Page 451 - 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

Feed and expansion cistern

Room thermostat

15 mm cold 22 mm vent pipe
feed pipe

Boiler 22 mm flow and return pipes with
heat emitters fed by 15 mm pipe

p Figure 7.2 Pumped central heating only

There are three basic semi-gravity systems and each is an advance on the previous
system. These are: INDUSTRY TIP

1 the one-pipe system These systems are often
2 the two-pipe system (C-plan system) referred to as ‘gravity’ or
3 the C-plan plus system. ‘pumped’ systems because
they incorporate old heating
The one-pipe system
technology in the form of
This is a simple ring circuit of pipework to and from the boiler and, for this gravity circulation and forced
reason, there are no separate flow and return pipes. The main ‘ring’ is pumped circulation via a central
and the water circulates through the radiators by gravity circulation. The size of heating circulating pump.
the radiators is calculated from the temperature drop at each successive radiator,
with the last radiator always being around 15°C cooler than the first. Balancing
the flow of water to each radiator is a simple process by the use of radiator
valves, but this increases system resistance and slows the heating process.
One-pipe systems will not work effectively when installed with combination
boilers, so these systems are not recommended for ‘combi’ boiler conversions.
The C-plan (two-pipe) system

Like the one-pipe system, the C-plan system has gravity circulation to the hot
water circuit and pumped circulation to the central heating circuit. The system
differs considerably from the one-pipe system by having two pipes: a flow pipe
from the boiler to the heat emitters and a return pipe from the emitters back
to the boiler. The heat emitters are connected to separate branches of the
main flow and return pipes so, in effect, each heat emitter has its own flow and
return pipework to the boiler. This means that all of the heat emitters achieve

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