Oversink and Undersink Water Heater
- 11-
3.55
Any discharge should be visible at the
tundish
. In addition, where discharges from safety devices may not be
apparent, e.g. in dwelling occupied by people with impaired vision or mobility, consideration should be given to the
installation of a suitable safety device to warm when discharge takes place, e.g. electronically operated.
Discharge pipe D2
3.56
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 pipework
(see Diagram 1); and
b.
Be installed with a continuous fall of at least 1 in 200 thereafter.
3.57
The discharge pipe (D2) should be made of:
a.
Metal; or
b.
Other material that has been demonstrated to be capable of safely withstanding temperatures of the water
discharged and is clearly and permanently marked to identify the product and performance standard (e.g. as
specified in the relevant part of BS 7291-1:2006
Thermostatic pipes and fittings for hot and cold water for domestic
purposes and heating installations in buildings. General requirement
).
3.58
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. for discharge pipes between 9m and
18m the equivalent resistance length beat least two 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
diagram 1, table 1 and worked example.
Note
: an alternative approach for sizing discharge pipes would be to allow Annex D, section D.2 of BS 6700:2006
Specification for design, installation, testing and maintenance of devices supplying water for domestic use within buildings and
their curtilages.
Diagram 1 –
Typical discharge pipe arrangement
Table 1 –
Sizing of copper discharge pipe ‘D2’ for common temperature relief valve outlet sizes
Valve outlet size
Minimum size of discharge to
Tundish (D1)*
Minimum size of discharge pipe
from Tundish (D2)*
Maximum resistance allowed
expressed as a length of straight
pipe, i.e. elbows or bends
Resistance created by each
elbow or bend
G ½
15mm
22 mm
38 mm
35 mm
up to 9 m
up to 18 m
up to27 m
0.8 m
1.0 m
1.4 m
G ¾
22 mm
28 mm
35 mm
42 mm
up to 9 m
up to 18 m
up to27 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 to27 m
1.4 m
1.7 m
2.3 m
*See 3.49 and 3.56 and Diagram 1
Note
: The above table is based on copper tube. Plastic pipes may be of different bore and resistance.
Sizes and maximum lengths of plastic should be calculated using data prepared for the type of pipe being used.
