Page 486 - 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

Position of an automatic bypass valve
p Figure 7.47 Automatic bypass valve

gravity primary circulation as part of an upgrade to give some control over
secondary hot water temperature. The Domestic Heating Compliance Guide
states that:
For replacement systems where only the hot water cylinder is being
replaced and where hot water is on a gravity circulation system, a
thermo-mechanical cylinder thermostat should be installed as a
minimum provision.

This means that if the hot water cylinder only is being replaced and no control
over the hot water temperature exists, then a thermo-mechanical thermostat is
the minimum standard of hot water control required to comply with Document
L of the Building Regulations.
Thermo-mechanical thermostats work on the principle of thermal expansion of
a liquid or gas, in much the same way as thermostatic radiator valves except,
with this valve, it is the temperature of the water that is sensed by a remote
sensor. The sensor should be placed about a third of the way up from the
bottom of the cylinder.

Anti-gravity valves
Anti-gravity valves prevent unwanted gravity circulation to the upstairs
radiators on semi-gravity systems when only the hot water is being heated.
They are essential on all semi-gravity systems and especially in those systems
fuelled by solid fuel. Anti-gravity valves should be positioned on the vertical
flow to the upstairs heating circuit.

Anti-gravity valves are very similar in design to the single check valves mentioned
in earlier chapters. They allow water to flow in only one direction and, when
the heating system is off, they are in the closed position. In this position, gravity
circulation cannot take place. As soon as the central heating circulating pump
switches on, the flow of the water opens the valve to allow heating circulation.

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