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PAW-VP•LDHW - Technical Manual – 2020-07
Appendix
A
A.8.
Alternative Discharge
Discharge pipes should be in metal and dedicated to the unvented cylinder. The pipe should have a
continuous fall and should terminate in a safe and visible place. Downward discharges at low level, i.e. up
to 100 mm above external surfaces such as car parks, hard standings, grassed areas etc. are acceptable
providing that where children may play or otherwise come into contact with discharges, a wire cage or
similar guard is positioned to prevent contact, whilst maintaining visibility.
Discharge at high level, i.e. into a metal hopper and metal down pipe with the end of the discharge pipe
clearly visible (tundish visible or not) or onto a roof capable of withstanding high temperature discharges
of water and 3 m from any plastics guttering system that would collect such discharges (tundish visible).
Where a single pipe serves a number of discharges, such as in blocks of flats, the number served should
be limited to not more than 6 systems so that any installation discharging can be traced reasonably eas-
ily. The single common discharge pipe should be at least one pipe size larger than the largest individual
discharge pipe to be connected. For further information contact your Building Control Office.
Valve
outlet size
Minimum size of
discharge pipe D1*
Minimum size of
discharge pipe D2*
from tundish
Maximum resistance
allowed, expressed as
a length of straight pipe
(i.e. no elbows or bends)
Resistance
created by
each elbow or
bend
22 mm
up to 9 m
0.8 m
G1/2
15 mm
28 mm
up to 18 m
1.0 m
35 mm
up to 27 m
1.4 m
28 mm
up to 9 m
1.0 m
G3/4
22 mm
35 mm
up to 18 m
1.4 m
42 mm
up to 27 m
1.7 m
35 mm
up to 9 m
1.4 m
G1
28 mm
42 mm
up to 18 m
1.7 m
54 mm
up to 27 m
2.3 m
Safety device
(e.g. temperature
relief valve)
Metal discharge pipe (D1) from
temperature relief valve to tundish
600mm maximum
Tundish
300mm
minimum
Discharge pipe (D2) from tundish,
with continuous fall.
Discharge below
fixed grating
(3.61 gives
alternative points
of discharge)
Fixed grating
Trapped gulley
