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3.5.6 Alternative discharge
Discharge pipes must be metal, change to dis-
charge pipes should be suitably temperature
rated as defined by G3 building Regulations.
The pipe must 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 stand-
ings, grassed areas etc. are acceptable providing
that where children may play or otherwise come
into contact with discharges, a wire cage or simi-
lar 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 withstand-
ing 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
easily. 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 elbown or bends)
Resistance created by each
elbow or bend
G 1/2
15 mm
22 mm
28 mm
35 mm
up to 9 m
up to 18 m
up to 27 m
0.8 m
1.0 m
1.4 m
G 3/4
22 mm
28 mm
35 mm
42 mm
up to 9 m
up to 18 m
up to 27 m
1.0 m
1.4 m
1.7 m
G 1
28 mm
35 mm
42 mm
54 mm
up to 9 m
up to 18 m
up to 27 m
1.4 m
1.7 m
2.3 m
Safety device
(e.g. temperature
relief valve)
Metal discharge pipe (D1) from
temperature relief valve to tundish
600 mm.
maximum
Tundish
300 mm.
minimum
Discharge pipe (D2) from tundish,
with continous fall. See table and
worked example.
Discharge below
fixed grating
(see text for alternative
points of discharge)
Fixed grating
Trapped gulley
