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4. The chimney
The optimum combustion is achieved at a constant chimney draft of 10-14 Pascal (Pa) measured in the flue
pipe above the stove (when having a smoke exhauster). However, with natural draft (chimney without smoke
exhauster) the draft in the chimney will be between 18-25 Pa when conditions are optimum. There are many
factors that affect the chimney draft, including the outside temperature, wind strength, and surrounding
buildings. There are no requirements with respect to specific chimney heights, but a chimney must be tall
enough to provide a good draft. If the recommended chimney draft is not achieved, there may be problems
with smoke out of the door by firing.
4.1 Inadequate draft in the chimney
If the chimney is too short, leaks, or is inadequately insulated, there may be problems with the chimney draft
(let the chimney sweep assess this). The draft should be sufficient during the ignition phase when the
stove/chimney is cold to provide a clean combustion and prevent smoke leakage. Therefore, we recommend
a chimney draft around 5 Pa in the ignition phase.
Once the ignition is a success and the smoke temperature is rising, it is important that the chimney draft
reaches a stable area around 18-25 Pa in operation (1 hour after ignition) when the chimney/stove is hot.
The draft in the area of 18-25 Pa when the stove is hot is essential in order to achieve an optimum and clean
combustion. Therefore, we recommend that you measure your chimney draft when the chimney is cold and
when it is hot after 1 hour in operation.
In practice, the chimney draft will differ – especially in cold condition. Therefore, the best indication of the
chimney draft is to measure the draft when it is hot. If the chimney sweep judges the draft to be sufficient, but
you still have problems lighting a fire, try extending the fire lighting phase by using more thin pieces of
kindling and 1-2 firelighters, so that the chimney gets thoroughly warm. The chimney will not draw at
optimum effect until it is warm. When a layer of embers has been formed, add 2-3 dry logs.
If it is impossible to establish a sufficiently natural chimney draft, the chimney can be fitted with a smoke
exhauster, e.g. Aduro DraftOptimizer.
4.2 Chimney and Smoke
If you experience smoke coming out of the stove or the start section of the smoke pipe, it could be due to
certain weather conditions. On stormy or cold winter days or if the chimney has not been used in a while, a
cold plug can be created in the chimney, meaning the natural draft will be at its minimum.
If the chimney is cold and cold plugs are created, you may experience smoke leaks during the ignition and
startup phase. In this situation there can be more smoke than usually in the combustion chamber and some
of it might leak out of the stove. Therefore, if you have a cold chimney that has not been used for a while, we
recommend that you use some wood sticks to heat the chimney and eliminate the cold plug.
