4.Chimney cowl
A properly installed chimney cowl ensures optimum stove functioning. The anti-downdraught chimney cowl consists of a
number of components whose outlet section sum always doubles the vent pipe section. Make sure the chimney cowl is
at least 150cm above the roof top so that it is fully exposed to the wind.
5.Draught
Fumes heat up during combustion, increasing their volume. Their density is therefore lower than the one of the
surrounding colder air.
This difference between the inside and outside temperatures of the chimney results in a negative pressure which
increases proportionally to the vent pipe length and the temperature.
The draught must be stronger than the fume circulation resistance so that all exhaust fumes generated during combustion
inside the stove are drawn upwards through the outlet and the vent pipe. Many weather conditions affect the vent pipe
functioning, such as rain, fog, snow, altitude, and wind being the most important since it can create both negative pressure
and dynamic loading.
The wind action varies depending on whether it is ascending, descending or horizontal.
Ascending wind always results in an increased negative pressure and draught.
Horizontal wind results in an increased negative pressure as long as the chimney cowl was properly installed.
Descending wind always diminishes the negative pressure, sometimes inverting it.
Excess draught causes an increase in the combustion temperature and consequently a loss in stove efficiency.
A part of the combustion fumes are drawn up through the vent pipe together with small pellet particles before combustion
reducing stove efficiency, increasing fuel consumption and resulting in the emission of polluting fumes.
At the same time the high fuel temperature, due to an excess amount of oxygen, wears down the combustion
chamber sooner than expected.
On the other hand, poor draught slows down combustion resulting in a decrease in the stove temperature, fume spillage
inside the room, a loss of stove efficiency and dangerous build-up in the vent pipe.
6.Stove efficiency
Highly efficient stoves may pose difficulties for fume extraction.
In order for a vent pipe to work properly its internal temperature must increase as a consequence of the fumes
generated during combustion.
The stove efficiency instead depends on its capacity to deliver most of the generated heat to the surrounding
environment. As a consequence the more efficient the stove, the colder the combustion exhaust fumes, resulting in a
reduced draught.
A traditional chimney flue, with a rough design and insulation, is more efficient if used with a traditional open fireplace or
a poor quality stove where most of the heat is lost with the fumes.
Therefore, purchasing a quality stove often entails modifying the existing chimney flue to obtain a better insulation, even
when it already works properly with old appliances.
