Safety Device Discharge Arrangements (G3) - Mitsubishi Electric ecodan EHPT20Q-VM2EA Installation Manual

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Installation

4.4 Safety Device Discharge Arrangements (G3)

The following instructions are a requirement of UK Building Regulations and MUST be adhered
to. If you are in any doubt please seek advice from local building planning offi ce.
Metal discharge pipe (D1)
pressure relief valve to tundish
Safety device
(e.g. pressure
relief valve) 500 mm maximum
Tundish
300 mm
minimum
Metal discharge pipe (D2) from tundish,
with continuous fall. See Point 4.D i-iv,
Table 4.4.1 and worked example
Valve outlet Minimum size of Minimum size of discharge pipe
size discharge pipe D1 D2 from tundish
G 1/2 15 mm 22 mm
28 mm
35 mm
G 3/4 22 mm 28 mm
35 mm
42 mm
G1 28 mm 35 mm
42 mm
54 mm
1. Connect the tundish and route the discharge pipe as shown in Figure 4.4.1.
2. The tundish should be fi tted vertically and as close to the safety device as possible and within
500 mm of the device.
3. The tundish should be visible to occupants and positioned away from electrical devices.
4. 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 device 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 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 fl ow resistance.
Refer to Figure 4.4.1, Table 4.4.1 and the worked example. An alternative approach for sizing
discharge pipes would be to follow BS 6700: 1987 specifi cation for design installation, testing
and maintenance of services supplying water for domestic use within buildings and their
cartridges.
B) Have a vertical section of pipe at least 300 mm long, below the tundish before any elbows or
bends in the pipework.
C) Be installed with a continuous fall.
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Discharge below fi xed grating
(Point 4.D gives alternative points of
discharge)
Fixed grating
Trapped gulley
<Figure 4.4.1>
Maximum resistance allowed, expressed as a Resistance created by
length of straight pipe (no elbows or bends) each elbow or bend
Up to 9 m 0.8 m
Up to 18 m 1.0 m
Up to 27 m 1.4 m
Up to 9 m 1.0 m
Up to 18 m 1.4 m
Up to 27 m 1.7 m
Up to 9 m 1.4 m
Up to 18 m 1.7 m
Up to 27 m 2.3 m
<Table 4.4.1>
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Installation
D) Have discharges visible at both the tundish and the fi nal point of discharge but where this is
not possible or is practically diffi cult there should be clear visibility at one or other of these
locations. Examples of acceptable discharge arrangements are:
i. Ideally below a fi xed grating and above the water seal in a trapped gully.
ii. Downward discharges at low level; i.e. up to 100 mm above external surfaces 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 fl ats,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 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 dwellings occupied by blind, infi rm 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, roofi ng felt and
nonmetallic rainwater goods may be damaged by such discharges.
Discharges MUST be positioned in a frost-free location.
Worked example:
The example below is for a G½ pressure relief valve with a discharge pipe (D2) having 4 No.
elbows and length of 7 m from the tundish to the point of discharge.
Discharges MUST be positioned in a frost-free location.
From Table 4.4.1: Maximum resistance allowed for a straight length of 22 mm copper discharge
pipe (D2) from a G½ pressure relief valve is: 9.0 m subtract 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 pressure 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.
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