APR 2018
9
Hot Water Energy Storage
Unvented Cylinders
GB & NI
Installation & Maintenance Instructions for an unvented
hot water cylinder with external thermal expansion
The two safety valves will only discharge water under fault
conditions. When operating normally water will not be
discharged. The tundish should be located in the same space
as the unvented hot water storage system and be fitted as
close as possible to, and lower than, the safety device, with
no more than 600mm of pipe between the valve outlet and
the tundish. The tundish should be positioned away from
electrical devices.
Any discharge should be visible at the tundish. The tundish
should be located such that any discharge is visible. In
addition, where discharges from safety devices may not
be apparent, extra consideration should be given, e.g. for
people with impaired vision or mobility. This could be via
the installation of a suitable electronically operated or other
safety device to warn when discharge takes place.
The discharge pipe (D2) from the tundish should:
A) Have a vertical section of pipe at least 300mm long,
below the tundish before any elbows or bends in the
pipework.
B) Be installed with a continuous fall of at least 1 in 200
thereafter.
The discharge pipe (D2) from the tundish should be of metal
or other material that has been demonstrated to be capable
of withstanding temperatures of the water discharged.
The discharge pipe (D2) should be at least one pipe size
larger than the nominal outlet size of the safety device,
unless its total equivalent hydraulic resistance exceeds
that of a straight pipe 9m long. Therefore, discharge pipes
between 9m and 18m equivalent resistance length should be
at least two sizes larger than the nominal outlet size of the
safety device; between 18 and 27m at least three sizes larger.
Bends must be taken into account in calculating the flow
resistance. Refer to the diagram, Table 2 and the
worked example.
An alternative approach for sizing discharge pipes would be
to follow BS EN 806:2 specifications for design, installation,
testing and maintenance of services supplying water for
domestic use within buildings and their curtilages.
The discharge pipe (D2) should terminate in a safe place
where there is no risk to persons in the vicinity of the
discharge. Examples of acceptable discharge arrangements
are:
A) To a trapped gully with the end of the pipe below the fixed
grating and above the water seal.
B) Downward discharges at a low level; i.e. up to 100mm
above external surfaces such as car parks, hard standings,
grassed areas etc. are acceptable, providing that – where
children play or otherwise could come into contact with
discharges – a visible wire cage or similar guard is positioned
to prevent contact.
C) Discharges at a high level; e.g. into a metal hopper
and metal down pipe with the end of the discharge pipe
clearly visible; or onto a roof capable of withstanding high
temperature discharges of water and 3m from any plastic
guttering systems that would collect such discharges.
D) Device to warn when discharge takes place.
Discharge worked example
The example below is for G1/2 temperature relief valve with
a discharge pipe (D2) having four elbows and a length of 7m
from the tundish to the point of discharge.
Maximum resistance allowed for a straight length of 22mm
copper discharge pipe (D2) from a G1/2 temperature relief
valve is: 9.0m.
Subtract the resistance for four 22mm elbows at 0.8m each
= 3.2m.
Therefore the maximum 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:
14m.
As the actual length is 7m, a 28mm (D2) copper pipe will be
satisfactory.
Table 2:
Sizing of copper discharge pipe ‘D2’ for a
temperature relief valve with a G1/2 outlet size (as supplied).
Size of
discharge
pipework
Maximum length
of straight pipe
(no bends or
elbows)
Deduct the figure
below from the
maximum length for
each bend or elbow in
the discharge pipe
22mm
Up to 9m
0.8m
28mm
Up to 18m
1.0m
35mm
Up to 27m
1.4m
INSTALLATION INSTRUCTIONS (CONTINUED)
