Page 391 - 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

Unvented hot water storage systems require the installation of a discharge
pipework to safely convey any water that may be discharged as the result of a
defect or malfunction. Discharge pipework will be discussed later in this section.

The use of cold water accumulators in unvented hot water
systems
The use of cold water accumulators is becoming increasingly popular, especially
in areas where the water pressure is exceptionally low. Accumulators and
boosting pumps, as we saw in Unit 5, Cold water systems, offer a positive
solution to the problem of low water pressure and poor low flow rate by storing
water at night for use during the day. Both flow rate and pressure are critical
factors when fitting unvented hot water storage systems as these rely on a good
flow rate and pressure to provide a satisfactory operation. It should be borne in
mind, however, that boosting pumps that deliver more than 12 litres per minute
are not allowed under the Water Supply (Water Fittings) Regulations when the
cold water supply is being taken direct from a water undertaker’s mains supply.

The issue of poor mains supply
Water supply pressures have consistently diminished over the past 30 years. As
more and more homes, factories, offices and shops are built, the loading on the UK
water system has increased, with little or no upgrading of the water mains supply
network. The pipework that serves our towns and cities is now supplying more
properties than ever before, and this has resulted in a gradual degradation of both
pressure and flow rate. In some areas of the UK, the supply pressure can be as little
as 1 bar, which is unsatisfactory for an unvented hot water storage system.
Pressure of water takes two forms:

1 static pressure – this is the water pressure when no flow is occurring;
this is always greater than the dynamic pressure
2 dynamic pressure (also known as ‘running pressure’) – this is the water
pressure when outlets are open and water is flowing.

During periods of peak use, both static and dynamic pressures will decrease.
If, during this time, a property has a static pressure of, say, 2 bar, then the
dynamic pressure could drop to below 1 bar. At off-peak times, say, during the
night, this could rise significantly, to 3 bar static and 2 bar dynamic, simply
because less water is being used in the surrounding area. An accumulator
would take advantage of the night-time rise in pressure to replenish its storage
capacity while the mains pressure is at its highest. With the accumulator fully
replenished, a good pressure and flow rate would be available throughout the
day, provided that the accumulator has been sized correctly.
Figure 6.25 shows a typical unvented hot water storage system with an
accumulator installed to increase both the pressure and the flow rate. An
important factor here is the use of two pressure reducing valves (PRVs). The
first PRV regulates the pressure entering the property so that any pressure
fluctuations can be controlled to a predetermined pressure at night when the
accumulator is replenishing. The second PRV reduces the pressure to that of
the UHWSS manufacturer’s recommendations.

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