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4.7 Air Supply
Free Air Requirements - Provision for air for combustion and ventilation:
General:
A sufficient permanent air supply to the appliance will be required for:
(a) Correct combustion of fuel an effective discharge of combustion products to the open
air.
(b) The ventilation of any confined space in which an appliance is installed to prevent
overheating of the appliance and any equipment in and near the appliance.
(c) The satisfactory operation of any draught break or stabiliser which may be fitted.
It should be both the Architect’s and the installer’s concern that the air required for
these functions be introduced so as to cause as little discomfort as possible to building
occupants and thus to offer them the least temptation to obstruct ventilation.
Air for Combustion:
Example
• Boiler rating 20 kW
• Air for Combustion 550 mm2/kW = 550 * 150 = 82,500 mm2
= 825 mm * 825 mm
• Air for Combustion 850 mm2/kW (with stabiliser) = 850 * 150 = 127.500 mm2
= 1275 mm * 1275 mm
This requirement will be satisfied if the room or space in which the appliance is installed
has an opening or duct which is designed to allow the passage of air at all times
equivalent, in total free area, to 550 square mm/kW(2.5 sq. inches per 10,000 Btu's)
minimum of the appliance rating.
Note ; It is not recommended to fit an extractor fan in the room which contains the
Boiler.
Air for Ventilation:
For an appliance in a confined space, care should be taken to provide air for
ventilation. A working guide to the minimum free area of openings to be provided in
addition to that for combustion air is as follows:
(a) Where the air is taken from a heated space, 1100 sq. mm/kW (5.0 sq.
inches per 10,000 Btu's) at low level.
(b) Where the air is taken from outside, 550 sq. mm/kW at both high level
and low level (2.5 sq. inches per 10,000 Btu's)
For boilers fitted with a flue draught stabiliser – first 5kW=300mm2 +
850mm2 per kW above 5kW
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