Page 373 - 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

Hot water draw-off connection 1" male thread KEY TERM
Immersion heater connection Immersion heater: an
electrical element that
sits in a body of water,
just like in a kettle. When
switched on, the electrical
current causes the electrical
element to heat up, which
in turn heats up the water.
Most immersion heaters
elements are rated at 3 kW
Primary flow connection but cylinders can have
1" male thread
1, 2, 3 or 4 elements. All
immersion heaters must
comply with BS EN 60335–
2–73 and have a resettable
double thermostat (RDT)
as standard. This enables
problems with overheating
Primary return connection to be recognised quickly.
1" male thread

Cold feed connection
1" male thread
p Figure 6.4 An indirect cylinder

In a double feed indirect system, two cisterns are used: a large cistern for the
domestic hot water and a smaller one for the heating. It is now general practice
to install indirect cylinders in preference to direct types, even if the indirect
flow and return are capped off.

The double feed indirect hot water storage cylinder allows the use of boilers
and central heating systems that contain a variety of metals, such as steel and
aluminium because the water in the cylinder is totally separate from the water
in the heat exchanger. This means that there is no risk of dirty or rusty water
being drawn off at the taps. The system is designed in such a way that the water
in the boiler and primary pipework is hardly ever changed, the only loss of water KEY TERM
being in the feed and expansion cistern through evaporation. Heat exchanger: a device
or vessel that allows heat
The secondary water is that which is drawn from the hot water storage cylinder to be transferred from one
to supply the hot taps. It is heated by conduction as the water in the cylinder is water system to another
in contact with the heat exchanger. without the two water
systems being allowed to
A feed and expansion cistern feeds the primary part of the system, and this must
be large enough to accommodate the expansion of the water in the system when come into contact with each
other. The transfer of heat
it is heated. The vent pipe from the primary system must terminate over the feed between the two systems
and expansion cistern. An alternative method would be to use a sealed heating takes place via conduction
system, which is fed with water from the cold water main via a filling loop. (see Chapter 3, Scientific
Expansion of water is accommodated in an expansion vessel. principles).

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