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Code Violation: Flue Undersizing
Result: Smoke Wafting Into Room
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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September 22, 2007
Q: I recently read your article on proper chimney sizing,(very informative). Two years ago, I had a certified sweep install a new stainless steel liner
in our chimney, and the firebrick inside the masonary was 6"x12", so a 6" liner wouldn`t fit. However, fortunately for us, we have a 21 foot draw,
(height), so he installed a 5" diameter liner, and we have had no problems with backdraft. Just curious, but wondering if we are lucky because of the
extreme height of our chimney. Also, I always start our fires with bone-dry western red cedar, gets hot really quick. Any comments on this?
Thanks
George
Hi George,
Thanks for the input! Chimney flow rate is affected by both flue size and draft velocity. If the goal is to evacuate x amount of exhaust per hour from
a given firebox at a given flow rate, a basic calculation can be used to project the optimum flue size that will be needed (to be verified in laboratory
testing). If testing determines that the chosen flue size is insufficient to evacuate the exhaust from that stove, something has to change: either a
larger flue must be provided, or the draft velocity increased. In your case, it sounds like the extreme height of your chimney is creating enough
updraft (flow rate) to compensate for the undersized flue. However, chimney flow rate being an unpredictable variable in the real world, the larger
flue size is the manufacturer’s only sure bet.
It might also be that your particular stove calculated for a 5” flue in the first place, but wasn’t officially tested for that size. Given that the smallest
manufactured wood chimney is 6” round, a manufacturer might opt to test to that size only, as (a) passing would be a sure thing and (b) additional
testing on a 5” flue would add considerable expense at the listing lab. I’m told that Pacific Energy, for example, has performed in-house testing on
several of their models with a 5” flue liner and found that size to be adequate, but opted to test only with a 6” flue for listing purposes.
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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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