“I have a chimney in my old house with a diameter of 120 mm. I’ve decided on a new stove whose flue connection has a diameter of 150 mm. Reducers are available for purchase. Can’t I simply connect the two?”
This is a common problem when replacing old stoves with modern ones.
An older appliance may have had a smaller flue and could be operated with the existing chimney without any apparent problems. However, a newer stove may have different flue requirements.
A smaller chimney diameter does not automatically mean that the chosen stove cannot be connected. However, it is not enough to simply buy a reducing ring and assume that everything will work.
Why do new ovens have a larger flue connection?
The manufacturer designs the combustion system, including the exhaust pipe, according to the design and operating parameters of the device.
The outlet diameter is therefore not a randomly chosen dimension.
Depending on the model, modern wood-burning stoves have a power outlet such as 130, 150 or 180 mm.
However, the heating output of the stove alone is not sufficient to determine the required chimney diameter. The exhaust gas parameters, the required draft, and the design of the entire flue system also play a role.
Can I reduce the exhaust connection from 150 to 120 mm?
Not without a professional assessment.
Reducing the diameter also reduces the flow area.
For example, a circular cross-section with a diameter of 120 mm has an area that is approximately 36% smaller than a cross-section with a diameter of 150 mm.
This can increase the flow resistance of the exhaust gases and impair the proper functioning of the oven.
This can result in lighting problems, inadequate exhaust gas extraction, or smoke entering the room.
However, the claim that a reduction in diameter is impossible under all circumstances is incorrect.
The permissibility of a specific reduction depends on the manufacturer’s requirements, the parameters of the exhaust system, and the result of a professional assessment, including the necessary calculations.
Isn’t a tall chimney enough?
“The chimney is eight meters long, so it needs to have a good draft. Why can’t I use a smaller diameter?”
The height of the chimney is important, but it alone does not guarantee proper exhaust gas removal.
It also depends on the cross-section, the design, the exhaust gas temperature, the exhaust pipe length, the number of manifolds, and other conditions.
Even a tall chimney may be unsuitable for a particular stove.
Therefore, the connection cannot be assessed solely based on height and diameter.
Attention: Does the chimney actually have the marking 120 mm or is only the opening for the smoke vent that large?
That is a fundamental difference.
In older houses, the chimney opening itself may be larger, while the opening through which the flue is connected is only 120 mm in size.
This connecting hole is called a sonophonuch.
For example, if the chimney has a sufficiently large vent opening and the problem is only a slight draft, a professional adjustment of the connection may be the solution.
Therefore, it is not a good idea to buy a reducer before you know the actual size of the chimney connection.