Page 456 - 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

An outline of the operating sequence of the Y-plan system

1 At a set time, the programmer activates the system calling for both hot
water and heating.
2 With the motorised valve in the mid-position, 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 valve is energised by the
cylinder thermostat, which closes the hot water port, preventing water
flowing to the hot water cylinder heat exchanger, or
b When the room reaches its set temperature, the valve is energised by the
room thermostat, which closes the heating port, preventing water flowing
to the heating circuit.

A B A B A B

In the mid-position, HEATING ONLY
the valve allows the With the ball shutting off CENTRAL HEATING ONLY
With the ball shutting off
water to circulate port B, water is allowed
AB around both AB to circulate around the AB port A, water is allowed
heating 'A' and hot central heating circuit to circulate around the
hot water circuit (port B)
water 'B' circuits (port A)
p Figure 7.7 The mid-position valve in the p Figure 7.8 The mid-position valve with the p Figure 7.9 The mid-position valve with the
mid-position serving heating and hot water hot water port closed heating port closed
KEY TERM 4 With both the room thermostat and the boiler thermostat satisfied, the pump

Boiler interlock: ‘locking and the boiler shut down and the valve returns to the mid-position. In this
out’ on the overheat high- condition, the system will only operate should either the room thermostat or
limit thermostat. cylinder thermostat call for heat. This is known as boiler interlock.

The system contains a system bypass fitted with an automatic bypass valve
that 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 temperatures. 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.
KEY POINT
Locking out on the overheat high-limit thermostat
Modern boilers contain two thermostats. The first controls the temperature
of the water inside the boiler and can be set by the user up to a maximum of
82°C. The second is for protection of the boiler and is known as the ‘high-
limit’ thermostat. Its job is to protect the boiler from overheating by shutting
it down or ‘locking out’ when a temperature of around 85°C is reached.
High-limit thermostats are manually resettable by pushing a small button on
the boiler itself.

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