Page 457 - 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

The Honeywell W-plan utilising a three-port motorised diverter valve

The W-plan is very similar to the Y-plan. The main difference is that the system
uses a three-port motorised diverter valve. This means that either the hot water
circuit or the heating circuit can be opened but not both circuits at the same
time. It is known as a hot water priority system. If both circuits are calling for
heat, the heating circuit will not open until the hot water circuit is satisfied.
The W-plan system is not recommended where a high hot water demand is
required as it would lead to the space heating temperature dropping below
comfort levels. The pipework layout is identical to that of the Y-plan.
The Honeywell S-plan utilising two two-port motorised zone valves
KEY TERM
The S-plan has two two-port motorised zone valves to control the primary and
heating circuits separately by the cylinder and room thermostats respectively. Zoning: a process where
This system is recommended for dwellings with a floor area greater than 150 m living spaces and sleeping
2
because it allows the installation of additional two-port zone valves to zone the spaces are individually
controlled via independent
upstairs heating circuit from the downstairs circuit. A separate room thermostat time clocks, room
and possibly a second time clock/programmer would also be required for thermostats and motorised
upstairs zoning. zone valves.

This system uses two two-port zone valves to control the flow of water to the
central heating circuit and the hot water circuit. They are controlled by a
Feed and cylinder thermostat and a room thermostat. Individual thermostatic radiator
expansion cistern
valves independently control the temperature of each room.
Automatic Cylinder Room
air valve thermostat thermostat
22 mm vent pipe
15 mm cold feed
Two-port
zone valve

Programmer

System
bypass Wiring
centre

Two-port
zone valve

22 mm flow and return pipes
p Figure 7.10 S-plan system

As with the Y-plan, a system bypass is required for overheat protection of the
boiler.
An outline of the operating sequence of the S-plan system

1 At a set time, the programmer activates the system calling for both hot
water and heating.

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