Page 471 - 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 boilers were always wall mounted, Combustion air in
very light in weight and, as a consequence, Flue gas outlet
often quite small in size, designed for fully Return Fan Combustion air in
pumped S- and Y-plan heating systems only. Flow
They were the first generation of central
heating boilers to use a high temperature
limiting thermostat (or energy cut-out) to
guard against overheating, and often used a High-limit thermostat
basic printed circuit board to initiate a pump Low water heat exchanger
overrun, which kept the pump running for a
short period after the boiler had shut down. Combustion chamber
It was required to dissipate any latent heat Pilot light
build-up in the water in the heat exchanger as
Gas burner
this could ‘trip’ the energy cut-out resulting in Thermocouple
boiler lock-out.
Interrupter
Low water content boilers can be found with a
variety of flue types: Multi-function control

l open flued
l room sealed (natural draught) Gas pipe
l fan-assisted room sealed (forced draught). Boiler thermostat

p Figure 7.29 A fan-assisted low water content boiler

 Table 7.10 Advantages and disadvantages of low water content boilers
Advantages Disadvantages
Light in weight Not energy efficient
Often a cheaper appliance Do not comply with Building Regulations Document L
Relatively fast water-heating times Could be very noisy
Relatively short working life
High maintenance compared with other boilers

Combination boilers (non-condensing)
Combination boilers that supply instantaneous hot water as well as central
heating have been around for many years. Early models, although wall mounted,
were very large. Most had a sealed (pressurised) heating system but some were
of the low pressure, open vented type. Hot water flow rates were often poor by
comparison to modern condensing types.
Early combination boilers can be found with a variety of flue types:

l open flued
l room sealed (natural draught)
l fan-assisted room sealed (forced draught).

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