8
400
Discharge pipework
1) The tundish must be installed directly below the discharge outlet connection of the
boiler. The tundish must also be in a position visible to the occupants, and positioned
away from any electrical devices. The discharge pipe from the tundish should termi-
nate in a safe place where there is no risk to persons in the vicinity of the discharge
and to be of metal.
2) Discharge pipes from the temperature & pressure relief are expansion relief valve are
joined together within the appliance.
3) The pipe diameter must be at least one pipe size larger than the nominal outlet size of
the safety device unless it's 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 2 sizes larger than the nominal outlet size of the safety device. Between 18m
and 27m at least 3 sizes larger, and so on.
Bends must be taken into account in calculating the flow resistance.
See fig. 2.4.1 and Table 1.
4) The discharge pipe must have a vertical section of pipe at least 300m in length, below
the tundish before any elbows or bends in the pipework.
5) The discharge pipe must be installed with a continuous fall.
6) The discharge must be visible at both the tundish and the final point of discharge, but
where this is not possible or practically difficult; there should be clear visibility at one
or other of these locations. Examples of acceptance are:
i) Ideally below a fixed grating and above the water seal in a trapped gully.
ii) Downward discharges at a low level; i.e. up to 100mm above external surfaces such
as car parks, hard standings, grassed areas etc. These are acceptable providing that
where children may play or otherwise come into contact with discharges, a wire cage or
similar guard is positioned to prevent contact, whilst maintaining visibility.
iii) Discharges at high level; I.e. into a metal hopper and metal down pipe with the end
of the discharge pipe clearly visible (tundish visible or not). Or onto a roof capable of
withstanding high temperature discharges of water 3m from any plastic guttering sys-
tems that would take such a discharge (tundish visible).
iv) Where a single pipe serves a number of discharges, such as in blocks of flats, the
number served should be limited to not more than 6 systems so that any installation
can be traced reasonably easily. The single common discharge pipe should be at
least one pipe size large than the largest individual discharge pipe to be connected. If
unvented hot water storage systems are installed where discharges from safety
devices may not be apparent I.e. in dwellings occupied by the blind, infirm or disabled
people, consideration should be given to the installation of an electronically operated
device to warn when discharge takes place.
Note: The discharge will consist of scalding water and steam. Asphalt, roofing felt
and non-metallic rainwater goods may be damaged by such discharges.
Warningr
The outlet from the temperature & pressure relief valve must not be used for any other
purpose.
The temperature & pressure relief valve must not be removed in any circumstances.
Any of the above will totally invalidate the warranty.
Note
The discharge from the central heating and domestic hot water systems may be
joined together after the tundish.
500 mm Max.
300 mm
Min.
Temperature & pressure
relief valve
Metal discharge pipe (D1) from
temperature & pressure relief valve.
to tundish.
Metal discharge pipe (D2) from tundish
with continuous fall. See Table 2 and worked
example.
Tundish
Fixed grating
Trapped gulley
Discharge below
fixed grating.
(see page 6 for
alternative points
of discharge).
Table 1. Sizing of copper discharge pipe
“D2” for common temperature
valve outlet sizes.
WORKED EXAMPLE
The example below is for a G 1/2 Temperature & Pressure Relief valve with a dis-
charge 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 G 1/2 T & P valve is 9m.
Subtract the resistance for 4 no. 22mm elbows at 0.8m each = 3.2m.
Therefore the maximum permitted length equates to: 5.8m.
As 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 G 1/2 T & P
valve equates to: 18m.
Subtract the resistance for 4 no. 28mm elbow
at 1.0m each = 4m.
Therefore the maximum permitted length
equates to: 14m
As the actual length is 7m, a 28mm (D2) cop-
per pipe will be satisfactory.
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 pipe (i.e.
no elbow or bends)
Resistance created
by each elbow or
bend
G 1/2
15 mm
22 mm
28 mm
35 mm
Up to 9 m
Up to 18 m
Up to 27 m
0.8 m
1.0 m
1.4 m
G 3/4
22 mm
28 mm
35 mm
42 mm
Up to 9 m
Up to 18 m
Up to 27 m
1.0 m
1.4 m
1.7 m
G 1
28 mm
35 mm
42 mm
54 mm
Up to 9 m
Up to 18 m
Up to 27 m
1.4 m
1.7 m
2.3 m
Fig. 2.4.1
SH005A
TB004A
