A properly installed chimney cowl ensures optimum stove operation. The anti-downdraught chimney cowl
consists of a number of components whose outlet section sum always doubles the flue pipe section. Make
sure the chimney cowl is at least 150cm above the roof rise 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 flue 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.
05.2 DRAUGHT
As they heat up, the gases formed during combustion undergo an increase in volume and, as a result, have a lower density than the cooler surrounding air.
This difference in temperature between the inside and outside of the flue results in a negative pressure which increases proportionally to the flue 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. There are many meteorological factors that influence the operation of a flue pipe, rain, fog, snow, altitude, but the most
important is the wind, which can cause negative thermal pressure as well as dynamic negative pressure.
The wind action varies depending on whether it is ascending, descending or horizontal.
An ascending wind always has the effect of increasing pressure and draft.
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 gases, as well as small particles of combustible material, are drawn into the flue 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.
Instead, poor draught slows down combustion resulting in a decrease in the stove temperature, fume spillage inside the room, a
loss of stove efficiency and a dangerous build-up in the flue pipe.
To avoid excessive draught it is advisable to use a draught regulator (see figure to the side).
5.3 EFFICIENCY OF THE HYDRO STOVES
Paradoxically, highly efficient stoves may pose difficulties for fume extraction.
In order for a flue 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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