Page 388 - 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

Unvented hot water storage systems and pipework arrangements
Many installers claim that an unvented hot water storage system is the best
type of system for any domestic situation, but this is far from the case. There
are many factors that must be considered before this arrangement is installed
into a property:
● Available pressure and flow rate – this is probably the most important
factor, simply because poor pressure and flow rate will affect the operating
performance of the installation. Pressure and flow rate readings should be
taken at peak times to ensure adequate water supply before recommending
this type of system.
● The route of the discharge pipework, termination and discharge pipework size.
● The type of terminal fittings to be used. This is especially important when
retro-fitting unvented installations onto existing hot water systems as the
existing taps etc. may not be suitable.
● Cost – unvented systems tend to be very expensive.

The types of unvented hot water storage cylinder
There are two types of unvented hot water storage cylinder; both are
manufactured to BS EN 12897:2006 – Specification for Indirectly Heated
Unvented (Closed) Storage Water Heaters and Available as Direct Fired/
Heated or Indirectly Heated Vessels:
● unvented hot water storage cylinders using an external expansion vessel
● unvented hot water storage cylinders incorporating an internal expansion
air gap.
Most unvented cylinders are manufactured from high-grade duplex stainless
steel for strength and corrosion resistance. Some older cylinders may be
manufactured from copper or steel with a polyethylene or cementitious lining.

Isolation valve

In-line strainer
Hot water draw-off Expansion vessel
Temperature Pressure reducing valve
relief valve
Balanced cold connection
Check valve

Expansion (pressure)
Immersion heaters relief valve
D1 discharge pipework
Tundish
D2 discharge pipework
Cold feed

p Figure 6.21 A typical unvented cylinder with external expansion vessel

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