Installation & Maintenance Instructions for an unvented
hot water cylinder with internal thermal expansion
Hot Water
Energy
Storage
Unvented Cylinder
SEPT 2018
9
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 have 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 i.e. 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 3 sizes larger, and so
on. Bends must be taken into account in calculating the
flow resistance. Refer to diagram 1, Table 1 and the worked
example. An alternative approach for sizing discharge
pipes would be to follow BS6700 Specification 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 come into contact with discharges,
a wire cage or similar guard is positioned to prevent contact
whilst maintaining visibility.
c. Discharges at a high level; e.g. in to 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.
WORKED EXAMPLE
The example below is for 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. Subtract the resistance for 4 No. 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 1
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
1m
35mm
Up to 27m
1.4m
