Page 453 - 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

An improvement on the one-pipe system, the general layout of the two-pipe
heating circuit is still used in modern systems. Heating temperature is
Feed and expansion cistern controlled by a room thermostat but water temperature is controlled by
the boiler thermostat.

22 mm vent pipe
Room thermostat
controlling the pump

15 mm cold Programmer
feed pipe

22 mm flow and return
Boiler

28 mm gravity
primaries

p Figure 7.4 Two-pipe semi-gravity system

One of the biggest problems with older central heating systems was the lack KEY POINT
of temperature control on both the hot water and heating circuits, which
meant that the hot water and the radiators became as hot as the water in the The two-pipe semi-gravity
system is no longer
boiler. The C-plan went some way towards addressing this problem with the installed as it does not
inclusion of a room thermostat that simply switched off the pump when the comply with Building
desired room temperature was reached. The secondary water, however, was still Regulations Document L.
uncontrolled and was often too hot. Systems of this type
must be updated to
include full thermostatic
 Table 7.2 Advantages and disadvantages of the C-plan (two-pipe) system control over both hot
water temperature and
Advantages Disadvantages room temperatures by
All of the heat emitters reach Uncontrolled heating of the primary circuit leading to the inclusion of separate
the same temperature overheating of the domestic secondary hot water controls. The updated
Constant boiler cycling even when the hot water and heating system is known as the
are up to temperature leads to wastage of fuel energy C-plan plus.
The two-pipe system is much The system is not Building Regulations Document L
quicker at heating up than the compliant and must be updated to C-plan plus system as a KEY TERM
one-pipe system; this saves minimum standard
on fuel usage Semi-gravity system: a
The boilers fitted to this type of system are only about 78%
efficient or less central heating system
that has pumped heating
Condensing-type boilers cannot be fitted to this type of
installation because of the gravity circulation needed by the circulation but gravity hot
hot water storage cylinder water circulation.

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