Johnson & Starley 42-451-15, Installation, Commissioning & Servicing Instructions

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5.1.9 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).
5.1.10 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 5.1.9 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 5.1.11. 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 3 of these instructions) and upgraded as
necessary.
5.1.11 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 ).
4 25
Fig. 6a, MODAIRFLOW FUNCTIONAL DIAGRAM
Component Internal Size Resistance Component Internal Size Resistance
(mm) Factor (mm) Factor
Flue Blocks 197 x 67 0.85 per meter 45
O
Bend 100 mm pipe 0.61 per
231 x 65 0.65 run 125 mm pipe 0.25 fitting
317 x 63 0.35 150 mm pipe 0.12
140 x 102 0.60 197 x 67 0.30
200 x 75 0.60 231 x 65 0.22
183 x 90 0.45 317 x 63 0.13
Pipe 100 0.78 Raking block Any 0.30 per block
125 0.25
150 0.12 Adaptor block Any 0.50
Chimney 213 x 213 0.02 Terminal 100 mm ridge 2.5
125 mm ridge 1.0
90
O
Bend 100 mm pipe 1.22 per 150 mm ridge 0.48
125 mm pipe 0.50 fitting 100 mm GCI 0.6
150 mm pipe 0.24 125 mm GCI 0.25
150 mm GCI 0.12
Table 2
Resistance factors for use in calculating equivalent heights
Appliance Inlet Flue Outlet
Resistance (K
i
) Resistance (K
o
)
100 mm dia spigot 2.5 100 mm flue 2.5
125 mm dia spigot 1.0 125 mm flue 1.0
150 mm dia spigot 0.48 150 mm flue 0.48
Table 3
Inlet and outlet resistance
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
K
ie
Inlet resistance of equivalent flue = 2.5
K
oe
Outlet resistance of the actual flue = 2.5
From table 2:
Other resistances of actual flue:
Terminal
= 2.5
Pipe bend ( 2 x 0.61) = 1.22
Pipe (4 x 1m @ 0.78) = 3.12
(5 x 0.3m @ 0.78) = 1.17
Sum K = 8.01
Equivalent height :
From the formula
(2.5 + 2.5)
H
e
= 6.2 x ______________________
(2.5 + 2.5) - (0.78 x 6.2) + 8.01
H
e
= 3.793 This flue exceeds 1.0m equivalent height and is therefore satisfactory.
5 24
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