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85
Flue systems for gas appliances
Gas appliances can be flued by a wide variety of methods, as the products
of combustion are mainly water vapour, carbon dioxide, nitrogen and oxygen,
at a temperature-of about 95 °C after the draught diverter. The function of the
draught diverter is to discharge flue products into the boiler room during a
down-draught through the chimney. Such reverse flows occur infrequently for a
few seconds during adverse wind conditions. Diversion ensures that the correct
combustion process is not interrupted. It stops the pilot flame from being blown
out, with consequential shut-down of the appliance until manual ignition is ar-
ranged. The draught diverter also dilutes the products of combustion by en-
trainment of room air into the flue. A carbon dioxide concentration of 4% by
volume is found in the secondary flue after the diverter. The primary flue pipes
are those before the diverter.
Brick chimney
New masonry chimneys must be lined with vitrified clay or stainless steel
pipe. Existing chimneys may incorporate a stainless steel flexible flue liner,
which can be pulled through an existing chimney with a rope and rounded plug.
The liner has the same diameter as the appliance flue outlet, often 125 mm for
domestic appliances, and is built into the top of the chimney with a plate to
form a sealed air space between the liner and the brickwork. This acts as ther-
mal insulation to maintain flue gas temperature. If the flue gases were allowed
to cool to below about 25 °C condensation of the water vapour would occur
and deterioration of the metal and brickwork would reduce serviceability. As-
bestos cement or glazed earthenware pipes can be built into new chimneys for
protection of the brickwork. A cowl is fitted to the flue to reduce the ingress of
rain and the possibility of down-draughts.
The pipe will be either asbestos cement or double-walled stainless steel
with thermal insulation between the inner and outer pipes. A flue pipe taken
through a roof is fitted with a lead slate to weatherproof the junction. The ter-
minal should be 600 mm from the roof surface and clear of windows or roof-
lights. An internal flue from a small domestic appliance can be connected to a
ridge terminal. An externally run asbestos cement flue pipe has a branch tee
junction at its emergence through the wall. A 25 mm copper drain pipe takes
condensation to a drain gulley.
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