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TEMPERATURE AND PRESSURE RELIEF VALVE
A temperature and pressure relief valve is factory fitted as part of the unvented water system kit. This valve has a lift pressure of 7 bar and a lift
temperature of 90°C.
This should be checked for transit damage prior to filling the water heater.
The storage vessel relief valve connection should not be used for any other purpose.
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 500mm 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
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.
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.
RELIEF VALVE DISCHARGE PIPEWORK
METAL DISCHARGE PIPE (D1) FROM
TEMPERATURE RELIEF VALVE
TO TUNDISH
SAFETY DEVICE
(e.g. TEMPERATURE
RELIEF VALVE)
500mm MAXIMUM
TUNDISH
300mm MINIMUM
METAL DISCHARGE PIPE (D2) FROM
TUNDISH, WITH CONTINUOUS FALL
DISCHARGE IDEALLY BELOW
FIXED GRATING
FIXED GRATING
TRAPPED GULLY
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