Page 458 - 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

OPEN 2 Both of the two-port motorised zone valves open, and water from the boiler
circulates around both primary and heating circuits. The boiler fires up and
the circulating pump begins to circulate the water.
3 a or b:
a When the cylinder reaches temperature, the two-port zone valve
is energised by the cylinder thermostat, which closes the hot water
zone valve preventing water flowing to the hot water cylinder heat
p Figure 7.11 Zone valve open
exchanger, or
b When the room reaches its set temperature, the two-port zone valve
CLOSED is energised by the room thermostat, which closes the valve preventing
water flowing to the heating circuit.
4 With both the room thermostat and the boiler thermostat satisfied, the
pump and the boiler shut down. In this condition, the system will operate
only should either the room thermostat or cylinder thermostat call for heat.
This is known as ‘boiler interlock’.
p Figure 7.12 Zone valve closed
 Table 7.4 The comparisons between the Y-plan, W-plan and S-plan systems

Full thermostatic control Building Regulations Document L compliant Recommended for larger properties Can be used with sealed (pressurised) systems Can be used with system Can be zoned Anti-cycling boiler interlock

Y-plan system ü ü û ü ü boilers û ü

W-plan system ü ü û ü ü û ü

S-plan system ü ü ü ü ü ü ü

S-plan plus
system ü ü ü ü ü ü ü

The open vent, cold feed and circulating pump position for fully
pumped systems
The position of the open vent pipe, the cold feed pipe and the circulating pump
to a fully pumped system is an important part of the system design. If the open
vent pipe, the feed pipe and circulating pump are positioned onto the system
incorrectly, the system will not work properly and may even induce system
corrosion due to constant aeration of the system water.

The open vent and the cold feed should be positioned on the flow from the
boiler on the suction side of the circulating pump with a maximum of 150 mm
distance between them. This is called the neutral point, as the circulating pump

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