Page 384 - 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

The F and E cistern must be capable of accommodating an expansion of
4 per cent of the total amount of water contained in the primary system
and any heating system installed.

Unvented hot water storage systems

An unvented hot water storage system is simply a sealed system of pipework
and components that is supplied with water above atmospheric pressure. The
system does not require the use of a feed cistern. Instead, it is fed with water
direct from a water undertaker’s mains supply, or with water supplied by a
booster pump and a cold water accumulator if the mains pressure is low.
An unvented hot water system differs from open vented types because there is
no vent pipe. Expansion of water due to the water being heated is accommodated
KEY POINT in either an external expansion vessel or an expansion bubble within the storage
Unvented systems require cylinder.
safety discharge pipework The system also requires other mechanical safety devices for the safe control of
that must be correctly
sized and positioned in the expansion of water and to ensure that the water within the storage cylinder
accordance with Building does not exceed 100°C. There are two categories of centralised unvented hot
Regulations Document water storage systems:
G3. This is discussed
later in the chapter. 1 directly fired/heated storage systems
2 indirectly fired/heated storage systems.

The various types of unvented hot water system
There are three basic types of unvented hot water system. They are defined by
how the water is heated. These are:
1 indirect storage systems
2 direct storage systems:
● electrically heated
l gas or oil fired
3 small point of use (under sink).

Indirect storage systems
Indirect unvented hot water storage systems utilise an indirect unvented
hot water storage cylinder at the heart of the system. As with open vented
systems, the cylinder contains a coiled heat exchanger to transfer the heat
indirectly from the primary system to the secondary system. This can be done
in one of two ways:

1 by the use of a gas-fired condensing boiler
2 by the use of an oil-fired condensing boiler.

Older, non-condensing boilers may be used if the boiler is an existing appliance,
provided that the boiler contains both a control thermostat and a high energy
cut-out (high-limit) thermostat to limit the water temperature at the coil should
the control thermostat fail. On no account must solid fuel appliances and
boilers be used to provide heat to the coil. The primary hot water system may
either be an open vented or sealed system.

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