Page 407 - 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

2 Overheat thermostat (thermal cut-out 90°C maximum but more usually
factory set at between 85°C and 89°C): again, this can take two forms:
l with direct heated systems, it is incorporated into the immersion heater
thermostat
l with indirectly heated systems, it is a separate component factory
wired into the vessel and designed to operate the motorised valve at the
primary hot water coil.
3 Temperature/pressure relief valve (95°C): a standard component used
on most vessels that is designed to discharge water when the temperature
exceeds 95°C. Most types have a secondary pressure relief function.

Functional controls

The functional controls of an unvented hot water storage system are designed
to protect the water supply.

● To avoid contamination, the storage cylinder or vessel must be of an
approved material, such as copper or duplex stainless steel, or have an
appropriate lining that will not cause corrosion or contamination of the
water contained within it. Where necessary it must be protected by a
sacrificial anode.
● A single check valve (often referred to as a non-return valve) must be fitted
to the cold water inlet to prevent hot or warm water from entering the
water undertaker’s mains supply.
● A means of accommodating and containing the increase in volume of water
due to expansion must be installed. This can either be by the use of an
externally fitted expansion vessel or via an integral air bubble.
● An expansion valve (also known as a pressure relief valve) must be installed,
and should be designed to operate should a malfunction occur with either
the pressure reducing valve or the means of accommodating the expanded
water. The expansion valve must be manufactured to BS EN 1491:2000 –
Building Valves. Expansion Valves. Tests and Requirements.

The Water Supply (Water Fittings) Regulations also state that:
Water supply systems shall be capable of being drained down and fitted with
an adequate number of servicing valves and drain taps so as to minimise the
discharge of water when water fittings are maintained or replaced.

To comply with this requirement, a servicing valve should be fitted on the cold
supply close to the storage vessel, but before any other control. The valve may be
a full-bore spherical plug, lever action-type isolation valve or a screw-down stop
valve to BS 1010. Any drain valves fitted should be manufactured to BS 2879
and be ‘type A’ drain valves with a locking nut and an ‘O’ ring seal on the spindle.
The functional controls of an unvented hot water storage system are listed below.
We will look at each one in turn and identify its position within the system.

395

9781510416482.indb 395 29/03/19 9:02 PM