Page 485 - 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 7 Central heating systems

Automatic air valves
Automatic air valves are fitted where air is expected to collect in the system,
usually at high points. They allow the collected air to escape from the system
but seal themselves when water arrives at the valve.
When water reaches the valve, the float arm raises, closing the valve. As more
air reaches the valve, the float momentarily drops, allowing the air out of the
system. These valves are often used with a check valve that prevents air from
being drawn into the system backwards through the valve.

Air outlet

Air cap
Rubber
sealing
washer
Float

p Figure 7.46 Automatic air valve

Automatic bypass valves
The automatic bypass valve controls the flow of water across the flow and
return circuit of fully pumped heating systems by opening automatically as
other paths for the water close, such as circuits with motorised valves and
radiator circuits with thermostatic radiator valves. This occurs as the hot water
circuit and heating circuit/thermostatic radiator valves begin to reach their full
temperature. As the circuits close, the bypass will gradually open, maintaining
circulation through the boiler and reducing noise in the system due to water
velocity. Most boiler manufacturers require a bypass to be fitted to maintain a
minimum flow rate through the boiler, to prevent overheating.

Automatic bypass valves are much better than fixed bypass valves, as these,
being permanently open, take the flow of hot water away from the critical parts
of the system, which increases the heating time for both hot water and heating
circuits. This reduces the efficiency of the system and increases fuel usage.

Thermo-mechanical cylinder control valves
Thermo-mechanical cylinder control valves are non-electrical valves used to
control the temperature of a hot water cylinder. They are mainly used with

473

9781510416482_Ch7.indd 473 29/03/19 9:41 PM