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3.11 DISCHARGE PIPEWORK
It is a requirement of Building Regulations that any discharge from an unvented system is conveyed
to where it is visible, but will not cause danger to persons in or about the building. The tundish
and discharge pipes should be fitted in accordance with the requirements and guidance notes
of Building Regulations. Building Regulation G3 Requirements and Guidance section 3.9 are
reproduced in the following sections.
Information Sheet No. 33 available from the British Board of Agrément gives further advice on
discharge pipe installation. For discharge pipe arrangements not covered by G3 Guidance or BBA
Info Sheet No.33 advice should be sought from your local Building Control Officer.
Any discharge pipe connected to the pressure relief devices (Expansion Valve and Temperature/
Pressure Relief Valve) must be installed in a continuously downward direction and in a frost free
environment.
G3 REQUIREMENT
"...there shall be precautions...to ensure that the hot water discharged from safety devices is
safely conveyed to where it is visible but will not cause danger to persons in or about the build-
ing."
G3 GUIDANCE SECTION 3.9
The discharge pipe (D1) from the vessel up to and including the tundish is generally supplied by
the manufacturer of the hot water storage system. Where otherwise, the installation should include
the discharge pipe(s) (D1) from the safety device(s). In either case the tundish should be vertical,
located in the same space as the unvented hot water storage system and be fitted as close as
possible and within 500mm of the safety device e.g. the temperature relief valve.
The discharge pipe (D2) from the tundish should terminate in a safe place where there is no risk
to persons in the vicinity of the discharge, preferably be of metal and:
a. be at least one pipe size larger than the nominal outlet size of the safety device unless its
total equivalent hydraulic resistance exceeds that of a straight pipe 9m long i.e. discharge pipes
between 9m and 18m equivalent resistance length should be at least two sizes larger than the
nominal outlet size of the safety device, between 18 and 27m at least 3 sizes larger , and so on.
Bends must be taken into account in calculating the flow resistance. Refer to Diagram 8, Table 1
and the worked example.
An alternative approach for sizing discharge pipes would be to follow BS 6700:1987 Specification
for design, installation, testing and maintenance of services supplying water for domestic use within
buildings and their curtilages, Appendix E, section E2 and table 21.
b. have a vertical section of pipe at least 300mm long below the tundish before any elbows or
bends in the pipework.
c. be installed with a continuous fall.
d. have discharges visible at both the tundish and the final point of discharge, but where this is not
possible or is practically difficult there should be clear visibility at one or other of these locations.
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