Page 459 - 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

acts on both the feed pipe and the open vent pipe with equal suction. If they
are any further apart, the neutral point becomes weak and the pump will act
on the feed pipe with a greater force than the open vent pipe. This creates an
imbalance, which leads to a lowering of the water in the feed and expansion
cistern. When the pump switches off, the water returns to its original position.
The constant see-sawing motion aerates the water creating corrosion within the
system.

150 mm

The neutral point

KEY POINT
What is aeration of the
water?
Central heating systems
do not like air. Air is one
p Figure 7.13 Position of the cold feed and open vent pipes of the biggest causes
of corrosion in heating
The circulating pump systems because the air
The circulating pump must also be positioned with care to avoid design faults in the water contributes
that could lead to problems with corrosion by aeration of the water due to to rust occurring
water movement in the feed and expansion cistern. This occurs when water throughout the system
is either pushed up the cold feed pipe and the open vent pipe or is circulated and the formation of
between the cold feed pipe and the open vent pipe. red oxide sludge. Water
alone will not cause
corrosion, even with
ferrous metals present,
such as radiators and
convectors. It is the air
present in the water
that causes metals to
System under positive
System under negative System under positive pressure. Correct rust and constant water
pressure. pressure. arrangement showing the
Pushing into the cistern Pumping over the vent position of the vent and feed movement at the feed
pipe behind the pump. This is and expansion cistern
known as the neutral zone will aerate the water in
p Figure 7.14 The position of the circulating pump the system enough for
corrosion to take place.

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