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1.9 Draft conditions
If smoke spillage occurs when the fire door is opened, it is probably due to poor chimney
draft. This type of stove requires at least 12 pa of chimney draft to achieve satisfactory com-
bustion and smoke spillage prevention. However, in cases where the stove door is opened
too vigorously you could expect that slight smoke spillage may occur. The flue gas tempera-
ture at the rated output is 243˚ C relative to 20˚ C. The flue gas flow is 5.43 gr/sec.
If you have any doubts, you may want to have your installer measure the draft in the chimney.
Draft conditions
The chimney’s draft is the resulting effect within the flue caused by the difference
in temperature within the flue and the cooler temperature outside. Other factors
that can influence the level of draft include the length of the flue, insulation of the
chimney, adverse weather conditions or tall buildings or trees nearby the flue ter-
minal.
Poor draft occurs when:
• The atmospheric temperature difference is too low, e.g. a poorly insulated chim-
ney. If the chimney is excessively cool, it may help to ‘prime’ the flue before the
fire is lit. Just place a screwed-up sheet of newspaper in the flue-ways of the sto-
ve and ignite.
• The outside temperature is too high, e.g. during the summer months.
• There is no air movement (wind) outside.
• The chimney is not tall enough, with the result that the terminal sits in the lee of
the roof surface or in the vicinity of tall trees or neighbouring buildings. These
conditions are also associated with downdraft where the flue gases are pushed
back down the chimney.
• Flue draft is diluted by residual air entering the chimney, e.g. due to inadequate
flue-pipe joints or leaks at the cleaning door or flue collar.
• Unsealed, unused fireplaces are connected to the chimney.
• The flue is blocked, e.g. by soot, due to inadequate cleaning, loose debris or even
a birds nest.
• The house is too tightly sealed (see section on Fresh air supply).
A good draft is achieved when:
• The temperature difference between the chimney and outside is high. This also
applies during firing, when the need is greatest.
• The weather is clear and there is a good wind.
• The chimney is of the proper height, i.e. minimum 4 meters over the stove and the
termination adequately clear of the roof line.
Fresh air supply
A wood-burning stove requires air for combustion and therefore you may need to
install additional ventilation to the room, especially so in well insulated houses. If
the air supply is inadequate the chimney draft may be too weak, with the possible
result that the stove will not burn properly; smoke spillage may also occur during
the refuelling process. On a positive note, a properly installed wood-burner will
give even the most efficient houses a pleasant and natural ventilation.
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