Theory of Natural Draft Wood Stove Flue Design
Installing a wood stove isn’t just about picking the right parts and following a diagram. Behind every recommendation is a bit of science, and knowing the “why” can help you both design a better system and troubleshoot problems when things aren’t working as expected.
This article takes a deeper look at the science behind natural draft wood stove flue systems. If you’re comfortable with DIY installation and want to understand the theory that drives good flue design, you’re in the right place.
What's In This Article
- Key terms in flue design
- How draft works in a natural draft stove
- Roof exits vs wall exits
- Pipe orientation and air leaks
- Why flue systems should be tall, hot, and straight
- How elevation and pipe size affect performance
- Industry standards for chimney termination
Industry Terms
Before diving in, let’s define some common terms:
- Flue: The complete pipe system routing combustion gases from the stove to the outdoors.
- Chimney: The portion of the flue visible outside your structure.
- Class A Chimney: Insulated double-wall or triple-wall pipe rated to penetrate roofs and walls.
- Connector Pipe (Stovepipe): Single or double-wall non-insulated pipe inside the space, connecting the stove to the Class A chimney. Not rated for roof or wall penetration.
- Draft: The force created by a column of hot gases rising inside the flue, which pulls combustion gases out of the stove and draws fresh air in.
How Natural Draft Actually Works
A common misconception is that the stove “pushes” smoke up the chimney. In reality, the flue is the "engine". Natural draft wood stoves depend on the draft created by the flue system to function.
The column of hot gases rising inside the chimney creates a negative pressure zone that pulls more gases up from below. This suction effect is what draws fresh air through the stove’s air intakes and carries smoke outside. Without a functioning flue, a wood stove will smolder, no matter how well-built it is.
Flue System Design Considerations
Roof Exit vs Wall Exit
Your first major design choice is whether the chimney exits through the roof or a side wall. Each approach has trade-offs.
Roof Exit Advantages
- Lower cost (fewer parts, less Class A pipe)
- Stronger draft with fewer horizontal sections
- Easier cold starts since more pipe stays warm inside the structure
Wall Exit Advantages
- Works around lofts, beams, or cabinets
- Keeps most stovepipe out of the living area (aesthetic preference)
- Sometimes the only option in certain layouts
Challenges of Wall Exits
- More expensive (higher percentage of insulated pipe)
- Draft is reduced by horizontal sections unless chimney height is added
- Slower to warm up when cold, making starts more difficult
- Can be harder to design since insulated pipe can’t be cut to length
If you’re weighing options, check out our roof vs wall exit guide for more detail, or use the Kit Builder Tool to compare layouts in your specific structure.
Pipe Orientation: Why Male Ends Point Down
Many new installers assume stovepipe male ends should face up to keep smoke from leaking out. In reality, the male ends of single wall stovepipe (and the inner wall of double wall pipe) should always point down toward the stove.
Here’s why:
- Draft creates negative pressure inside the flue, so leaks pull room air in rather than pushing smoke out.
- Pointing male ends down ensures condensation and creosote stay inside the pipe instead of dripping onto the exterior.
Air Leaks and Airtightness
A flue system doesn’t need to be perfectly airtight to work. Minor leaks at joints usually pull air in, not push smoke out. Over-sealing pipe joints with caulk or cement can fail quickly as metal expands and contracts.
If you need to tighten things up in an airtight structure, a flexible gasket material is a better option.
Keep the Flue Straight and Vertical
Since the rising hot column of flue gases creates the draft, any component of the flue that resists the vertical movement of gases will negatively impact the draft. A 90-degree elbow adds more resistance than a 45, and any horizontal run slows gases down.
Best practice is to keep the system as straight and vertical as possible. If offsets are necessary, use gradual angles and keep horizontal sections short.
Chimney Height Matters
The taller the chimney, the stronger the draft. That’s because the pressure difference between the bottom and top increases with height. A 20-foot chimney can draft nearly twice as strongly as a 10-foot chimney.
At higher elevations, air pressure is lower, so chimneys draft less effectively. For example, at 5,000 feet a system will draft weaker than the same setup at sea level. Often, the solution is simply to add more height.
Keep Flue Gases Hot
Draft strength also depends on temperature difference. Hotter gases rise faster, while cooler gases slow down and condense.
- Use insulated Class A pipe outside to keep gases hot.
- Keep as much of the chimney as possible inside the structure (roof exits help with this).
- Never run single-wall pipe outdoors. It will chill gases too quickly, causing poor draft and creosote buildup.
Matching Flue Size to Stove
Bigger isn’t better. Folks sometimes expect that a larger flue system will create less resistance to draft, and therefore result in stronger draft. In fact, a flue that’s too large for the stove will hold gases longer, giving them more time to cool and weaken draft.
Whenever possible, match chimney size to the stove’s flue collar. If you need to connect a small stove to a larger chimney, follow the rules carefully. See our article on adapting to larger chimneys for details.
Chimney Termination
A properly terminated chimney helps draft and keeps smoke out of your living space.
- NFPA 211 requires the chimney top to be 3 feet above the roof line, or 2 feet above anything within 10 feet (whichever is higher)
- We recommend a detachable chimney above the roof penetration for easy disassembly during transport. Learn more about detachable chimneys for mobile wood stoves.
