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Page 15

15 mm
discharge
pipe

Expansion
relief valve

Typical Discharge Pipe Arrangement

Dotted line showing
alternative route with
single tundish being used

600 mm max.

P & T

Relief

Valve

300 mm

min.

22 mm metal pipe with continuous
fall up to 9m equivalent length (D2).

NOTES

The discharge will consist of scalding
waterand steam. Asphalt, roofing felt
and non-metallic rainwater goods may
be damaged by such discharges.

It is not acceptable to discharge straight
into a soil pipe.

Discharge below
fixed grating

Fixed Grating

Trapped gully

Pressure & Temperature/expansion Relief
Valve Pipework

The relief valve should be installed to discharge
in accordance with G3 of the Approved
Document of the Building Regulations and
should be piped to where it is visible, but will
not cause danger to persons or damage to
materials.

The following information is taken from
Approved Document G3 of the Building
Regulations and is provided to assist with
the design and installation of the discharge
pipework. However, the information is not
exhaustive and reference should always be
made to Approved Document G3 of the Building
Regulations. The fi nal decision regarding any
arrangements rests with Building Control and
it is recommended that their advice is sought
if you have any concerns regarding this aspect
of the installation.

The two safety valves will only discharge
water under fault conditions. When operating
normally water will not be discharged.

The tundish should be vertical, located in the
same space as the unvented hot water storage
system and be fi tted as close as possible and
within 600mm of the safety device e.g. the
temperature relief valve.

The discharge pipe (D2) from the tundish should
terminate in a safe place where there is no risk
to persons in the vicinity of the discharge, be
of metal and:

a) 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 fl ow resistance. Refer to the table and
the worked example.

An alternative approach for sizing discharge
pipes would be to follow BS EN 806-2:2005
Specifi cation for design installation, testing
and maintenance of services supplying water
for domestic use within buildings and their
curtilages.

b) Have a vertical section of pipe at least

300mm long, below the tundish before any
elbows or bends in the pipe work.

c) Be installed with a continuous fall.

Worked Example

The example below is for G1/2 temperature relief valve with a discharge pipe (D2)
having 4 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: 9m subtract the resistance for
4 x 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 fi gure