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Flue Undersizing

Q: I recently bought a wood stove with an 8" flue collar, and my local stove shop refuses to hook it up to my existing 6" chimney. They say this would constitute "flue undersizing," which they claim is a code violation. I need the installation to pass inspection for my homeowner's insurance company, so before I decide to tackle the job myself, I need to know if this would indeed constitute a code violation. And just for my own education/edification, could you also explain why there are different sizes in the first place?

Sweepy Flue undersizing is indeed a code violation, regardless of who performs the installation. Specifically, the stovepipe, liner and chimney used to vent a wood stove must be at least as large as the exhaust opening on the stove.

Why there are different sizes:
Wood stove manufacturers know that shoppers are motivated to pay as little as possible for the installation. Since 6" stovepipe, chimney liner and manufactured chimney are significantly less expensive than 8" venting, they strive mightily to design fireboxes that will operate safely and efficiently with 6" venting. In cases where no amount of tweaking will enable the prototype to burn properly on a 6" flue, the flue opening must be enlarged, and the stove re-tested with larger venting. This process automatically ensures that each wood stove comes to the marketplace with the smallest flue size needed to pass safety testing.

Flue undersizing = insufficient updraft
The difference between 6" and 8" venting might seem fairly small, until we examine the math. An 8" flue has a cross-sectional area (CSA) of 50, while the CSA of a 6" flue is just over 28,
barely over half the size. If you've ever tried to glug down a 32 oz. soda through one of those cocktail-size drinking straws, you'll appreciate the affect an undersized flue has on chimney flow rate.

The consequences:
So, what would happen if you went ahead and vented your 8" stove into your 6" chimney? Most likely, you'd have a dickens of a time establishing an updraft every time you tried to start a fire. And even if you succeeded, you'd suffer from the tendency for odorous, poisonous wood exhaust to billow into the house. And, as you mention above, you'd have the constant worry that the insurance adjuster might notice the code violation in the event of a fire claim, and drive off into the cold, cold night.


Q: I recently read your article on proper chimney liner sizing. Very informative, but possibly not all-inclusive. 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've 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?

Sweepy Hi George,
Thanks for the input! Hot kindling fires can certainly speed the startup process, but once your chimney is primed, flow rate is primarily determined by
flue size and draft velocity. When the goal is to evacuate x amount of exhaust 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 during 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 is an unpredictable variable in the real world, so specifying the tested venting size is the manufacturer’s safest 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 most commonly available manufactured stove pipe, chimney and liner size is 6” round, a manufacturer might opt to test to that size only, as (a) passing the smoke-back test would be a sure thing and (b) additional testing on a smaller flue would add considerable expense at the listing lab. We understand that Pacific Energy, for example, has performed in-house testing with a 5" diameter flue on several of their models and found that size to be adequate, yet they opted to test only with a 6” flue for UL listing purposes.


Q: I understand the physics of getting by with an undersized chimney flue if the chimney height develops sufficient updraft, but I wonder about the legality. As a WETT Certified Chimney Sweep, It is my understanding that the codebooks all prohibit flue downsizing, regardless of functionality. Yet some manufacturers of stainless chimney liners offer a 5-1/2" size, and even a 5" size, to be used on stoves with 6" flue collars! I understand that installation of a 5-1/2" liner would vastly improve the safety of a chimney with cracked 6" x 6" liner tiles (which have an opening too small to accomodate a 6" stainless liner), but in light of the code spec, how do the liner manufacturers get away with it?
WETT Certified in Canada

Sweepy Hi Fellow Sweep,
Your WETT certification training is serving you well on this point:
flue undersizing is indeed a code violation. However, what constitutes flue undersizing is a matter subject to interpretation. If you look down the inside of a typical 6" wood stove flue collar, you'll find that the actual exhaust opening in the firebox is usually only 5" or 5-1/2" in diameter! Even though the flue collar is sized to accomodate the standard 6" diameter pipe, it would be hard to argue that code requires the venting to be larger than the exhaust opening itself. We think that's how pipe and liner manufacturers are able to offer the smaller sizes.


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 liner 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. I lined the whole flue with insulated 5" and installed a cap. The chimney is not very tall; only about 13 feet from smoke shelf to top. 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 6 inch non-insulated liner? Can I improve things by extending the length of pipe sticking out of the chimney (lengthening the chimney to improve the flow rate, as mentioned by "George" on your website)? Would a VacuStack help? I would appreciate your opinion.
Vince Roscigno

Sweepy Hi Vince,
As discussed in the letters above, 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.

The loophole that allows installation of smaller venting for many wood stoves doesn't apply to Hearthstone's cast iron flue collars, because they are the same size as the exhaust opening in the firebox. This means the exhaust opening in the firebox of your Hearthstone Homestead is the same 6" diameter as the flue collar, and has the same cross-sectional area (CSA) of 28.28 square inches. Your 5" liner has a CSA of only 19.64 sq. inches, which constitutes 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. Your combination of flue undersizing and barely-adequate chimney height is like a Chuck Norris one-two punch, knocking the breath out of your updraft (cute analogy courtesy of Master Sweep Howard Lopeman).

Extending your flue height with uninsulated liner pipe would most likely make the problem worse: insulation is required above the existing structure, to prevent the tremendous loss of updraft and excessive creosote formation that results when wood exhaust cools in an uninsulated 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 always windy where you live.

Had you consulted us prior to your Homestead purchase, we'd have recommended you chip out the existing terra cotta liner tiles to make room for an insulated 6" liner, 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.


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 derived from the most widely recognized codebooks, but 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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