15
Discharge Pipework
CAUTION: Safety Valve Connections
The 7 bar pressure relief valve on the domestic hot water store may discharge boiling water and should be piped to a safe
but visible drain point e.g. a gulley where any discharge will not cause damage or injury.
Typical Discharge Pipe Arrangement
CAUTION: A tundish (supplied) should be incorporated close to the appliance where any discharge will be visible
but not hazardous. The pipe-work should have a continuous fall to the drain and should be of minimum 28mm dia. In all
instances, installation of the discharge pipework must be in accordance with G3 of the Building Regulations and installed by a
Competent Person.
The information below is taken from Approved Document G3 of the Building Regulations and is provided to assist with the
design and installation of the discharge pipework. However, the information is not exhaustive and reference should always be
made to Approved Document G3 of the Building Regulations. The final decision regarding any arrangements rests with
Building Control and it is recommended that their advice is sought if you have any concerns regarding this aspect of the
installation.
The two safety valves will only discharge water under fault conditions. When operating normally water will not be
discharged.
Table 1 – Sizing of copper discharge pipe “D2” for common temperature relief valve outlet sizes
Refer to paragraphs 3.5, 3.9, and 3.9(a) of Approved Document G
Pressu
re &
Tempe
rature/
expansi
on
Relief
Valve
Pipewo
rk
P&T
Relief
Valve
600 mm max.
300 mm min.
NOTES:
The discharge will consist of scalding
water and steam. Asphalt, roofing felt
and non−metallic rainwater goods may
be damaged by such discharges,
It is not acceptable to discharge
straight into a soil pipe.
15 mm
discharge
pipe
Expansion
Relief Valve
Dotted line showing
alternative route with
single tundish being
used
Discharge below
fixed gratting
Fixed gratting
Trapped gully
22 mm metal pipe with continuous
fall up to 9 m equivalent length (D2)
Valve
Outlet
Size
Minimum size if
discharge pipe
D1*
Minimum size of
discharge pipe
D2* from tundish
Maximum resistance allowed,
expressed as length of straight
pipe (i.e. no elbows or bends)
Resistance created by
each elbow or bend
G1/2
15mm
22mm
28mm
35mm
Up to 9m
Up to 18m
Up to 27m
0.8m
1.0m
1.4m
G3/4 22mm 28mm
35mm
42mm
Up to 9m
Up to 18m
Up to 27m
1.0m
1.4m
1.7m
G1 28mm 35mm
42mm
54mm
Up to 9m
Up to 18m
Up to 27m
1.4m
1.7m
2.3m