Page 449 - 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

By far the most popular heating system in the UK is the ‘wet’ system, whether
supplying radiators, convectors or underfloor heating, but while wet systems have
enjoyed the monopoly thus far, other systems, such as electric storage heaters
and warm air, continue to be available. In some areas of the UK, district heating,
supplied from a central source and serving many properties, is also commonplace.
Here, we will look at the following central heating options that are available for
today’s homeowner, their layouts and operating principles:
l wet central heating
l warm air systems
l electric storage heaters
l district heating installations.
Types of system

Wet central heating
Domestic wet central heating systems fall into two different categories, based
upon the way the system is filled with water and the pressure at which it operates:
1 Low pressure, open vented central heating systems, fed from a feed and
expansion cistern in the roof space. These can be both modern fully pumped
systems and existing gravity hot water/pumped heating installations.
2 Sealed, pressurised central heating systems, fed direct from the mains cold
water supply and incorporating an expansion vessel to take up the expansion KEY TERMS
of water due to the water being heated. These are generally more modern
fully pumped and combination (‘combi’) boiler systems. Combination (‘combi’) boiler:
a boiler that provides
The water in low pressure open vented central heating systems is kept below central heating and
100°C. For existing systems the flow water from the boiler is usually about instantaneous hot water.
80°C and the return water temperature is usually 12°C to 15°C lower. Gravity circulation:
circulation that occurs
Circulation of the water can be either by: because heat rises through
the water. No pump is
l gravity circulation to the heat exchanger in the hot water cylinder and required.
pumped heating to the heat emitters, or Fully pumped system: a
l by means of a fully pumped system where both the hot water heat heating system that uses
exchanger and heat emitters are heated using a circulating pump. pumped circulation to
both heating and hot water
Fully pumped systems have the advantage that system resistance created by circuits.
the pipework, fittings and heat emitters can be overcome much more easily and
this enables the system to heat up faster, giving the occupants a much more
controllable system.
Sealed heating systems operate at a higher pressure, with modern systems
incorporating condensing boilers operating at a slightly lower temperature of
65°C for the flow temperature with a return temperature 20°C lower at 45°C.
In both cases the difference between the flow and return temperatures is the
amount of heat lost to the heated areas.
Figure 7.1 illustrates the development of central heating, from the open vented
one-pipe system through to the more modern sealed combination boiler
systems and fully pumped systems using system boilers.
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