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7
Water to Water Heat Pump (WTW)
7.6.6 Safety Device Discharge Arrangements (G3)
The following instructions are a requirement of UK Building Regulations
and must be adhered to. For other countries please refer to local legisla-
tion. If you are in any doubt please seek advice from local building plan-
ning office.
1. Position the inlet control group so that discharge from both safety valves can
be joined together via a 15 mm end feed Tee.
2. Connect the tundish and route the discharge pipe as shown in Figure 7.6.7.
3. The tundish should be fitted vertically and as close to the safety device as
possible and within 500 mm of the device.
4. The tundish should be visible to occupants and positioned away from electri-
cal devices.
5. 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, be of metal
construction and:
A) Be at least one pipe size larger than the nominal outlet size of the safety de-
vice unless its total equivalent hydraulic resistance exceeds that of a straight
pipe 9 m long i.e. discharge pipes between 9 m and 18 m equivalent resist-
ance 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. Refer to
Figure 7.6.7, Table 7.6.4 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 services supplying water for
domestic use within buildings and their cartilages.
B) Have a vertical section of pipe at least 300 mm long, below the tundish be-
fore 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
visibility at one or other of these locations. Examples of acceptable discharge
arrangements are:
Safety device
(e.g. temperature
relief valve)
500 mm maximum
Metal discharge pipe (D2) from tundish,
with continuous fall. See Point 5.D i-iv,
Table 7.6.4 and worked example
Valve outlet
Minimum size of
size
discharge pipe D1
G 1/2
15 mm
G 3/4
22 mm
G1
28 mm
Metal discharge pipe (D1)
temperature relief valve to tundish
Tundish
300 mm
minimum
Minimum size of discharge
pipe D2 from tundish
22 mm
28 mm
35 mm
28 mm
35 mm
42 mm
35 mm
42 mm
54 mm
<Table 7.6.4>
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 100 mm above external sur-
faces such as car parks, hard standings, grassed areas etc. are acceptable
providing that where children may play or otherwise come into contact with
discharges 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 temperature discharges of water and 3 m
from any plastic 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 installation discharging can be traced reasonably easily. The single
common discharge pipe should be at least one pipe size larger than the larg-
est 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 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,
roofing felt and nonmetallic rainwater goods may be damaged by
such discharges.
Worked example: The example below is for a G½ temperature relief valve with
a discharge pipe (D2) having 4 No. elbows and length of 7 m from the tundish
to the point of discharge.
From Table 7.6.4: Maximum resistance allowed for a straight length of 22 mm
copper discharge pipe (D2) from a G½ temperature relief valve is: 9.0 m sub-
tract the resistance for 4 No. 22 mm elbows at 0.8 m each = 3.2 m. Therefore
the maximum permitted length equates to: 5.8 m. 5.8 m is less than the actual
length of 7 m, therefore calculate the next largest size. Maximum resistance
allowed for a straight length of 28 mm pipe (D2) from a G½ temperature relief
valve equates to: 18 m
Subtract the resistance for 4 No. 28 mm elbows at 1.0 m each = 4 m. Therefore
the maximum permitted length equates to: 14 m. As the actual length is 7 m, a
28 mm (D2) copper pipe will be satisfactory.
Discharge below fixed grating
(Point 5.D gives alternative points of
discharge)
Fixed grating
Trapped gulley
<Figure 7.6.7>
Maximum resistance allowed, expressed as a
length of straight pipe (no elbows or bends)
Up to 9 m
Up to 18 m
Up to 27 m
Up to 9 m
Up to 18 m
Up to 27 m
Up to 9 m
Up to 18 m
Up to 27 m
Resistance created by
each elbow or bend
0.8 m
1.0 m
1.4 m
1.0 m
1.4 m
1.7 m
1.4 m
1.7 m
2.3 m
B-231
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