14
c) Discharge pipe work
NOTE:
The following guidelines refer to Building Regulation G3. This
regulation only applies to an unvented system over 15 litres. It is good
practice to adopt the same principles for the EW and MW range.
1) The tundish must be vertical and fitted within 500 mm of the temperature &
pressure relief valve and must be located with the water heater. The tundish must
also be in a position visible to the occupants, and positioned away from any
electrical devices. 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 and to be of
metal.
2) Discharge pipes from the temperature & pressure relief and pressure relief
valve may be joined together.
3) The pipe diameter must 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 2 sizes larger than the nominal outlet size of the safety device.
Between 18 m and 27 m at least 3 times larger, and so on.
Bends must be taken into account in calculating the flow resistance.
See diagram.6 and Table 2.
4) The discharge pipe must have a vertical section of pipe at least 300 mm in
length, below the tundish before any elbows or bends in the pipe work.
5) The discharge pipe must be installed with a continuous fall.
6) The discharge must be visible at both the tundish and the final point of
discharge, but where this is not possible or practically difficult; there should be
clear visibility at one or other of these locations. Examples of acceptance are:
i) Ideally below a fixed grating and above the water seal in a trapped gully.
ii) Downward discharges at a low level; i.e. up to 100 mm above external surfaces
such as car parks, hard standings, grassed areas etc. These 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; i.e. 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 3 m from any
plastic guttering systems that would collect such a discharge (tundish visible).
