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Discharge
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 extract is taken from the latest G3
Regulations
Discharge pipes from safety devices
Discharge pipe D1
3.50 Safety devices such as temperature relief valves
or combined temperature and pressure relief valves
(see paragraphs 3.13 or 3.18) should discharge either
directly or by way of a manifold 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 safety device,
e.g. temperature relief valve.
3.52 Where a manifold is used it should be sized to
accept and discharge the total discharge from the
discharge pipes connected to it.
3.53 Where valves other than a temperature and
pressure 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 fitted as close as possible to, and lower than,
the safety device, with no more than 600mm of pipe
16
between the valve outlet and the tundish (see Fig. 9,
page 18).
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 pipework (see Fig. 9); 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 as specified in the relevant part of
BS 7291- 1:2006 Thermostatic pipes and fittings for
hot and cold water for domestic purposes and heating
installations in buildings.General requirements).
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 calculating
the flow resistance. See Fig. 9, Table 5 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 + A1:2009
Specification for design, installation, testing and
maintenance 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 connected.
3.60 The discharge pipe should not be connected to a
soil discharge stack unless it can be demonstrated that
the soil discharge stack is capable of safely withstanding
temperatures of the water discharged, in which case, it
should:
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