It’s in BS 6798:2009
Here it is:
From current BS 6798:2009
Specification for installation
and maintenance of gas-fired
boilers of rated input not
exceeding 70 kW net
5.3.3 Airing cupboard installations
5.3.3.1 An airing cupboard adapted to house a boiler shall conform to 5.3.2. Additionally, the airing spaces shall be separated from the boiler compartment by a non-combustible partition, which may be perforated, if necessary, by apertures having a major dimension no greater than 13 mm.
5.3.3.2 Any flue duct which passes through the airing space shall be double-walled or thermally insulated, unless the duct is surrounded by an air inlet duct.
5.3.3.3 Where the boiler is of the open-flued type, the draught diverter and the air vents shall be in the boiler compartment and shall connect to the room and not the airing space.
COMMENTARY AND RECOMMENDATIONS ON 5.3.3.2 and 5.3.3.3
For a double-wall chimney conforming to BS EN 1856-1, or for a flue duct concentric with an air inlet duct, the external skin and the air gap provide sufficient insulation and no extra precautions are necessary other than normal installation tolerances.
A single-wall chimney should be protected by an air gap of at least 25 mm for a distance of at least 1 m from the draught diverter outlet connection.
This air gap can be provided by a non-combustible guard which forms an annular space around the chimney of not less than 25 mm.
Any clearance between the chimney guard and the compartment partition where the chimney passes through should not exceed 13 mm. (See Figure 5.)
Expanded metal or rigid wire meshes are suitable materials for the partition and guard.