Adapting a Tiny Wood Stove to a Larger Flue Size
Key Points
- Wood stoves need properly sized chimney systems to function.
- Using the same size pipe as the stove's outlet works best.
- Chimney sizes smaller than the stove's outlet are almost never allowed.
- Larger chimney pipe can work as long as the chimney is otherwise well-designed.
- A 3" stove can use a 4" or 5" roof exit.
- A 4" stove can use a 5" or 6" roof exit, or a 5" wall exit.
- A 5" stove can use a 6" or 8" roof exit, or a 6" wall exit.
Why Consider Larger Sizes?
Does your structure already have an existing solid fuel rated flue system in good shape? Is your current stove too large for your space, or just needs to be upgraded? Attaching a new small stove to your existing flue can be an attractive option. Reusing existing pipe significantly decreases the difficulty of your project, avoids cutting any new holes in your structure, and can cut the overall project cost in half or more.
Are you installing a small stove in a structure subject to building codes? Most building codes require using UL Listed Class A pipe, and the smallest available pipe meeting that requirement is 5 inch (see our 5" Ventis parts). If you're using a 3" or 4" stove, you'll need to make sure your stove can handle adapting to the larger size.
Are you looking to save money on your project by using locally available chimney pipe in a larger size?
Can you safely use a larger chimney with your small stove?
Chimney Size and Draft
The size of a wood stove chimney is critical to its performance. The flue is the "engine" of a natural draft wood stove. The rising column of hot flue gas actively pulls combustion products from the stove, and pulls fresh air in through the intakes. This phenomenon is commonly referred to as "draft," and without it, a wood stove will not function.
Decreasing the flue size smaller than the stove's collar (or flue flange) is almost never permitted. Doing so would restrict the flue system to less than required for proper draft, causing smoke to spill out of the air intakes. Per NFPA-211 (the industry standard for residential style wood stoves):
12.4.4 (1) The cross-sectional area of the flue shall not be less than the cross-sectional area of the appliance flue collar, unless specified by the appliance manufacturer.
However, increasing the flue to a size larger than the stove's collar (or flue flange) is sometimes permitted. A larger chimney does have a negative effect on draft, since it allows the flue gases to slow and cool before they escape the top of the chimney. But as long as the chimney isn't too large, the stove will still draft just fine.
How large is too large?
Acceptable Flue Size Increases for Roof Exits
- Roof exits provide flue gases with a straighter vertical route. A roof exit with no elbows, or at most two 45 degree elbows, creates very little resistance to the rising column of hot flue gases.
- Roof exits keep the majority of the stovepipe in the heated envelope of the structure, so the flue gases stay hot on their way out.
12.4.4 (2) The cross-sectional area of the flue of a chimney with no walls exposed to the outside below the roof line shall not be more than three times the cross-sectional area of the appliance flue collar.
So, according to industry standard, the following size increases are acceptable:
- A 3" stove can use a 3", 4" or 5" roof exit. We carry a 3x4" adapter.
- A 4" stove can use a 4", 5" or 6" roof exit. We carry a 4x5" adapter and a 4x6" adapter.
- A 5" stove can use a 5", 6" or 8" roof exit. We carry a 5x6" adapter.
Acceptable Flue Size Increases for Wall Exits
Wall exits don't produce as strong of a draft as roof exits, so your options for increasing flue size in a wall exit are limited.
- A wall exit adds two sharp 90 degree turns and a horizontal section that resists the vertical movement of hot flue gases
- Because most of the pipe in a wall exit is outdoors, the hot flue gases have to move through a large quantity of cold pipe, which can cause them to cool and slow down.
12.4.4 (3) The cross-sectional area of the flue of a chimney with one or more walls exposed to the outside below the roof line shall not be more than two times the cross-sectional area of the appliance flue collar.
So, according to industry standard, the following size increases are acceptable:
- A 4" stove can use a 5" wall exit. We carry a 4x5" adapter.
- A 5" stove can use a 6" wall exit. We carry a 5x6" adapter.
Exceptions to the Rules
The NFPA guidelines are based on an otherwise well-designed flue system, and a normal household stove. There are a few considerations that may make a size increase not function well for a small stove, even if it is theoretically acceptable.
- When stoves become very small, heat output is limited, and the ratio of pipe surface area to volume increases. The household rules of thumb start to break down at 3". In our experience, most 3" stoves will not function reliably well with a wall exit, even if the pipe size matches the stove. The smallest stove we've seen that drafts reliably well with a wall exit is our Dwarf 3kW Standard, which uses a 4" flue system.
- Adding "heat robbing" appliances to the flue can negatively impact a stove's draft, and cause an otherwise sound flue system to fail. Flue water jackets and "heat reclaimer" systems cool and slow flue gases, and can interfere with proper draft.
- The oven on the Dwarf Cookstove Combo also robs significant heat from the Dwarf 5kW stove, flue systems larger than 5" are not recommended for this model.
How to Avoid Replacing the Entire Flue System
If you have an existing flue system that is too large for your stove, but you don't want to replace the whole flue, adding a liner inside your stovepipe might be an option.
If you run single-wall stovepipe inside of your existing chimney from the flue flange all the way to the chimney cap, you can effectively reduce the size of your existing stovepipe to match the stove.
Make sure you're using solid fuel rated stovepipe or a solid fuel rated flexible chimney liner inside your existing flue system. For best results, the space between the liner and the existing stovepipe should be filled with ceramic stovepipe insulation.
i have a 7 inch insulated chimney. Can i use a 6 inch stove with wall exit?
