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The Chimney Effect which makes a fire work only operates when there is a permanent fresh air supply from the outside into the space in which the fire is burning. Without an appropriate air supply the fire will not burn effectively and there will be a severe risk of dangerous Carbon Monoxide and fumes escaping. All fires require a supply of air from outside the building - there is absolutely no exception to this rule under any circumstances whatsoever.

For ordinary enclosed stoves, cookers and boilers it has customarily been taken that a suitable air vent will have an open area of 550mm² for every kW of heat output, so:

Appliance nominal Heat Output in kWArea of vent required in mm²Equal to square hole with sides (mm)Equal to round hole of diameter (mm)
1 550 23 26
2 1100 33 37
3 1650 41 46
4 2200 47 53
5 2750 52 59
6 3300 57 65
7 3850 62 70
8 4400 66 75
9 4950 70 79
10 5500 74 84
11 6050 78 88
12 6600 81 92
13 7150 85 95
14 7700 88 99
15 8250 91 103
20 11000 105 118
25 13750 117 132
30 16500 128 145
35 19250 139 157
40 22000 148 167
45 24750 157 178

This outside air can be supplied in different ways. Air leaking around door frames etc. can often add up to the area in the table above and may suffice alone for small appliances. It has commonly been accepted that, in a draughtier house, enough air for about the first 5kW of output can often be supplied by such leakage, but this is by no means a hard-and-fast rule, there will be many circumstances where even older properties admit insufficient air even for a tiny 2 or 3kW appliance.
● An extractor fan, or another open vented appliance (such as an open fire, even an unused one) in the same building can remove this air.
● Some appliances have provision for air to brought directly to them though a tube
Open Fires have a different, often very large, air requirement.
● Where the appliance is fitted with a draught diverter (a device to reduce excess flue draught by pulling extra cool air directly into the flue) extra air inlets are required.
● For more information, refer to the Building Rules.

A table showing the amount of air actually required for combustion is on the Fuel Properties page.

Where it is necessary to actually calculate the air entry requirement, for instance if types of heat-recovery ventilation are being used, then how much air passes through a vent is a function of the size of the orifice and the difference in pressure from the low pressure (out) side to the high pressure (in) side. The pressure difference is commonly measured in Pascals (Pa). If a fan is being used, the manufacturers will usually provide information about the pressure difference. If it is just a passive vent in a wall, it is customary to assume that the pressure difference is about 3Pa. The velocity it flows at is roughly given, for air in ordinary situations, from:
V = (Pa x 0.5) + 0.8

V = Velocity in metres per second
Pa = Pressure Difference in Pascals
The volume of air passing is then given from the velocity (V) multiplied by the area of the hole, in the same units, in this case metres. It is customarily taken that about FOUR times as much air must be supplied as is actually needed, as much of the air entering a fire doesn't actually take part in combustion but simply passes through unchanged.

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