MEGAFLO 400L Product Manual - page 15
Unvented Hot Water Cylinder
15
INST
ALLA
TION AND
COMMISSIONING (2)
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 17) 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”.
VALVE OUTLET
SIZE
MINIMUM SIZE OF
DISCHARGE PIPE D1
MINIMUM SIZE OF
DISCHARGE PIPE D2
FROM TUNDISH
MAXIMUM RESISTANCE
ALLOWED, EXPRESSED
AS A LENGTH OF
STRAIGHT PIPE (I.E. NO
ELBOWS OR BENDS
RESISTANCE CREATED BY
EACH ELBOW OR BEND
G 1/2
15mm
22mm
28mm
35mm
UP TO 9m
UP TO 18m
UP TO 27m
0.8m
1.0m
1.4m
G 3/4
22mm
28mm
35mm
42mm
UP TO 9m
UP TO 18m
UP TO 27m
1.0m
1.4m
1.7m
G 1
28mm
35mm
42mm
54mm
UP TO 9m
UP TO 18m
UP TO 27m
1.4m
1.7m
2.3m
Fixed grating
Discharge below
fixed grating
(Building Regulation
G3 section 3.61 gives
alternative points
of discharge)
Trapped
gully
Discharge pipe (D2) from tundish,
with continuous fall. See Building
Regulation G3 section 3.56,
Table 4 and worked example
300mm
minimum
600mm maximum
Metal discharge pipe (D1) from
Temperature relief valve to tundish
Tundish
Safety device
(e.g. Temperature
relief valve)
Figure 7 - D1 and D2 schematic
Worked example of discharge pipe sizing
The example on page 18 (G3 Guidance notes) is for a G
1
/
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 5:
Maximum resistance allowed for a straight length of 22mm copper discharge pipe (D2) from a G
1
/
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 G
1
/
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.