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Installation Discharge
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 extract 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 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 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
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.
8
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 valve, with no more than 600 mm of pipe between
the valve outlet and the tundish (see Fig. 4, page 9).
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 300 mm
long below the tundish before any elbows or bends in
the pipework (see Fig. 4, page 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 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 9 m long, i.e.
for discharge pipes between 9 and 18 m the equivalent
resistance length should be at least two sizes larger
than the nominal outlet size of the safety device;
between 18 and 27 m at least 3 sizes larger, and so
on; bends must be taken into account in calculating
the flow resistance. (See Table 4, page 10), 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 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.
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