Page 472 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

 Table 7.11 Advantages and disadvantages of non-condensing combination boilers

Advantages Disadvantages
Instantaneous hot water supply Not energy efficient
Sealed system means no F and E cistern Very large in size
required in the roof space Do not comply with Building Regulations
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Can be very noisy
High maintenance compared with other
boilers
Poor hot water flow rates
Difficult to maintain

Condensing boilers
A more recent addition to the gas central heating family is the condensing
boiler. These work in a very different way from the traditional boiler.

Natural gas, when it is combusted, contains CO , nitrogen and water vapour. As
2
the flue gases cool, the water vapour condenses to form water droplets. It is this
KEY TERM process that condensing boilers use.
Dew point: the temperature With a condensing boiler, the flue gases first pass over the primary heat
at which the moisture within
a gas is released to form exchanger, which extracts about 80 per cent of the heat. The flue gases, which
water droplets. When a still contain 20 per cent of latent heat, are then passed over a secondary heat
gas reaches its dew point, exchanger where a further 12–14 per cent of the heat is extracted. When this
the temperature has been happens, the gases cool to their ‘dew point’, condensing the water vapour inside
cooled to the point where the boiler as water droplets, which are then collected in the condensate trap
the gas can no longer hold before being allowed to fall to drain via the condensate pipe. The process gives
the water and it is released
in the form of ‘dew’, or condensing boilers their distinctive ‘plume’ of water vapour during operation,
water droplets. which is often mistaken for steam.

Flue gas outlet

Combustion air in
Flow
Primary heat
exchanger
Secondary
heat
exchanger
Return

Fan

Condensing trap
p Figure 7.30 How a condensing boiler works

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