- A vertical termination is critical. Angled caps can let wind push air back down, causing backdraft.
- Marine setups have looser standards (NFPA 302), but detachable chimneys above the deck are recommended for both performance and safety.
Putting the Science Into Practice
Understanding the science helps you see why the rules exist and how to make smart adjustments. For example:
- If you must use a wall exit, add chimney height to restore draft.
- If you're using your stove in a cold climate with a wall exit, keep more of the chimney inside and add height if it struggles with cold starts
- If you’re at high elevation, plan for extra chimney height from the start.
The better your system design, the less you’ll struggle with smoky starts, backdraft, or creosote buildup.
Next Steps
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Use our Kit Builder Tool to design your flue system and get a custom parts list.
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Read our full roof vs wall exit guide to choose the right approach for your setup.
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Browse our parts shop to order Class A chimney, stovepipe, and accessories.
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Not sure which stove fits your space best? Check out our Which Stove is Right for Me guide.
A wood stove depends on its flue system to work well. By designing with draft theory in mind, you’ll set yourself up for safer burns, cleaner glass, and fewer headaches down the road.
Would outside air pressure make any difference? Right now we have an insane 1035 mBar and the wood stove seems to be struggling .
Lutz-
Good question. Elevation is known to affect draft due to differences in ambient barometric pressure, but generally higher elevation and lower ambient pressure is bad for draft, not the other way around. Weather conditions can also influence draft, where stoves tend to struggle on more rainy, foggy, humid days, which are also normally associated with low-pressure systems.
If you notice your stove is struggling, now would be a good time to inspect your chimney. The most common reason for a wood stove to start behaving differently is if the chimney is in need of sweeping. I recommend having a rotary cleaning kit on-hand so you can quickly touch up our chimney as often as it needs it.
The flue would remain inside the 2 storey dwelling over a length of about 25 feet from the top of the wood heater. Inside the ground floor the pipe would be single skin with a mesh sleeve on top of the heater unit covering the first section of the flue. Upstairs, after floor penetration arrangements, would it be wise to return to a single flue pipe to dissipate heat into the room?
Also, I have seen water heating jackets used on the first section of the flue above the heater unit. Do you think this would be OK with the upstairs flue being single skin?
Steve-
Thanks for the question. It’s generally not permitted to change back to single-wall pipe once you have transitioned to insulated pipe. You are meant to continue in insulated pipe all the way to the chimney cap.
In this instance, you would also be creating an unsafe condition by having single-wall pipe exposed through the second story. All the pipe running through the second story must be Class A chimney pipe, and enclosed in a chase to enforce clearances around the pipe. Exposed single-wall pipe on the second story that’s sometimes room temperature and sometimes screaming hot is likely to cause injury to an occupant, or a fire when combustible items are placed too close to the pipe.
The traditional solution to heating both levels of a two-story structure with a wood stove is simply to place the wood stove on the bottom floor, and make provisions for air circulation between the floors. Since hot air rises, warm air will naturally travel upstairs.
Thanks for the question, Bob! In general, NFPA-211 requires:
In other words, the overall height of the insulated portion of your chimney needs to be at least twice the length of the horizontal run.
Because our stoves are used in relatively short structures like tiny houses and RVs without much vertical space, we tend to be very conservative about the horizontal distance recommended. We like folks to keep the horizontal distance to the bare minimum required to penetrate a wall in a wall exit configuration, in the range of 10 to 40 inches depending on the structure and overall flue design.
Unfortunately, 30 feet horizontal is far too much of a horizontal run for a natural draft wood stove to function. If you want to install a wood stove in this structure, you’ll need to rethink either the wood stove placement or the routing of the flue system.
Hello, I am considering placing a wood furnace next to my propane furnace so that I can save some on propane. What I want to know is if I put this furnace next to my old furnace I would have to run the flue about 6 or 7 feet across the basement ceiling until it gets to a window that I am thinking of using to bring the flue outside. Then it will be straight for another 4 or 5 feet until it can go up next to my deck. I would use the deck to anchor the flue. I guess what I am asking is can this work? Do I have to go straight up to make it work? Just wanted to know what is the best way to make this work for me. I need to know if I need to place the wood furnace near the window and make the flue shorter or can it be in the middle of the room and have it be longer because of the distance from the middle of the room. Any info would be great. Thank you!
Hi Patrick,
You would need to contact the wood furnace manufacturer to see how many inches of horizontal run they allow. Our stoves work best with a maximum horizontal run of 20″. It will vary from wood stove to wood stove.
-Kayla
Interesting article and quite informative.
What effect does having an external flue have. On the one hand, it would be colder as it’s outside and exposed to the wind..
On the other hand, the pressure difference between top and bottom, should be greater as the air at the top has called a bit more.
This doe also mean the pressure difference between the top and outside is also not so high (tho still be a big difference).
Can’t quite work that one out.
Hi Chris. Thanks for the question.
Pipe inside the heated envelope of the structure is always better insulated than pipe outside. Hotter flue gases are at a lower pressure relative to ambient pressure, so they will rise more forcefully and produce a stronger draft. This is one of the reasons why roof exits generally perform better than wall exits. It’s also why wall exits draft more strongly if you exit as high on the wall as possible.