Johnson & Starley HI-SPEC M31, Installation, Commissioning And Servicing Instructions, Page: 6 / 36

www.johnsonandstarley.co.uk
6.1.6 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.
6.1.7 All materials shall be in accordance with Building Regulations requirements.
6.1.8 The flue should run as vertically as possible. Horizontal runs should be avoided if at all possible and any
directional change should be as gentle as possible. If there is any doubt about the flue configuration, the
equivalent flue height should be determined (see 6.1.10).
6.1.9 If the appliance to be fitted is a replacement, the old appliance should be checked for signs of spillage prior to
commencement of the installation and appropriate action taken, (i.e. check flue system and renew as necessary).
6.1.10 It is recommended that at least 600mm of vertical flue should be provided from the top of the draught diverter
(for new installations this shall be incorporated into the flue design). However, when carrying out replacement
installations, an existing flue system may be encountered, where the vertical flue above the appliance to the first
bend is less than 600mm. In the first instance, the installer must judge whether this distance can be achieved
practicably by some means. Where this is not practicable, the existing flue system may be used, providing there
is no evidence of spillage from the old appliance (see 6.1.9 above). Every effort must be made, however, to
ensure that the existing flue complies in every other way to BS 5440 Part 1, including the visual inspection,
flue flow and spillage test described in 4.3.2 of the above standard. Flue configurations may be assessed in
terms of equivalent vertical height - details are given in Section 6.1.11. For air heaters, the minimum equivalent
vertical height is 1 metre. The installer must make a judgement based on his knowledge and experience and the
examination and testing described above as to whether an existing flue system can be used.
Note: Ventilation of the compartment, room or internal space in which the appliance is to be installed must be checked
for compliance with the requirements of BS 5440 Part 2 ( Ref. Section 4 of these instructions) and upgraded as
necessary.
6.1.11 Calculation method for flue sizing: ( from BS 5440: Part 1, Appendix A)
a. This appendix provides a procedure for estimating whether a given flue design is likely to ensure full
clearance of combustion products.
b. The procedure is based on calculating the ‘equivalent height’ of the flue under consideration, i.e. that height
of the straight vertical circular flue pipe of specific size which will produce the same flow rate as the flue
under consideration. The equivalent height is calculated from the formula:
(K
i
+ K
o
)
e
H
e
= H
a
x
(K
i
+ K
o
)
a
- K
e
H
a
+ Sum K
where:
H
e
is the height of the equivalent flue;
H
a
is the vertical height of the actual or proposed flue;
K
i
is the inlet resistance of the flue;
K
o
is the outlet resistance from the flue;
subscript (e) refers to the equivalent flue diameter;
subscript (a) refers to the actual or proposed flue diameter;
K
e
is the resistance per unit length of the equivalent flue;
Sum K is the resistance (other than the inlet and outlet resistance) of the actual or proposed flue.
Note: K and Sum K are obtained from Table 2. K
o
and K
i
are obtained from Table 3.
c. Table 2 gives resistance factors for common flue components for use in the formula. Table 3 contains the
appropriate inlet and outlet flue resistances, (the flue is likely to be satisfactory if its equivalent height
exceeds 1m) .
d. Worked Calculation Example:
A warm air unit with a 100 mm diameter flue spigot, fitted with a pre-fabricated flue system leading to a ridge
tile in the loft (refer Fig. 2):
From Table 3
K
ia
Inlet resistance of actual flue = 2.5
K
oa
Outlet resistance of actual flue = 2.5
Kie Inlet resistance of equivalent flue = 2.5
K
oe
Outlet resistance of the actual flue = 2.5