© Baxi Heating UK Ltd 2008
14
Vertical
Flues
(Twin Pipe)
Y
X
Vertical
Flues
Y
X
The total equivalent length for this
example is = 5.5 metres.
1m extension
135°bend
91.5°bend
1m
0.25m
0.5m
4
2
2
4.0m
0.5m
1.0m
AIR DUCT
N
o
of
fittings/pipes
Sub total
Equivalent
Length Value
Equivalent Length Air Duct =
5.5m
1m extension
135°bend
91.5°bend
1m
0.25m
0.5m
4
2
2
4.0m
0.5m
1.0m
FLUE DUCT
N
o
of
fittings/pipes
Sub total
Equivalent
Length Value
Equivalent Length Flue Duct =
5.5m
Y
X
C
Roof
Terminal
A
B
This bend is
equivalent to 1 metre
Total Equivalent Length =
A+B+C+1x90°Bend
All vertical and angled runs must be included,
measured from the boiler adaptor (point X) to
the joint with the flue terminal (point Y). One
91.5° bend or two 135° bends can be
included without reduction of the flue length.
If further elbows are required the flue
length must be reduced by the following
amounts:-
1 metre for each 91.5° bend
0.5 metre for each 135° bend
3.8.3
Twin & Vertical Flue Systems
1. Maximum permissible equivalent flue lengths are:-
Vertical Concentric
1.9m
Vertical Twin Pipe
6m*
* Both the air and flue duct can be up to 6 metres
long each.
2. Any additional “in line” bends in the flue system must
be taken into consideration. Their equivalent lengths
are:-
Concentric Pipes:
135° bend
0.5 metres
93° bend
1.0 metres
Twin Flue Pipe
135° bend
0.25 metres
91.5° bend
0.50 metres
The elbow supplied with the standard horizontal flue is
not included in any equivalent length calculations
NOTE: Flue length is measured from point X to Y
as shown.
Y
X
