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 tted 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 Ofcer. 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 specied 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 tted 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 tted 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 Fig. 05 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 300mm
long below the tundish before any elbows or bends in
the pipework (see Fig. 05 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 specied 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 calculating
the ow resistance (see Table 03 page 9) 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 Specication 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.
Installation Discharge
