Heatrae Sadia ELECTROMAX SOLAR Installation & Servicing Instructions Manual page 19

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 two pipe sizes larger,
and so on. Bends must be taken into account in calculating the flow resistance. Refer to
table 2 and the worked example.
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 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.
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 cartilages, appendix E, section E2 and table 21.
Examples of acceptable discharge arrangements are:
i. Ideally below a fixed grating and above the water seal in a trapped gully.
ii. Downward discharges at 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
may 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 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 plastics guttering system 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 discharging
can be traced reasonably easily. The single common discharge pipe should be at least one
pipe size larger than the largest individual discharge pipe (D2) 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.
Note: The discharge will consist of scalding water and steam. Asphalt, roofing felt and non-
metallic rainwater goods may be damaged by such discharges.
Worked example of discharge pipe sizing:
The example below is for a G1/2 temperature relief valve with a discharge pipe (D2) having
4 No. elbows and length of 7m from the tundish to the point of discharge.
From Table 2:
• Maximum resistance allowed for a straight length of 22mm copper discharge pipe (D2)
from a G1/2 temperature relief valve is 9.0m.
• Subtract the resistance allowed for 4 No. 22mm elbows at 0.8m each = 3.2m
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