Heatrae Sadia ELECTROMAX Installation & Servicing Instructions Manual page 17

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 1:
Maximum resistance allowed for a straight
length of 22mm copper discharge pipe
(D2) from a G1/2 temperature relief valve
is 9.0m.
Table 1 Sizing of Copper Discharge
Pipes (D2) for Common T&P Relief Valve
Sizes
Valve outlet size
G1/2
G3/4
G1
Safety device
(e.g. Temperature
relief valve)
Diagram 11
Schematic Discharge
Pipe Arrangement
Minimum size of Minimum size of
discharge pipe D1 discharge pipe D2
from tundish
22mm
15mm 28mm
35mm
28mm
22mm 35mm
42mm
35mm
28mm 42mm
54mm
Metal discharge pipe (D1) from
Temperature relief valve to tundish
500mm maximum Tundish
300mm
minimum
Discharge pipe (D2) from tundish,
with continuous fall. See Building
Regulation G3 section 3.9d i-iv,
Table 4 and worked example
Subtract the resistance allowed 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 valve equates to 18m.
Subtract the resistance of 4 No. 28mm
elbows at 1.0m each = 4.0m
Therefore the maximum permitted length
equates to 14.0m
As the actual length is 7m, a 28mm (D2)
copper pipe will be satisfactory.
Maximum
resistance
allowed,
expressed as a
length of straight
pipe (I.e. no
elbows or bends)
up to 9m
up to 18m
up to 27m
up to 9m
up to 18m
up to 27m
up to 9m
up to 18m
up to 27m
Discharge below
fixed grating
(Building Regulation
G3 section 3.9d gives
alternative points
of discharge)
Fixed grating
Trapped
gully
Installation & Servicing Instructions
Resistance
created by each
elbow or bend
0.8m
1.0m
1.4m
1.0m
1.4m
1.7m
1.4m
1.7m
2.3m
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