According to the NFPA-211 guideline of “no more than two times the cross-sectional area,” a 6″ stove should work with an otherwise well-designed 7″ or 8″ wall exit. Since codes vary depending on your locality, whether the increase is permitted will be up to your local authority having jurisdiction.
Can I go from 6 in stove pipe to a 4in chimney pipe for small wood stove. Chimney is inside wall
It’s possible but not ideal. Give us a call at 208-352-3417 or email us at support@TinyWoodStove.com and we’d be happy to discuss your specific set up and options. Thank you!
I have and 6in flue in my home for a wood stove I’m replacing with a pellet stove 3in flue. ? Is can I connect 3in to 6in flue?
A pellet stove has different flue requirements than a wood stove, so you’re going to need to consult with your pellet stove manufacturer to determine the best course of action. Pellet stoves are typically positive-pressure appliances that actively push their exhaust up the chimney. A wood stove is a negative-pressure appliance, one that uses the natural draft of the chimney draft to pull the combustion products out of the stove, and pull fresh air in through the intakes. Factory built chimneys designed for wood stoves are typically not totally airtight at the joints because they don’t need to be. If you hook up a positive pressure appliance to a factory built chimney designed for operating at negative pressure, you’ll likely spill combustion products into your living space. If your chimney is a site-built masonry chimney, then it might be OK, but you should check with the stove manufacturer to be sure.
I am putting a smaller wood stove in my existing masonry fire place, can I run single wall stove pipe all the way up the existing brick chimney?
Hi David,
Yes, single-wall stovepipe can work well as a liner when installing a small wood stove inside a masonry fireplace. It’s best practice to use a solid fuel rated liner size matching the stove’s flue flange and to insulate the liner with stovepipe insulation. DIY kits are available elsewhere online that include flexible pipe, insulation, a cap and flashing for the chimney, and an optional tee to connect to the stove’s rear exit.
I have a 7” insulated chimney/ceiling box. Do you have a stove pipe adapter that will work for 5” stove pipe. The chimney pipe is Duravent 7”
Dave-
Thanks for the question. Unfortunately, 7″ is not a very popular size, so I’m not aware of anyone who makes a solid-fuel rated 5×7″ increasing adapter. There are some 6×7″ adapters from other vendors online, so it would be possible to stack a 5×6″ and a 6×7″ adapter. But the cleanest install would be to get a metal shop to fabricate an adapter for you from 24 gauge or thicker steel or stainless steel, and paint it with Stove Bright paint to match the rest of your stovepipe.
Dan; I searched around the internet and came up with the adapter made by Imperial which lists part number BM0068.
Ecco Supply or leacock coleman might have stock but haven’t contacted them.
Thanks for sharing your findings, Dave! That part looks right to me.
I have a 10” flue and a 6” wood stove pipe……Can I do 10-8 and then 8-6?
Hi Sean, as a rule, you don’t want to adapt your stove up more than 2″ for a roof exit and 1″ for a wall exit. Going from 6 to 10″ (even using some pipe with an 8″ diameter), will cause major draft issues for your stove.
Ugh thanks for the feedback…that’s such a bummer….I have a very steep roof line, and was hoping not to have to put myself in a position to have to run a new 6” pipe down thru the 10” pipe…..
Hi there, I am trying to do 3″ to 8inch chimney exit – is this possible? I saw maybe I can do a liner? how do I go about this without it being totally sketchy? Thanks.
Thanks for the question, Cydney. Yes, the only way to do that would be to run a 3″ liner from your wood stove, through the existing chimney, to the top of the chimney.
Hello, I have an 8” flue wood stove in a cinder block garage. So it’s a 2 foot 8” vertical section then a 8” 90 through the block wall to another 2 foot 8” stove pipe section then it has a 8” to 6” reducer and then a 6” 90 with a 6 foot vertical 6” stove pipe going up. Is that 8 to 6” going to cause huge issues. It was originally all 8” but after it rusted out I couldn’t find everything I needed to keep it all 8” to I decided to reduce to 6” but now reading some stories that has me thinking I may have screwed up here. Smh. Any suggestions?
Hi Paulie-
Thanks for the question. What you’re describing is an 8″ wall exit reduced to a 6″ chimney on the outside. Unless your stove manufacturer specifically says it’s OK to reduce to 6″ (very unlikely) then this configuration is not going to work. While it’s sometimes OK to use a larger chimney than the flue flange size, it’s almost never permitted to reduce the chimney to a smaller size. Unfortunately, you will need to remove the 6″ pipe and replace it with a properly sized 8″ Class A chimney system.
i have a old french stove with a elliptical hole for flue is it safe to blank plate the shape of this hole and increase from the stove a 4 inch flue collar adapt
to 5 inches to go up a flue pipe straight out to chimney
Thanks for the question, David.
If you have an elliptical flue flange, the best practice is to take a piece of single-wall pipe and bend the bottom end to conform to the elliptical shape. To do this with our pipe, you need to cut the male end off the bottom to make the pipe flexible enough to bend. Keep in mind, sometimes the stove’s flange is male (the pipe goes outside of the flange), sometimes it’s female (the pipe goes inside), so you may or may not need to re-crimp your pipe once it’s the right shape.
Regarding your proposal, I would be concerned that adding a blank plate with a round hole would effectively reduce the size of your stove’s flue flange opening, which could prevent the stove from functioning properly. It would be much better to modify a pipe to fit your stove rather than modifying your stove to fit the pipe.