and smokeback into the house.
You can't extend your flue height with liner pipe: lined masonry or insulated manufactured chimney pipe is required above the existing structure, to
prevent the tremendous loss of updraft and excessive creosote formation that results when wood exhaust cools in the flue. You could add courses of
brick to the chimney to allow extension of your 5" insulated liner within, but in our experience, it would likely require an extreme height addition to
compensate for a 30% reduction in liner size.
The Vacu-Stack draft cap works on Bernoulli's principle, and will only function when wind is blowing directly onto it. The rest of the time, it has no
affect on chimney draft. The only way we'd recommend a Vacu-Stack to improve your situation is if it is windy all the time where you live.
Had you consulted us prior to your Homestead purchase, we'd have recommended 6" liner with a tight-sealing top plate to provide dead air
insulation, and would have advised you that even then, you might have to extend your chimney height to ensure adequate draft, especially during
times when flue gas temperatures and/or atmospheric conditions aren't ideal.
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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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