3
30
4.
DUCT SYSTEM
(See British Design Manual - Gas fired Warm Air Heating)
4.1
RETURN AIR
4.1.1
All return air shall be POSITIVELY ducted from outside the compartment to the top of the unit via a return air duct or, if
appropriate, using a Side Return Air Kit SR50, and mechanically secured. It is recommended that the return air duct not
be routed directly from the main living area, but from a convenient central area serving the remainder of the dwelling.
4.1.2
The return air system should be constructed of fire-resistant material. The flue shall not be run through an area serving
as a return air path. It is extremely important that the correct size of return air grilles and ducting is used. For heaters on
maximum output the return air duct size should not be less than the equivalent of 300mm x 250mm (12" x 10"). If flexible
duct is used the duct diameter should not be less than 350mm (14") dia. The return air grille should have a free area of
not less than 1266cm
2
(196in
2
).
4.1.3
An adequate and unobstructed return air path is essential from areas not served by a directly ducted return and to
which warm air is delivered. All such rooms should be fitted with relief grilles which have a free area of 0.0088m
2
/kW
(1in
2
/250Btu/h) of heat supplied to the room. The only exceptions are kitchens, bathrooms and WC.’s.
4.1.4
The return air duct should allow for ease of removal for access to the flue.
4.1.5
All duct work in the room or internal space in which the heater is installed shall be mechanically secured, and sealed
with ducting tape.
4.2
WARM DELIVERED AIR
4.2.1
All duct work, including riser ducts, should be fully insulated with 50mm (2in) fibreglass or similar. If short extended
duct runs are taken below floor level these should be similarly insulated, and in addition wrapped with a sound vapour
proof barrier, and protected from crushing.
4.2.2
The duct system should be carefully designed to suit the needs of its specific heating requirements and building
layout. The type of duct system, (i.e. radial/extended plenum/stepped) should be installed using the least number of
fittings to minimise airflow resistance. The base duct, which equalises the air pressure to supply ducts, shall be
constructed to support the weight of the heater, which shall be secured to the plenum with screws on at least two
sides, and sealed using self-adhesive foam strip, ducting tape or sealing compound. All ducting and blanking plates
shall be mechanically secured and sealed.
5.
INSTALLATION REQUIREMENTS
Note: For ELJAN 6 circulator Installation Instructions, see separate instructions ZZ778.
5.1
FLUES
(see British Standards BS 5440 Pt. 1 Flues)
5.1.1
All joints shall be soundly sealed.
5.1.2
The flue should be kept as short and warm as possible.
5.1.3
Sufficient support brackets shall be installed to bear the weight of the total flue system.
5.1.4
The spigot connection of the heater draught diverter will accept internally the spigot end of a non-asbestos flueto BS
567 or twin wall metal flue to BS 715 of nominal 100mm (4in) diameter.
5.1.5
A split collar should be fitted to provide for flue maintenance or inspection.
5.1.6
The flue shall be in accordance with the Building Regulations and British Gas Materials and Installations specification
3rd edition) with regard to clearance and shielding from combustible materials.
NOTES:
Table 1
Minimum Effective Areas
