9
6.3.3
TEMPERATURE AND PRESSURE RELIEF VALVE
A temperature and pressure relief valve is provided in the unvented water system kit. This valve has a
lift pressure of 7 bar and a lift temperature of 90°C. The valve must be fitted to the relief valve tapping.
The storage vessel relief valve connection should not be used for any other purpose.
6.3.4
RELIEF VALVE DISCHARGE PIPEWORK
It is important that any discharge water does not collect in the discharge pipe-work and can run freely
to the tundish. The tundish should be mounted in a vertical and visible position 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.
The discharge pipe 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 or other material that has been demonstrated to be capable
of safely withstanding temperatures of the water discharged and is clearly and permanently marked to
identify the product and performance standard, 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.
An alternative approach for sizing discharge pipes would be to follow BS6700 Specification for design
installation, testing and maintenance of services supplying water for domestic use within buildings and
their curtilages.
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 of at least 1 in 200.
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.
Examples of acceptable discharge arrangements are:
I.
Ideally below the fixed grating and above the water seal in a trapped gulley.
II.
Downward discharges at a low level; i.e. up to 100mm above external surfaces such as car
parks, hard standings, 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.
III.
Discharges at a high level; e.g. into a 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 3m from any plastic guttering systems that would
collect such discharges (tundish visible).
IV.
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.
