5
16
Fig. 7
Functional Diagram
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
).
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.
5.1.12 Where flue blocks are used, builders should ensure that no obstruction is created during erection. The installer
should ensure that the connection flue does not project beyond the internal wall of the flue blocks and that there is
provision for examination and servicing.
5.1.13
Important:
Before installing the appliance, carry out a visual check of the flue system as directed in the relevant
section of BS 5440 Pt. 1, then check the flue performance as follows:-
a.
Close all doors and windows in the room in which the appliance is to be installed.
b.
Introduce some heat into the flue, using a blow torch or other means.
