Page 498 - 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

l Three-port diverter valve: very similar in appearance to the three-port mid-
position valve, this valve is designed to control the flow of water on fully pumped
central heating/hot water systems, where hot water priority is required.
l Three-port mid-position valve: used on fully pumped central heating/hot
water systems to provide full temperature control of both the hot water and
heating circuits when linked to cylinder and room thermostats. The circuits
can operate together or independently of each other.
l Two-port motorised zone valve: these can be found on both C-plan plus
systems, where a single valve linked to a cylinder thermostat controls the hot
water temperature, and S-plan fully pumped systems where two two-port
zone valves control the heating and hot water circuits via room and cylinder
thermostats. They can also be used to zone different parts of the heating circuit.

Advanced controls: weather compensation, optimum
start and delayed start
Domestic central heating systems can benefit from more advanced controls,
especially when a condensing boiler is fitted. Condensing boilers respond
better to lower flow and return temperatures than non-condensing appliances.
Advanced controls enhance system efficiency and include:
l weather compensation controls
l delayed start controls
l optimum start controls.

Weather compensation controls
This type of control uses an externally fitted temperature sensor fitted on a north-
or north-east-facing wall so as not to be in the direct path of solar radiation. As
the external temperature rises, the weather compensator reduces the circulation
temperature of the flow from the boiler to compensate for the warmer outside
temperature. Similarly, the reverse occurs if the weather gets colder.

External
temperature (°C)
20
Warmer
16

12
Cooler Compensation
8 slope

4

0

−4
0 20 40 60 80 100
Boiler flow temperature (°C)
p Figure 7.57 A weather compensation graph

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