Page 406 - 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

5

4

3
Bar gauge

2

1

0

100 110 120 130 140 150 160

Degrees Celsius 120.2 133.5 143.6 151.8 158.8
p Figure 6.41 Boiling point/pressure relationship

On the graph it can be seen that at the relatively low pressure of 1 bar the
boiling point of the water has risen to 120.2°C! If a sudden loss of pressure at
the hot water storage vessel were to occur due to vessel fracture, at 120.2°C
the entire contents of the cylinder would instantly flash to steam with explosive
results, causing structural damage to the property. Calculating how much steam
would be produced illustrates the point further.
1 cm of water creates 1600 cm of steam; if the storage vessel contains 200
3
3
litres of water and each litre of water contains 1000 cm , then the amount of
3
steam produced would be 200 × 1000 × 1600 = 320,000,000 cm of steam!
3
The Building Regulations Approved Document G3 states that unvented hot
water storage systems must have a three-tier level of safety built in to the
system. This takes the form of three components that are fitted to the storage
vessel. The aim of these components is to ensure that the water within the
system never exceeds 100°C. These components are:

1 Control thermostat (set to 60°C to 65°C): this can take two forms
depending on the type of storage vessel:
● with direct heated vessels, this is the immersion heater user thermostat
l with indirectly heated vessels, it is the cylinder thermostat wired to the
central heating wiring centre. Indirectly fired systems are also controlled,
in part, by the boiler thermostat (82°C maximum setting) and the boiler
high limit stat, designed to operate at typically 90°C.

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