Potterton Gold Installation And Servicing Instructions page 7

DISCHARGE PIPEWORK
It is a requirement of Building Regulation G3
that any discharge from an unvented system
is conveyed to where it is visible, but will not
cause danger to persons in or about the building.
The tundish and discharge pipes should be
fitted in accordance with the requirements and
guidance notes of Building Regulation G3. The G3
Requirements and Guidance section 3.50 - 3.63
are reproduced in the following sections of
this manual. For discharge pipe arrangements
not covered by G3 Guidance advice should be
sought from your local Building Control Officer.
Any discharge pipe connected to the pressure
relief devices (Expansion Valve and Temperature/
Pressure Relief Valve) must be installed in a
continuously downward direction and in a frost
free environment.
Water may drip from the discharge pipe of the
pressure relief device. This pipe must be left open
to the atmosphere. The pressure relief device is
to be operated regularly to remove lime deposits
and to verify that it is not blocked.
G3 REQUIREMENT
"...there shall be precautions...to ensure that the
hot water discharged from safety devices is safely
conveyed to where it is visible but will not cause
danger to persons in or about the building."
The following exract is taken from the latest G3
Regulations
Discharge pipes from safety devices
Discharge pipe D1
3.50 Each of the temperature relief valves or combined
temperature and pressure relief valves specified in 3.13 or
3.17 should discharge either directly or by way of a mani-
fold via a short length of metal pipe (D1) to a tundish.
3.51 The diameter of discharge pipe (D1) should be not
less than the nominal outlet size of the temperature relief
valve.
3.52 Where a manifold is used it should be sized to accept
and discharge the total discharge form the discharge pipes
connected to it.
3.53 Where valves other than the temperature and pres-
sure relief valve from a single unvented hot water system
discharge by way of the same manifold that is used by the
safety devices, the manifold should be factory fitted as part
of the hot water storage system unit or package.
Tundish
3.54 The tundish should be vertical, located in the same
space as the unvented hot water storage system and be fit-
ted as close as possible to, and lower than, the valve, with
no more than 600mm of pipe between the valve outlet and
the tundish (see Diagram 1).
Note: To comply with the Water Supply (Water Fittings)
Regulations, the tundish should incorporate a suitable air
gap.
3.55 Any discharge should be visible at the tundish. In
addition, where discharges from safety devices may not
be apparent, e.g. in dwellings occupied by people with
impaired vision or mobility, consideration should be given
to the installation of a suitable safety device to warn when
discharge takes place, e.g. electronically operated.
Discharge pipe D2
3.56 The discharge pipe (D2) from the tundish should:
(a) have a vertical section of pipe at least 300mm long
below the tundish before any elbows or bends in the pipe-
work (see Fig. 5); and
(b) be installed with a continuous fall thereafter of at
least 1 in 200.
3.57 The discharge pipe (D2) should be made of:
(a) metal; or
(b) 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 (e.g. as
specified in the relevant part of BS 7291).
3.58 The discharge pipe (D2) should 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. for discharge pipes between
9m and 18m the 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 calculat-
ing the flow resistance. See Fig 1, Table 3 and the worked
example.
Note: An alternative approach for sizing discharge pipes
would be to follow Annex D, section D.2 of BS 6700:2006
Specification for design, installation, testing and mainte-
nance of services supplying water for domestic use within
buildings and their curtilages.
3.59 Where a single common discharge pipe serves more
than one system, it should be at least one pipe size larger
than the largest individual discharge pipe (D2) to be con-
nected.
3.60 The discharge pipe should not be connected to a soil
discharge stack unless it can be demonstrated that that
the soil discharge stack is capable of safely withstanding
temperatures of the water discharged, in which case, it
should:
(a) contain a mechanical seal, not incorporating a water
trap, which allows water into the branch pipe without al-
lowing foul air from the drain to be ventilated through the
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