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4.7
Discharge Arrangement (DHW)
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Fig. 16 Diagram of a typical discharge pipe arrangement (extract from
Building Regulation G3)
[1]
Safety device (e.g. temperature relief valve)*
[2]
Metal discharge pipe (D1) from temperature relief valve to
tundish*
[3]
Discharge pipe (D2) from tundish with continuous fall
[4]
Fixed grating
[5]
Trapped gulley
[6]
Tundish*
*The positions (1), (2) and (6) are already pre-installed on PPCy
Connect the Tundish and route the discharge pipe. The discharge
pipework must be routed in accordance with Part G3 of schedule 1 of the
Building Regulations. The information that follows is not exhaustive and
if you are in doubt you should seek advice.
The two safety valves will only discharge water under fault conditions.
When operating normally water will not be discharged.
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 600mm of the safety device e.g. the temperature relief valve.
(all the above is already preplumbed)
Pre plumbed cylinder – 6 721 852 928 (2023/04)
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6
4
3
0010017083-001
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
and:
• 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 9 m long i.e. discharge pipes between 9 m and
18 m equivalent resistance length should be at least two sizes larger
than the the nominal outlet size of the safety device, between 18 and
27 m at least 3 sizes larger, and so on. Bends must be taken into
account in calculating the flow resistance. Refer to Fig. 16, Table 8
and the worked example.
An alternative approach for sizing discharge pipes would be to follow
British specification standards for design installation, testing and
maintenance of services supplying water for domestic use within
buildings and their curtilages.
• Have a vertical section of pipe at least 300 mm long, below the
tundish before any elbows or bends in the pipework.
• Be installed with a continuous fall.
• It is preferable for the discharge to be visible at both the tundish and
the final point of discharge but where this is not possible or
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practically difficult there should be clear visibility at one or other of
these locations. Examples of acceptable discharge arrangements
are:
– Ideally below the fixed grating and above the water seal in a
trapped gulley.
– Downward discharges at a low level; i.e. up to 100 mm above
external surfaces such as car parks, hard standing, grassed areas
etc. are acceptable providing that where children play or
otherwise come into contact with discharges, a wire cage or
similar guard is positioned to prevent contact whilst maintaining
visibility.
– Discharges at a high level; e.g. in to 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 withstanding high
temperature discharges of water and 3 m from any plastic
guttering systems that would collect such discharges (tundish
available).
– 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 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. If unvented hot water storage systems are installed
where discharges from safety devices may not be apparent i.e. in
dwellings occupied by blind, infirm or disabled people,
consideration should be given to the installation of an
electronically operated device to warn when discharge takes
place.
NOTICE
▶ The discharge may consist of scalding water and steam. Asphalt,
roofing felt and non-metallic rainwater goods may be damaged by
such discharges.
▶ It is only acceptable to discharge into a soil pipe if certain conditions
stipulated in the G3 regulations are met.
Pre-Installation
15
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