Theory of Natural Draft Wood Stove Flue Design
Following manufacturer guidelines, local codes, and industry standards helps to ensure a successful wood stove installation. However, it can be helpful to understand why the rules are what they are.
Flue: The complete pipe system used to route combustion products from the stove to the outside, above the roof of the structure.
Chimney: The portion of the flue system that is visible from the outside.
Class A Chimney: Pipe appropriate for penetrating a roof or wall. Class A chimney pipe is either double-wall with a layer of insulation between the walls, or air-cooled triple wall.
Connector Pipe: The portion of the flue system connecting the stove to the Class A chimney. Connector pipe is generally single-wall or double-wall non-insulated, and is not permitted to penetrate the roof or wall. Also called "stovepipe."
Draft: The force of the rising hot column of flue gases pulling the gases below up the flue system. Also called "chimney effect" or "stack effect."
How a Natural Draft Flue System Functions
The flue system is the "engine" of the wood stove. Natural draft wood stoves depend on the draft created by the flue system to function. A wood stove without a flue system attached will not function properly. A fire inside will tend to smolder, and the air control settings will have little impact on the burn rate.
Beginners often assume that the flue system is acting like an exhaust pipe on a car, and the stove "pushes" smoke up through the chimney. In fact, the opposite is true.
The rising column of hot gases in a flue system pull the gases below up the chimney. This phenomenon, called "draft," results in a negative pressure zone inside the flue system, which actively pulls combustion products out of the stove, and pulls fresh air in through the air intakes.
Flue System Design Considerations
Understanding how draft works will lead to a few important design considerations when building your flue system.
Male ends point down
Beginners often assume that male ends of stovepipe should point up. After all, if flue gases are rising, wouldn't they be more likely to leak out of the stovepipe if female ends point up? This view is a result of a misunderstanding of how a flue system functions.
Since a properly functioning flue system will have a lower pressure inside than the air pressure in the room, any air leakage in the flue system should be into the flue, not out. In other words, since the rising gases in the pipe are pulling the gases below, any leaks at stovepipe joints should pull air into the pipe.
Orienting the male ends of single-wall pipe (and the inner wall of double-wall pipe) downward, toward the stove, ensures that any condensing liquid or creosote stays inside the flue system, instead of leaking out over the outside of the pipe. Water vapor is a large component of wood stove exhaust (both from residual moisture content of wood, and because water is a major combustion product of hydrocarbon fuel).
A Flue System Doesn't Need to be Totally Airtight to Function
Since any air leakage at gaps in the joints of the flue system will leak air into the pipe, rather than out, it's not generally necessary to have an airtight flue system.
Too much air leakage into the flue system at joints can weaken the draft, but flue components generally fit tightly enough not to cause a problem.
Since stovepipe gets very hot and expands and contracts through normal use, adding any kind of sealant between the stovepipe joints is generally not recommended. Even high temp sealant at pipe joints will quickly fail.
If the air-tightness of the flue system needs to be improved, for example in the construction of a passive house, sealing with a resilient, flexible material like stove gasket can provide the best results.
Flue Systems Should be as Straight and Vertical as Possible
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.
Elbows create resistance to draft relative to the angle of the elbow. 90-degree elbows create more resistance than 45-degree elbows, which create more resistance than 30-degree elbows. If an offset is required, using softer turns will help ensure better draft.
Non-vertical sections create some resistance to draft, relative to their length and the severity of the angle. Horizontal sections should be kept as short as possible for wall exits, and avoided entirely for roof exits.
The Chimney Must be Sufficiently Tall
Generally, the taller the chimney, the stronger the draft. How fast the flue gases rise, and how strongly they pull on the gases below them, is proportional to the air pressure difference between the bottom and the top of the chimney, versus the pressure outside.
Since air pressure is lower at higher elevation, the taller the chimney, the bigger pressure difference between the bottom and the top. A 20 foot chimney will produce approximately twice as much draft as a 10 foot chimney.
The Flue Gases Must Stay Hot
How fast the flue gases rise, and how strongly they pull on the gases below them, is proportional to the air pressure difference between the bottom and the top of the chimney, versus the pressure outside.
The hotter the flue gases are relative to the outside air, the stronger the draft. Hotter flue gases are less dense, so they will rise faster and more forcefully.
A better insulated chimney will keep flue gases hotter and moving faster, so it will produce better draft. A roof exit keeps more of the flue system inside of the heated envelope of the structure, so it will tend to keep flue gases hotter.
Single-wall pipe can be used inside the structure improve heat transfer into the living space, but it should never be used outside. Cold outside air will quickly cool and condense flue gases passing through a single-wall pipe, causing poor draft and creosote formation. All of the exterior chimney pipe should be insulated to ensure proper draft.
A Chimney May Need to be Taller at High Elevation
Another consequence of air pressure being lower at higher elevation is that the same chimney will not draft as well at high elevation as it does at sea level.
The pressure difference between the bottom and the top of the chimney is proportional to the total barometric pressure. So, a flue system 5,000 feet in elevation (typical barometric pressure of 85.91 kPa) will only draft 85% as strongly as the same flue system at sea level (101.33 kPa).
The Flue Size Should Match the Stove if Possible
Beginners sometimes expect that a larger flue system will create less resistance to draft, and therefore result in stronger draft. In fact, flue systems that are larger than the appliance requires tend to weaken the stove's draft.
Since a larger flue volume means flue gases take longer to escape, gases have more time to cool and slow. Since draft is proportional to flue gas temperature, cooler gases mean weaker draft.
While it's not ideal, it's sometimes possible to use a larger flue system with a small stove. Learn more about connecting a small wood stove to a larger chimney.
The Top of the Chimney Must Terminate Sufficiently High Above the Roof Line
NFPA 211 (the industry standard for residential wood stoves) requires the chimney opening to be 3' above the roof line, or 2' above the highest part of the building within 10', whichever is higher.
While there is currently no industry standard for wood stoves in vehicles like RVs, buses, and vans, we suggest using a detachable chimney to get as close as practical to 3' above the roof line. Learn more about detachable chimneys for mobile wood stoves.
NFPA 302 (the industry standard for boats) only requires that the chimney terminates above the deck. For best results, we suggest using a detachable chimney for boats where possible.
A chimney that is sufficiently high above the roof line will serve several functions.
- A taller chimney means a stronger draft. If the overall height of your chimney is too short, your stove won't draft well, and might spill combustion gases into your living space.
- In windy conditions, the roof of your structure can create turbulence that can push air downward into your chimney, causing combustion gases to spill into the living space. Having the chimney a couple of feet above the highest part of the structure helps to keep the chimney opening above any turbulence created by the structure.
- If your chimney is short enough that the smoke hits your roof, flammable creosote can condense on your roof, which is a fire hazard if it's allowed to accumulate.
- If there is any part of your structure that is taller than your chimney, the house stack effect can compete with your stove for fresh air.
The Top of the Chimney Must Terminate Vertically
Even the strongest draft can easily be overcome by wind pushing down the chimney. Wind tends to move laterally if the top of the chimney is sufficiently high above the roof line.
If the top of your chimney is not vertical, there will be a direction from which wind will push down the chimney, causing a backdraft. Backdrafting can be dangerous, since it causes flue gases containing carbon monoxide to spill into your living space.