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3.10 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 Tempera
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ture/Pressure Relief Valve) must be installed in a continuously downward direction and in a
frost free environment.
The water may drip from the discharge pipe of the pressure relief device and that this pipe
must be left open to the atmosphere. The pressure relief device is to be operated regularly
to remove lime deposits and to verify that it is not blocked.
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
building.”
G3 GUIDANCE SECTION 3.9
The discharge pipe (D1) from the vessel up to and including the tundish is generally sup
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plied 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 7, Table 1 and the worked example.
An alternative approach for sizing discharge pipes would be to follow BS 6700:1987 Specifica
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tion 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 pipe work.
c. be installed with a continous fall, and in a frost free enviroment.
