“I bought a flue pipe with a chimney damper. But when I close it, there’s still a large hole around it. Is the damper defective? And why doesn’t my stove have a damper?”
These questions are more common than you might think.
The chimney damper does not serve as a watertight seal. Its purpose is not to seal the chimney, but rather to… regulate the exhaust gas flow and influence the chimney draft.
Therefore, a throttle valve may not be necessary for every oven.
What is the purpose of a chimney damper?
The chimney creates a draft that directs the exhaust gases from the stove outside.
For the oven to function properly, the chimney draft must be within the limits specified by the manufacturer.
If the airflow is too strong, combustion can be more intense than desired. The wood burns faster and the stove can be subjected to excessive heat.
A flue gas damper installed in the flue pipe makes it possible to increase the flow resistance of the flue gases and thus reduce excessive draft under suitable conditions.
However, it is not intended to completely prevent the emission of flue gases during combustion.
My stove doesn’t have a chimney damper. Is that a defect?
NO.
Many modern wood-burning stoves do not have a flue damper and do not require one for proper operation.
The stove has its own combustion air supply control and is designed for a specific chimney draft range.
If the chimney meets the manufacturer’s requirements, there may be no reason to install an additional throttle valve.
The absence of a chimney damper is not in itself a manufacturing defect or a fault of the stove.
When is it advisable to replace the chimney damper?
Especially if the chimney draft is higher than prescribed by the stove manufacturer and the use of a throttle valve is appropriate for the respective installation.
A typical example would be a chimney that generates excessive draft under certain operating conditions.
The mere impression that the stove is burning the wood too quickly is not enough.
Similar behavior can have various causes, such as incorrect combustion air settings, leaks, or an unsuitable amount of fuel.
Therefore, before installing a throttle valve, it is important to know the actual chimney draft and the requirements of the respective stove.
A chimney damper is also not a solution for insufficient chimney draft. Closing it could even worsen the situation.
Why is the gap around the closed chimney damper so large?
“I turn the lever to the closed position, but when I look into the smoke vent, the flap doesn’t seal at all. There’s a large gap around it.”
This is the intention behind a conventional exhaust gas control valve.
Even in the most closed position, the prescribed exhaust gas flow must be guaranteed.
The reason for this is simple: Hot embers may still be present in the combustion chamber and combustion gases may be produced, even if no visible flames are seen.
If the smoke damper completely blocks access to the chimney, smoke and dangerous carbon monoxide can enter the room.
Therefore, the control dampers have a structurally ensured free cross-section.
A large gap around a closed flap doesn’t necessarily have to be a defect. On the contrary, it can be an important safety feature.
The exact size of the free cross-section depends on the type of damper, its intended use, and the specific technical requirements. It cannot be assumed that every damper must have the same opening.
Can I modify the flap so that it seals better?
NO.
If the throttle valve is designed to allow the passage of flue gases when closed, this space must not be arbitrarily reduced in size or covered.
Do not weld any additional sheet metal to the flap and do not attempt to modify it in such a way that it completely closes off the smoke vent.
This could impair the safe removal of flue gases.
How is a chimney damper used correctly?
The exact procedure must comply with the instructions of the oven and steamer manufacturer.
With a conventional manual draft damper, it should usually be fully open when lighting the fire so that the flue gases can escape unhindered into the chimney.
A possible shutdown is only considered in a stable operating state and only to the extent that the respective system allows it.
Before the door is opened to add more fuel to the fire, the damper is usually opened again to reduce the risk of smoke entering the room.
The throttle valve must never be used as a substitute for proper combustion air regulation.
And if the oven starts to smoke after closing, the flames become weaker, or the combustion becomes worse, then this setting is not correct.
Is a chimney draft damper the same as a chimney draft regulator?
NO.
These are two different devices, although they can solve a similar problem.
The exhaust flap creates an adjustable resistance directly in the exhaust path.
A chimney draft regulator, often also called a draft limiter, works on a different principle. In suitable systems, it allows additional air into the chimney draft, thus contributing to stabilizing the draft.
The choice of the appropriate solution depends on the specific oven, chimney and installation conditions.
Therefore, the two devices cannot be arbitrarily interchanged.
What if the airflow is too strong, but the manufacturer does not recommend the damper?
Then it is incorrect to simply install the damper contrary to the manufacturer’s instructions.
A solution must be found that meets the requirements of the device and the exhaust system.
For example, this could be a suitable technical solution for draught control, but the specific option must be based on an assessment of the entire system.
It is equally untrue that every stove with a tall chimney automatically needs a flue damper.
What can you take away from this?
A chimney damper is not a mandatory component of every wood-burning stove.
Its main function is to regulate the exhaust gas flow, especially in situations where excessive chimney draft needs to be reduced.
With a conventional control damper, the free space for the passage of flue gases is intended and important for safe operation, even in the closed position.
And above all: A throttle valve cannot replace a properly constructed chimney, a sufficient supply of combustion air, or the correct operation of the stove.
If the oven functions according to the manufacturer’s instructions and the chimney draft is sufficient, you may not need a chimney damper at all.