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DESIGN
Discharge Arrangements
The Pressure and Temperature Relief Valve (P&T) and Expansion Relief Valve (ERV) are
both provided with tundishes. It is normal for these to be connected into a single 22mm
discharge pipe but they can be run separately if required. At least the first 300mm of
pipework below the tundish should be vertical and not contain any elbows/bends
to ensure that if a fault occurs, the water does not back up and discharge from the
tundish. All elbows/bends should be large radius wherever possible.
The discharge from the P&T valve under a fault condition will be above 90°C. Because
of this, it is a requirement of Building Regulation Approved Document G3 that the
discharge from an unvented hot water storage system is conveyed to where it is visible
but will not cause danger to persons in or about the building. The discharge pipe from
the appliance tundish should be fitted in accordance with these requirements.
The discharge pipe MUST terminate in a SAFE and VISIBLE position. For a 22mm
discharge it must have an equivalent length of no more than 9 metres and it must
have a continuous fall (1:200 minimum) throughout its length. Above 9 metres
equivalent length, the pipe diameter must increase to meet the requirements of the
table shown opposite.
An example of how to calculate the size required is as follows:
A G½ temperature relief valve has a discharge pipe with 4 elbows and length of
7m from the tundish to the point of discharge. From the table opposite, maximum
resistance allowed for a straight length of 22mm copper discharge pipe from a G½
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 from a G½ temperature relief
valve equates to 18m. Subtract the resistance for 4 No. 28 elbows at 1.0m each = 4m.
Therefore the maximum permitted length equates to 14m. As the actual length is 7m,
a 28mm copper pipe will be satisfactory.
In apartment/flat situations the discharge pipes can be connected into a single pipe
which is discharged at low level. In this case the number should be limited to 6 to
allow the fault to be easily traced. The single pipe should be at least one size larger
than the largest individual discharge pipe connected to it.
The discharge can consist of scalding water and steam therefore the pipework should
be metal. The following locations for the discharge pipe are acceptable:
Low Level
•
Into a gully below the grating but above the water level (see diagram 1).
•
Onto the ground (drive, path or garden area). The pipe should discharge downwards
and be no more than 100mm above ground level. A wire cage should be provided
to prevent people coming into contact with scalding water (see diagram 2).
High Level
High level discharge is only acceptable if it is :
• onto a flat or pitched roof capable of withstanding high temperature water and
at least 3m away from plastic guttering.
or
•
into a metal hopper and down pipe which terminates at low level (as described above.)
Discharge into a soil or waste pipe (whether plastic or metal) is not acceptable.
The proposals for the discharge pipe/termination point should be discussed and
agreed with the Building Control Officer prior to commencing any work.
Further details are given in approved Document G3 of the Building Regulations.
Sizing of Discharge Pipe From Safety/
Temperature Relief Valve Tundish
Valve
outlet
size
G½
Page 24
Maximum
Min
resistance
Resistance
size of
allowed
created
discharge
shown as
by each
pipe from
straight
elbow or
tundish
pipe
bend
length
22mm
< 9m
0.8m
28mm
< 18m
1.0m
35mm
< 27m
1.4m
Diagram 1
Discharge into a gully
Diagram 2
Discharge onto the ground
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