To calculate the effective duct
length
Friction against the inner walls of the
vent ducting will slow down the
expelled air as it passes through the
ducting. An elbow, bend, non-return
flap or wall vent creates more friction
than a straight duct. The friction
created by each of these is expressed
as an
additional duct length factor
.
The additional duct length factors are
calculated by comparing the friction
created by, for example, an elbow to
that created by 1 metre of straight
plastic piping (see Table 1).
The additional duct length factor for
each item needs to be added together
to ascertain the
effective duct length
which will be needed for the venting
system. The effective duct length is a
means of calculating the friction
created by the venting system as a
whole.
A larger
internal duct diameter
also
helps reduce friction. If the effective
duct length is
more than 20 m
, the
duct diameter should be increased
(see Table II).
To calculate the effective duct length
1. First measure the distance requiring
straight ducting. Multiply this amount
by the corresponding additional
length factor in
Table I
.
2. Establish the number of angles,
bends etc., and add their additional
length factors together with the help
of
Table I
.
3. Add all the additional length factors
to give the effective duct length.
4. Refer to
Table II
for the appropriate
duct diameter required.
Table I
Component
Additional
duct
length
factor
Flexible ducting*
–
1 m laid straight
–
45° angle
(Bend radius = 0.25 m)
–
90° angle
(Bend radius = 0.25 m)
1.8 m
1.5 m
2.5 m
Flexible aluminium
ducting*/plastic piping
–
1 m laid straight, or 1 m
straight pipe
–
45° angle
(Bend radius = 0.25 m)
–
90° angle
(Bend radius = 0.25 m)
1.0 m
0.6 m
0.8 m
Wall vent kit* or window vent
kit*
–
with louvred grill
–
with non-return flap (hinged
flap)
3.8 m
1.5 m
Non-return flap* integrated into
the vent ducting
14.3 m
Flat ducting (for washer-dryer
stack)*
11.0 m
Venting connection to the
left/right
1.0 m
* accessory available to order
Table II
Maximum effective
duct length allowable
Internal duct
diameter required
20 m
40 m
80 m
100 mm
125 mm
150 mm
Installation - extraction ducting
43
Summary of Contents for PT 7136
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