Page 408 - 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

In-line strainer
The in-line strainer is basically a filter designed to prevent any solid matter
within the water from entering and fouling the pressure reducing valve and any
other mechanical components sited downstream. In modern storage systems,
this is incorporated into the composite valve, which will be discussed later in
this section.
Pressure reducing valve
Pressure reducing valves (PRVs) were looked at in detail in Chapter 5, Cold water
systems; however, they are of sufficient importance to warrant an explanation
here too.
The PRV of an unvented hot water storage system reduces the pressure of the
incoming water supply to the operating pressure of the system. In all cases this
will be set by the manufacturer and sealed at the factory. The outlet pressure
will remain constant even during periods of fluctuating pressures. Should the
pressure of the water supply drop below that of the operating pressure of the
PRV, it will remain fully open to allow the available pressure to be used.

Replacement internal cartridges are available and easily fitted without changing
the valve body should a malfunction occur.

Modern PRVs for unvented hot water storage systems are supplied with a
balanced cold connection already fitted.
Single check valve
The single check valve (also known as a non-return valve) is fitted to prevent
hot water from back-flowing from the hot water storage vessel, causing possible
fluid category 2 contamination of the cold water supply. The single check valve
also ensures that the expansion of water when it is heated is taken up within
the system’s expansion components or expansion bubble. Single check valves
are classified as either type EA or EB backflow prevention devices.

In most cases, the check valve will be part of the composite valve, to be
discussed later in this section.

Expansion device (vessel or integral to cylinder)
Water expands when heated. Between 4°C and 100°C it will expand by
KEY POINT approximately 4 per cent. Therefore, 100 litres of water at 4°C becomes 104

Backflow prevention litres at 100°C. It is this expansion of water that must be accommodated in an
devices were discussed unvented hot water storage system. This can be achieved in one of two ways:
in detail in Chapter 5, 1 by the use of an externally fitted expansion vessel, or
Cold water systems. 2 by the use of a purpose-designed internal expansion space or ‘expansion

bubble’.
Expansion vessels
An expansion vessel is a cylindrical-shaped vessel that is used to accommodate
the thermal expansion of water to protect the system from excessive pressures.
It is installed as close to the storage vessel as possible and preferably higher.
There are two basic types.

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