The formula for calculating the equivalent flue size is as follows:
(K
i
+ K
o
)
e
(K
i
+ K
o
)a - K
e
H
a
+
∑
K
where
H
e
is the height of the equivalent flue
H
a
is the vertical height of the actual or proposed flue measured from the flue spigot
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
∑
K is the resistance (other than the inlet and outlet resistances) of the actual or proposed flue
Note K and
∑
K are obtained from the table above
Example
100mm diameter
125 diameter
inlet resistance of actual flue
2.5
1
outlet resistance of actual flue
2.5
1
inlet resistance of equivalent flue
1
1
outlet resistance of equivalent flue
1
1
is the vertical height of the actual or proposed
flue measured from the flue spigot
7
7
other resistances of actual flue
terminal (1)
0.6
0.25
90˚ bend (0)
0
0
135˚ bend (0)
0
0
flue pipe
5.46
1.75
6.06
2
H
e
=
2.50
6.22
Note 1m of actual flue (with two 90˚ bends) gives the minimum equivalent flue height of 0.25m (for 100mm diameter flue) and 0.62m (for
125mm diameter flue).
The maximum equivalent flue height for 100mm diameter flue is 2.50m and for 125mm diameter flue the maximum equivalent flue height is
6.22m (with two 90˚ bends).
Ambi-Rad stocks an extensive range of flue pipe and accessories that are compatible with the cobra range of heaters. For details contact our
Sales Department.
Component
Internal Size (mm)
Resistance factor (Ke)
100
0.78
125
025
100
1.22
125
0.5
100
0.61
125
0.25
100GCI
0.6
125 GCI
0.25
Inlet resistance of flue (Ki)
Outlet resistance of flue (Ko)
100mm spigot
2.5
100mm flue
2.5
125mm spigot
1.0
125mm flue
1.0
Pipe
H
e
= H
a
x
90˚ bend
135˚ bend
Terminal
per fitting
per fitting
per fitting
per meter
18 Method for calculating equivalent flue resistance - Appendix 1
15
