DOWN DRAUGHTS
However well designed constructed and positioned,
the satisfactory performance of the flue can be
adversely affected by down draught caused by near-
by hills, adjacent tall buildings or trees. These can
deflect wind to blow directly down the flue or create
a zone of low pressure over the terminal. A suitable
anti-down draught terminal or cowl will usually effec-
tively combat direct down blow but no cowl is likely
to prevent down draught due to a low pressure zone.
(See Fig.9)
9. Fire Fence
Lay the fire fence into the slots provided ensuring
that they slope from front to back so that no embers
can fall out through the fire fence.
6
Direction of wind
Direction of wind
Pressure zone
Pressure zone
Suction zone
Suction zone
Pressure zone
Suction zone
Fig 9
Direction of wind
When calculating combustion air requirements for
this appliance use the following equation: 550mm
2
per each kW of rated output above 5 kW should be
provided, where a flue draught stabiliser is used the
total free area shall be increased by 300mm
2
for
each kW of rated output. If there is another appli-
ance using air fitted in the same or adjacent room, it
will be necessary to provide an additional air supply.
All materials used in the manufacture of air vents
should be such that the vent is dimensionally stable,
corrosion resistant, and no provision for closure.
The effective free area of any vent should be ascer-
tained before installation. The effect of any grills
should be allowed for when determining the effective
free area of any vent.
Air vents direct to the outside of the building should
be located so that any air current produced will not
pass through normally occupied areas of the room.
An air vent outside the building should not be locat-
ed less than the dimensions specified within the
Building Regulations and B.S. 8303: Part 1 from any
part of any flue terminal. These air vents must also
be satisfactorily fire proofed as per Building
Regulations and B.S. 8303: Part 1.
Air vents in internal walls should not communicate
with bedrooms, bedsits, toilets, bathrooms or rooms
containing a shower.
Air vents traversing cavity walls should include a
continuous duct across the cavity. The duct should
be installed in such a manner as not to impair the
weather resistance of the cavity.
Joints between air vents and outside walls should be
sealed to prevent the ingress of moisture. Existing
air vents should be of the correct size and unob-
structed for the appliance in use. If there is an
extraction fan fitted in adjacent rooms where this
appliance is fitted, additional air vents may be
required to alleviate the possibility of spillage of
products of combustion from the appliance/flue
while the fan is in operation. Refer to B.S. 8303 Part
1.
Where such an installation exists, a test for spillage
should be made with the fan or fans and other appli-
ances using air in operation at full rate, (i.e. extrac-
tion fans, tumble dryers) with all external doors and
windows closed.
If spillage occurs following the above operation, an
additional air vent of sufficient size to prevent this
occurrence should be installed.
VENTILATION AND COMBUSTION AIR
REQUIREMENTS
This appliance is rated less than 5kW, therefore
there is no mandatory requirements for additional air
unless a flue draught stabiliser is fitted then the air
requirement is 15cm
2
. However, we suggest that it is
advantageous to provide an air supply into the room
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