v) For units with secondary coil facilities, please discuss the use of the coil, and a suitable
configuration for use, with a suitably qualified plumber/engineer before proceeding to add a
heat source to the secondary coil.
6.11 Discharge Pipe Work:
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. Information Sheet No. 33, available from the British Board of Agreement 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 metal, in a continuously downward direction and in a frost free
environment. The water may drip from the discharge pipe of the pressure relief device 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.
Please see figure 7 for typical discharge arrangement.
6.12 Expansion Vessel:
An appropriately sized expansion vessel must be fitted to the system in which the unit is incorporated.
The Expansion vessel can be fitted to the cold feed in to the unit (see Figures 4 and 6).
6.13 G3 Requirement:
G3 (ENGLAND AND WALES), TECHNICAL STANDARD P3 (SCOTLAND) OR BUILDING
REGULATION P5 (NORTHERN IRELAND) AND THE WATER FITTING REGULATIONS (ENGLAND
AND WALES) OR WATER BYELAWS (SCOTLAND).
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.
6.14 G3 Guidance Section:
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(s) e.g. the TPRV. 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, 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 figure 7 and Table 1 below. An alternative
approach for sizing discharge pipes would be to follow BS6700:1987 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 pipe work.
c) Be installed with a continuous fall, and in a frost free environment.
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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