15
The chimney effect:
The following chapter has source from SOLIFTEC, and is 15uthorized15 information regarding
the chimney and it’s effect.
If the chimney is not appropriate, it will not generate enough upward draught, not enough air
will be sucked in towards the fuel for it to burn adequately, and smoke will not be properly
evacuated. Most closed stoves need an upward draught of about 16 Pascals.
Although a chimney appears to ‘suck’ and smoke appears to naturally rise, it is more accurate
to think of the weight of dense outside air pushing down to force air into the building, into
the fire and so push the hotter, lighter, waste gases out up the stack, somewhat like an air
bubble rises in water. This ‘Chimney Effect’ generates the ‘draught’ (or draft), the very tiny
difference in pressure inside and outside the flue.
How strong the draught is depends on four things:
HEAT
inside the chimney. The gases inside the chimney must stay VERY hot, or they will not
rise. Although a small length (up to about 1m) of uninsulated flue pipe may be acceptable
directly from a stove, the entire length of the chimney above must be insulated, typically by
thick masonry around an insulated ceramic liner or using prefabricated insulated components.
Large voids inside the flue mean a large area of masonry to cool down the gases, especially if
their outer surface is outside the building and exposed to the cold. Efficient modern stoves
release less heat into the flue and need especially strict attention to the chimney.
HEIGHT
of the chimney – the higher it is, the more draught will be generated. There is a
procedure for calculating chimney height in BS EN13384-1:2002 but it is extremely complex.
For typical domestic chimneys and appliances in Britain and Ireland both the British Standard
(BS 8303-1:1994) and Document J suggest a height of about 4.7 metres of straight, smooth,
insulated chimney above the appliance. Less than perfectly sound chimneys will need to be
taller to compensate. Special rules apply to Open Fires.
Path
The entire path from fire to chimney terminal needs to be smooth, straight and vertical
wherever possible. Where bends absolutely cannot be avoided, they should be very gentle
(say a maximum of 2x 45º) and the chimney height increased by at least a metre to
compensate. The flue needs to be absolutely sealed, without the slightest tiny crack at any
point which might let cool air in. The flue needs to be big enough for the appliance being
used – BS 8303 suggests a minimum of 125mm diameter for closed appliance up to 20 kW
rated output (150 mm if burning smoky fuels) – but not so big that its large surface area will
cool the smoke (say, bigger than about 375mm square). Special rules apply to open fires.
