Page 515 - 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 open vent and the cold feed should be positioned on the flow from the
boiler on the suction side of the circulating pump with a maximum of 150 mm
distance between them. This is called the neutral point, as the circulating pump
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.

The feed and expansion 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 m 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.

Positioning the circulating pump

The circulating pump must also be positioned with care to avoid design faults
that could lead to problems with corrosion by aeration of the water due to
water movement in the feed and expansion cistern. This occurs when water
is either pushed up the cold feed pipe and the open vent pipe, or is circulated
between the cold feed pipe and the open vent pipe.

System under negative System under positive System under positive pressure.
pressure. pressure. Correct arrangement showing
Pushing into the cistern Pumping over the vent the position of the vent and feed
pipe behind the pump. This is known
as the neutral zone
p Figure 7.74 The position of the circulating pump

Sealed systems: positioning the expansion vessel,
pressure gauge and filling loop

The expansion vessel is installed onto the return because the return water is
generally 20°C cooler than the flow water and this does not place as much
temperature stress on the expansion vessel’s internal diaphragm as the hotter
flow water. If installing the vessel on the flow is unavoidable, it should be
placed on the suction side of the circulating pump in the same way as the cold
feed and open vent pipe on the open vented system. Close to the expansion
vessel is the pressure relief valve (sometimes called the expansion valve) and a
pressure gauge.

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