Page 477 - 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

products of combustion up the flue. Forced draught open flues are not suitable
for all open flue types and their use will depend upon the boiler manufacturer
and the boiler/flue design.

Products of combustion Terminal
escape through the terminal

Secondary flue

Deflector plate

Draught diverter
Air drawn in through the
draught diverter Primary flue

Flow
Return
Combustion chamber

Combustion air
Gas burner
p Figure 7.34 The operation of an open flue

Room sealed (balanced) flues
This boiler draws its air for combustion direct from outside through the same
flue assembly used to discharge the flue products. This boiler is inherently safer
than an ‘open flue’ type, since there is no direct route for flue products to spill
back into the room. There are two basic types:

1 natural draught
2 fan assisted (forced draught).

Natural draught
Natural draught room sealed appliances have been around for many years and
there are still many thousands in existence. The basic principle is very simple –
both the combustion air (fresh air in) and the products of combustion (flue
gases out) are situated in the same position outside the building. The products
of combustion are evacuated from the boiler through a duct that runs through
the combustion air duct, one inside the other.

The boiler terminals are either square or rectangular and quite large in size.
Terminal position is critical to avoid fumes going back into the building through
windows and doors.

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