3.8
FLUE SYSTEM
NB:
When installing a replacement boiler a new flue system must be used. Do
not re-use the existing boiler flue installation.
3.8.1
Design
Individual air supply and flue outlet pipes are used. The material used for flue outlet &/or
air inlet must be muPVC to BS 5255 and/or BSEN1566-1 and BSEN1329 of an internal
diameter of 51 mm. (i.e. nominal 50 mm diameter muPVC solvent weld waste pipe)
Alternatively, where flue or air intake lengths of up to 60m are require, Keston Composite
75mm flue and air pipe can be used (contact your Keston stockist for details). Keston
Composite 75mm pipe MUST be painted where exposed to UV light.
Both 50mm flue outlet terminal and 50mm air inlet terminal are supplied and are illustrated
in Figure 3.8.1. Both terminals are identical. If 75mm terminals are required these can be
obtained from your Keston Boilers stockist. Request part numbers C.17.2.26.00.0
(terminals) and C.17.2.00.60.0 (50 to 75 adapters).
3.8.2
Maximum Lengths
The maximum lengths of both air inlet pipe and flue outlet pipe, when no bends are used,
are as detailed below.
However, each bend used has an equivalent length that must be deducted from the
maximum straight length stated above. Knuckle bends must not be used.
A 92.5
o
sweep elbow is equivalent to 1.0m straight length. A 45
o
bend is equivalent to
0.5m straight length
50mm
75mm
Maximum Air Inlet Length
:
39.0m
117.0m
Maximum Flue Outlet Length:
20.0m*
60m**
Maximum Total Flue and Air Intake Length :
40.0m
120.0m
* A flue length of between 16 and 20m in
50mm pipe will reduce the DHW output by
1%
** A flue length of between 48 and 60m in
75mm pipe will reduce the DHW output by
1%
Example (assuming 50mm muPVC flue
and air pipework)
Air inlet uses two one 92.5
o
sweep elbows.
Hence, maximum length permissible
(i.e. a+b in figure 2.8.2) = 39.0m - 1.0m -
1.0m = 37.0m
Flue outlet uses one 92.5
o
sweep elbow.
Hence, maximum length permissible (i.e.
c+d in figure 2.8.2 = 20.0m - 1.0 m =
19.0m
3.8.3
Slope
‘Horizontal' flue outlet pipework MUST slope at least 2 degrees (35 mm per metre run)
downwards towards the boiler. Pipework can be vertical. Only swept elbows can be used.
Air inlet pipework can be truly horizontal or vertical, or sloping in a downward direction
towards the boiler but in this case rain, etc., must be prevented from entering the pipe.
There must be no troughs in any of the pipework, whether it be air inlet or flue
outlet.
WD388/0/2004 Chapter 3 - Boiler Location
The Keston C36 Combi & C36P Combi Boilers
Installation & Servicing Instructions
Page : 15
AIR
FLUE
a
b
d
c
Figure 3.8.2 : Flue Air Maximum Length Example
KESTON COMBI
(COMBI P)
