Page 460 - 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

The circulating pump, or to give it its correct name the ‘hydronic central heating
circulator’, is a simple electric motor with a fluted water wheel-like impeller
that circulates the water around the system by centrifugal force. The faster the
impeller rotates, the greater the circulation that occurs in the system. For quiet
operation of the system, the flow rate should not exceed 1 litre per second and
1.5 litres per second for microbore systems (see page 452 of this chapter). Most
domestic circulating pumps have three speeds, which correspond to varying
circulatory pressures or ‘heads’. Domestic circulating pumps have either a 6 m
head to circulate up to a height of 6 m, or a 10 m head to circulate up to
a height of 10 m.

The use of air separators
The use of an air separator helps in the positioning of the feed and vent by
ensuring that the neutral point is built in to the system. The positioning
of the pipework on an air separator creates a turbulent water flow in
the separator body and this helps to remove air from the system, which
makes the system quieter in operation and significantly reduces the risk of
corrosion.
Air separator fitted to
ensure correct coupling The feed and expansion cistern
of the cold feed and Open vented systems contain a feed and expansion cistern, which fulfils
vent pipes
three important functions:
1 it is the means by which water enters the system for filling and top-up
2 it allows space for the system water to expand into when it is heated
3 it provides a static head (or water pressure) to the system.
Generally, the size of the F and E cistern will depend on the size of
the system, but for most domestic systems an 18-litre cistern is
recommended. The bigger the system, the more water it will contain

p Figure 7.15 The use of an air separator and so the water expansion will be greater. The water level in the cistern
should, therefore, be set at a low level.
The cistern must be located at the highest part of the system and must not be
affected by the operation of the circulating pump. For fully pumped systems,
the cistern must be at least 1 metre above the highest part of the pumped
primary flow to the heat exchanger in the hot water storage cylinder. For
gravity systems, the minimum height of the cistern can be calculated by taking
the maximum operating head of the pump and dividing it by 3.
The cold feed for the system for most domestic properties is 15 mm. The cold
feed pipe should not contain any service or isolation valves. This is to ensure
that there is a supply of cold water in the event of overheating and leakage,
p Figure 7.16 An air separator preventing the system from boiling.

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