6
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 tempera-
ture relief valve is 9.0m.
Subtract the resistance for 4 No. 22mm elbows at 0.8m
each = 3.2m
Therefore the permitted length equates to: 5.8m
5.8m is less than the actual length of 7m therefore
calculate the next largest size.
Maximum resistance allowed for a straight length of
28mm pipe (D2) from a G1/2 temperature relief valves
equates to 18m.
Subtract the resistance of 4 No. 28mm elbows at 1.0m
each = 4.0m
Therefore the maximum permitted length equates to: 14m
As the actual length is 7m, a 28mm (D2) copper pipe will
be satisfactory.
3.8 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 per-
sons in or about the building. The tundish and dis-
charge pipes should be fitted in accordance with the
requirements and guidance notes of Building Regula-
tion 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 Agrement 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 either your local Build-
ing Control Officer or Heatstore.
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 ter-
minate 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 equiva-
lent 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 calculat-
ing the flow resistance. Refer to Diagram 1, Table 1
and the worked example.
An alternative approach for sizing discharge pipes
would be to follow BS 6700:1987
Specification for
design installation, testing and maintenance of ser-
vices supplying water for domestic use within build-
ings and their curtilages,
Appendix E, section E2 and
table 21.
b. have a vertical section of pipe at least 300mm long,
below the 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 practically difficult there should be clear visibil-
ity at one or other of these locations. Examples of
acceptable discharge arrangements 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 stand-
ings, grassed areas etc. are acceptable providing that
where children may play or otherwise come into con-
tact with discharges a wire cage or similar guard is
positioned to prevent contact, whilst maintaining vis-
ibility.
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 temperature dis-
charges 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 eas-
ily. The single common discharge pipe should be at
least one pipe size larger than the largest individual
discharge pipe (D2) to be connected. If unvented hot
water storage systems are installed where discharges
from safety devices may not be apparent i.e. in dwell-
ings occupied by blind, infirm or disabled people, con-
sideration 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, roofing felt and non-metallic rainwa-
ter goods may be damaged by such discharges.
