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 thanthe cooler surrounding air.
This difference in temperature between the inside and outside of the flue results in a negative pressure which increases proportionally with 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 theoutlet and the flue 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.
Wind action varies depending on whether it is ascending, descending or horizontal.
•An ascending wind always results in increasing the pressure and draught.
•A horizontal wind results in an increased negative pressure as long as the chimney cowl is properly installed.
•A descending wind always reduces the negative pressure, and sometimes inverts 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, increasing the consumption of pellets and causing the emission of polluting smoke.
At the same time, the high fuel temperature, due to an excess of oxygen, wears the combustion chamber prematurely.
A poor draught, however, slows combustion resulting in a decrease in the stove temperature, fume egress inside rooms, a loss of
stove efficiency and hazardous build-up in the flue pipe.
To avoid excessive draughts, it is advisable to use a draught regulator (see figure to the side).
05.3 EFFICIENCY OF THE HYDRO/BOILER STOVE
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.
The efficiency of a stove is then determined by its ability to transfer most of the heat produced into the room to
be heated: it follows that the greater the efficiency of the stove, the cooler the combustion smoke residues are
and, as a result, the weaker the draught.
A traditional chimney flue, with an approximate design and insulation, is more efficient when 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 total fume load might exceed the existing chimney flue
capacity resulting in downdraught. If only one appliance is used, the fume heat will facilitate the draught
but the cold air from the other appliance not in use will cool down exhaust fume temperature again choking 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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