Page 402 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

Time clocks for secondary circulation

If secondary circulation is used on hot water systems, it should be controlled by
a time clock so that the circulating pump is not running 24 hours a day. The time
clock should be set to operate only during periods of demand, and should be
wired in conjunction with pipe thermostats (also known as aquastats) to switch
off the pump when the system is up to the correct temperature and circulation
is not required and to activate the pump when the water temperature drops.

Insulating secondary circulation pipework
If secondary circulation systems are installed, they should be insulated for the
entire length of the system. This is to prevent excessive heat loss through the
extended pipework due to the water being circulated by a circulating pump. The
insulation should be thick enough to maintain the heat loss below the values
shown in Table 6.11.

 Table 6.11 Insulation thickness for secondary circulation pipework

Tube/pipe size Maximum heat loss per metre
15 mm pipe 7.89 w/m
22 mm pipe 9.12 w/m
28 mm pipe 10.07 w/m

Secondary circulation on large open vented hot water
storage systems
Figure 6.38 shows a large domestic hot water system with secondary
circulation. As can be seen, there are some significant differences from other
secondary circulation systems, as discussed below.
● The hot water vessel includes a shunt pump. This is to circulate the water
within the cylinder to ensure that the varying temperature (stratification)
of the water inside is kept to a minimum, and to ensure an even heat
distribution throughout, thereby preventing the growth of Legionella
bacteria. Stratification is desirable during the day so that the draw-off water
is maintained at its hottest for the longest period of time. Because of this, the
shunt pump should operate only during periods of low demand, i.e. at night.
● The secondary circulation pump (component 5 on the drawing) is installed
on the secondary flow and not the secondary return as with other, smaller
systems.
l A non-return valve (component 6 on the drawing) is installed on the
secondary flow to ensure that reverse circulation does not occur.
● A cylinder thermostat (component 3 on the drawing) is provided to
maintain the temperature within the cylinder at a maximum of 60°C.
● A pipe stat (component 2 on the drawing) installed on the secondary flow
maintains the temperature at a minimum of at least 50°C.
l A motorised valve (component 4 on the drawing) is installed on the
secondary return close to the hot water storage vessel, to prevent water
being drawn from the secondary return when the pump is not operating.

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