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Unvented Hot Water Cylinder
19
INST
ALLA
TION AND
COMMISSIONING (2)
Discharge Pipe work
It is a requirement of Building Regulations that any discharge from an unvented system is conveyed
to where it is visible, but will not cause danger to persons in or about the building. The tundish and
discharge pipes should be fitted in accordance with the requirements and guidance notes of Building
Regulations. Building Regulation G3 Requirements and Guidance section 2 (page 18) are reproduced
in the previous sections.
For discharge pipe arrangements not covered by G3 Guidance advice should be sought from your
local Building Control Officer. Any discharge pipe connected to the pressure relief devices (Expansion
Valve and Temperature / Pressure Relief Valve) must be installed in a continuously downward direction
and in a frost free environment.
The water may drip from the discharge pipe of the pressure relief device and that this pipe must be
left open to the atmosphere. The pressure relief device is to be operated regularly to remove lime
deposits and to verify that it is not blocked.
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”.
Worked example of discharge pipe sizing (Taken from Building Regulations 2000, G3, 2010
edition)
The example below is for a G1” 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 15:
Maximum resistance allowed for a straight length of 35mm copper discharge pipe (D2) from a G1”
temperature relief valve is 9m.
Subtract the resistance for 4 No. 35mm elbows at 1.4m each = 5.6m
Therefore the permitted length equates to: 3.4m
3.4m is less than the actual length of 7m therefore calculate the next largest size.
Maximum resistance allowed for a straight length of 42mm pipe (D2) from a G1” temperature relief
valve equates to 18m.
Subtract the resistance of 4 No. 42mm elbows at 1.7m each = 6.8m
Therefore the maximum permitted length equates to: 11.2m
As the actual length is 7m, a 42mm (D2) copper pipe will be satisfactory.
Note:
The tundish must be installed away from any electrical devices