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.
The chimney cowls must:
•have useful outlet section that is at least twice that of the vent pipe.
•be made in such a way as to prevent the penetration of rain or snow.
•be constructed in such a way as to ensure, in the event of winds coming from any direction, the evacuation
of combustion products.
•be free of mechanical intake auxiliaries.
04.2 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 draw of the vent pipe must be able to overcome all resistance from the smoke circuit so that any smoke produced inside the stove during combustion is drawn
up and dispersed in the atmosphere through the discharge conduit and the vent pipe itself.. There are many meteorological factors that influence the operation of
the vent pipe, rain, fog, snow, altitude, but most of all is of course the wind, which can cause thermal depression as well as dynamic depression.
The wind action varies depending on whether it is ascending, descending or horizontal.
•Ascending wind always results inan 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.
Some of the combustion gas as well as small particles of combustible are drawn into the vent pipe before being burned, reducing
the stove's efficiency and increasing the consumption of pellets and causing the emission of polluting smoke.
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.
To avoid excessive draught it is advisable to use a draught regulator (see figure on the side).
04.3 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.
Now, the efficiency of a stove is determined by its capacity to transfer most of the head produced into the area to be
heated: the consequence of this is the greater the efficiency of the stove, the cooler the combustion smoke residues
are and as a result the lesser the 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.
Poor draught results in the stove not operating when hot or in smoke spillage.
•Connecting the stove pipe to an existing chimney flue that has already been used with an old appliance is a
common mistake. In this way two solid-fuel appliances share the same chimney flue, which is wrong and
dangerous.
•If the two appliances are used simultaneously, the fume load might exceed the existing chimney flue capacity resulting in downdraught. If only one appliance
is used, the fume heat will facilitate draught but the cold air coming from the other appliance not in use will cool down exhaust fume temperature again blocking
the draught.
•Besides the problems described so far, if the two appliances are placed on different levels the communicating vessel principle might be interfered with, causing
combustion fumes to be drawn in an irregular and unforeseeable way.
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