20
Discharge pipework
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 19) are reproduced in the following 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 on page 24 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 11:
Maximum resistance allowed for a straight length of 22mm copper discharge pipe (D2) from a G1/2 temperature relief
valve is 9m.
Subtract the resistance 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 1m each = 4m
Therefore the maximum permitted length equates to: 14m
As the actual length is 7m, a 28mm (D2) copper pipe will be satisfactory.
