Redring MW10 Handbook page 16

Table 2
Sizing of copper discharge pipe "D2" for common temperature valve outlets.
Valve outlet size
G 1/2
G 3/4
G 1
Worked example
The example below is for a G 1/2" temperature & 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.
From Table 2
Maximum resistance allowed for a straight length of 22 mm copper discharge pipe
(D2) from G 1/2 T & P valve is 9 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.
As 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 G 1/2"
T & P valve equates to: 18 m.
Subtract the resistance for 4 no. 28 mm elbow 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.
d) Electrical
WARNING:
The electrical installation must be in line with the current I.E.E. wiring regulations.
A mains supply of 230/240 VAC (BS 7671) 3 kW (13 amps) is required (diagram
7)
Heat resisting cable, round 3 core 1.5 mm (to BS 6141 table 8) should be used to
connect to the electrical supply through either:
- a 13 amp socket to BS 1363; or
Minimum size of
discharge pipe D1*
15 mm
22 mm
28 mm
The appliance must be earthed.
Minimum size of
discharge pipe D2* resistance allowed,
from tundish
length of pipe (i.e.
no elbow or bends)
22 mm
28 mm
35 mm
28 mm
35 mm
42 mm
35 mm
42 mm
54 mm
16
Maximum Resistance created
by each elbow or
expressed as a
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
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