
MHS Boilers Ltd ©
191007
29
Calculation of diameter and length
For the calculation and control of the inner diameter of a discharge system with mechanical discharge,
please refer to the applicable national and local standards and regulations.
Maximum Length of flue in m
Diameter
70 mm
Diameter
80 mm
Diameter
90 mm
Diameter
100 mm
Diameter
110 mm
Diameter
130 mm
WM/45 18
40*
n.a
n.a
n.a
n.a
WM/65
n.a.
12*
39
67
n.a.
n.a.
WM/85
n.a.
n.a.
18
32*
70
n.a.
WM/100
n.a.
n.a.
n.a.
22*
40
80
WM/120
n.a.
n.a.
n.a.
35*
38
72
Table 6 Length of flue
The above flue pipe lengths have been rounded down.
* Flue diameter on the boiler
These lengths apply to open type appliances.
For room sealed appliances, these lengths apply to inlets and outlets combined.
Flue losses of various flue components expressed in meters of straight pipe. The total loss should
be subtracted from the maximum permissible flue length from the previous table.
Type
Diameter in
mm
Pipe bend 90°
R/D =0.5
Pipe bend 90°
R/D = 1
Pipe bend 45°
R/D =0.5
T-piece
WM/45
70/80* 3.5/4.0 1.1/1.2 1.1/1.2 4.0/4.0
WM/65 80*/90/100 4.0/4.5/4.9 1.2/1.3/1.4 1.2/1.3/1.4 4.0/4.3/4.5
WM/85 90/100*/110 4.5/4.9/5.4 1.7/1.8/2.0 1.3/1.4/1.5 4.5/4.9/5.4
WM/100
100*/110/130
4.9/5.4/6.2 1.8/2.0/2.2 1.4/1.5/1.6 4.9/5.4/6.0
WM/120
100*/110/130
4.9/5.4/6.2 1.8/2.0/2.2 1.4/1.5/1.6 4.9/5.4/6.0
Table 7 Flue losses in metres of straight pipe
* Connection to the appliance
If concentric discharge from the appliance is used, the following maximum lengths (excluding roof
clearance) are permissible:
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