Page 401 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 6 Hot water systems

Secondary circulation installations on open vented hot
water storage systems

With secondary circulation on open vented systems, the return pipe runs from
the furthest hot tap back to the cylinder, where it enters at about a quarter of
the way down. A circulating pump is placed on the return, close to the hot water
cylinder, pumping into the vessel. As with all secondary circulation systems, the
pump must be made from bronze or stainless steel to ensure that corrosion does
not pose a problem. Isolation valves must be installed either side of the pump so
that the pump may be replaced or repaired. The system is shown in Figure 6.36.

Open vent pipe
Cold feed pipe

22 mm or 28 mm full-way gate or
lever-type spherical ball valve

Secondary return
1 / 4 of the way down
the cylinder

Bronze
pump

Secondary return connection at the furthest appliance
p Figure 6.36 Secondary circulation on an open vented hot water storage installation

Some open vented cylinders can be purchased with a secondary return
connection already installed on the cylinder. Alternatively, an Essex flange
(Figure 6.37) can be used on cylinders where no connection exists. p Figure 6.37 An Essex flange

Preventing reversed circulation in secondary
circulation systems
The secondary flow (the hot water draw-off), as we have already seen, should
have a temperature of at least 60°C. The secondary return of the secondary
circulation circuit should have a return temperature of 50°C when it reaches the
cylinder at the end of the circuit. In this way, the hottest part of the cylinder
will always be the top, where the hot water is drawn off. If reversed circulation
were to occur, the water in the cylinder would never reach the disinfecting
temperature of 60°C and so would always be at risk of a Legionella outbreak,
however remote.

By installing a single check valve on the return, and positioning it between the
pump and the cylinder, reverse circulation is prevented.

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