Page 463 - 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

Fully pumped systems with three-port mid-position valve
(known as the Y-plan) or a three-port diverter valve (known as
the W-plan)
The three-port mid-position valve (Y-plan) or diverter valve (W-plan) controls
the flow of water to the primary (cylinder) circuit and the heating circuit.
The valve reacts to the demands of the cylinder thermostat or the room
thermostat. This was discussed in detail earlier in the chapter (page 443).

Thermostat

Pressure gauge

Wiring centre Programmer
Temporary filling loop
with double check valve
arrangement

Pressure relief valve
and discharge pipework Expansion vessel
p Figure 7.19 The sealed Y-plan system

The system contains a system bypass fitted with an automatic bypass valve,
which simply connects the flow pipe to the return pipe. The bypass is required
when all circuits are closed either by the motorised valve or the thermostatic
radiator valves as the rooms reach their desired temperature. The bypass valve
opens automatically as the circuits close to protect the boiler from overheating
by allowing water to circulate through the boiler, keeping the boiler below its
maximum high temperature. This prevents the boiler from ‘locking out’ on the
overheat energy cut-out.

Sealed system components
As we have already seen, sealed systems do not contain a feed and expansion
cistern, nor open vent pipe. Instead, these systems incorporate the following
components:

l an external expansion vessel fitted to the system return
l a pressure relief valve
l the system is filled via a temporary filling loop or a CA disconnection device
l a pressure gauge.
These will be covered later in this chapter.

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