Page 385 - 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

Balanced cold Isolation valve
Hot water draw-off water connection
In-line strainer

Pressure reducing valve

Internal expansion Check valve
pocket or air bubbles
Expansion (pressure)
relief valve
Flow Temperature relief valve

Heat exchanger D1 discharge pipework
Immersion heater Tundish

D2 discharge pipework
Return
Cold feed

p Figure 6.16 Indirect-type unvented hot water storage cylinder with internal expansion

An immersion heater provides back-up hot water heating for use during the
summer or for when the boiler malfunctions.

Direct storage systems
The direct system uses a direct-type unvented hot water cylinder that does
not contain any form of heat exchanger. There are two very different types,
as described below.
● Electrically heated: this type of cylinder does not contain a heat exchanger.
Instead, the water is heated directly by two immersion heaters controlled
by a time switch. One immersion heater is located close to the bottom of
the cylinder to heat all of the contents of the cylinder at night and another
located in the top third to top up the hot water during the day if required
via a one-hour boost button on the time switch. Both immersion heaters
are independently controlled and cannot be used simultaneously. The
immersion heaters are manufactured to BS EN 60335–2–73 and must
contain a user thermostat usually set to 60°C and a non-resetting thermal
cut-out (high limit stat).
● Gas or oil fired: the design of these water heaters originated in North
America. They consist of a hot water storage vessel with a flue pipe that
passes through the centre. Expansion of the water is catered for by the use
of an external expansion vessel. Below the storage vessel is a burner to heat
the water; this can be fuelled by either gas or oil, depending on the type.
The burner is controlled by a thermostat and a gas/oil valve. An energy
cut-out prevents the water exceeding the maximum of 90°C. The safety
and functional controls and components layout is almost identical to other
unvented hot water storage systems.

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