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Worked example of discharge pipe sizing
The example below is for a G1/2 temperature relief
valve with a discharge pipe (D2) having 4 No. elbows
and length of 7m from the tundish to the point of
discharge.
From Table 4:
Maximum resistance allowed for a straight length of
22mm copper discharge pipe (D2) from a G1/2
temperature relief valve is 9.0m.
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 3.9 are reproduced in the following sections.
Information Sheet No. 33 available from the British
Board of Agrément gives further advice on discharge
pipe installation. For discharge pipe arrangements
not covered by G3 Guidance or BBA Info Sheet No.33
advice should be sought from your local Building
Control Officer.
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.”
G3 GUIDANCE SECTION 3.9
The discharge pipe (D1) from the vessel up
to and including the tundish is generally
supplied by the manufacturer of the hot
water storage system. Where otherwise,
the installation should include the
discharge pipe(s) (D1) from the safety
device(s). In either case the tundish
should be vertical, located in the same
space as the unvented hot water storage
system and be fitted as close as possible
and within 500mm of the safety device e.g.
the temperature relief valve.
The discharge pipe (D2) from the tundish
should terminate in a safe place where
there is no risk to persons in the vicinity of
the discharge, preferably be of metal and:
a. 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. discharge pipes between 9m
and 18m 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. Refer to Diagram 1, Table 1 and
the worked example.
An alternative approach for sizing dis-
charge pipes would be to follow BS
6700:1987 Specification for design instal-
lation, testing and maintenance of ser-
vices supplying water for domestic use
within buildings and their curtilages,
Appendix E, section E2 and table 21.
b. have a vertical section of pipe at least
300mm long, below
t
he tundish before
any elbows or bends in the pipework.
c. be installed with a continuous fall.
d. have discharges visible at both the
tundish and the final point of discharge
but where this is not possible or is practi-
cally difficult there should be clear
visibility at one or other of these locations.
Examples of acceptable discharge ar-
rangements are:
i. ideally below a fixed grating and
above the water seal in a trapped gully.
ii. downward discharges at low level;
i.e. up to 100mm above external surfaces
such as car parks, hard standings,
grassed areas etc. are acceptable pro-
viding that where children may play or
otherwise come into contact with dis-
charges a wire cage or similar guard is
positioned to prevent contact, whilst
maintaining visibility.
iii. discharges at high level; e.g. 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 tem-
perature discharges of water and 3m
from any plastics guttering system that
would collect such discharges (tundish
visible).
iv. where a single pipe serves a number
of discharges, such as in blocks of flats,
the number served should be limited to
not more than 6 systems so that any
instalation discharging can be traced
reasonably easily. The single common
discharge pipe should be at least one
pipe size larger than the largest indi-
vidual discharge pipe (D2) to be con-
nected. If unvented hot water storage
systems are installed where discharges
from safety devices may not be appar-
ent i.e. in dwellings occupied by blind,
infirm or disabled people, consideration
should be given to the installation of an
electronically operated device to warn
when discharge takes place.
Note:
The discharge will consist of
scalding water and steam. Asphalt, roof-
ing felt and non-metallic rainwater
goods may be damaged by such dis-
charges.