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Boilers used in domestic central heating systems
Unit 306: Central heating

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Boilers used in domestic central heating systems
Boilers used for central heating systems are generally heated by one
of four different types of fuel source:
Solid fuel (coal, coke anthracite, bio mass wood pellets)
Gas (Natural gas and LPG)
Oil (C2-grade 28-second viscosity oil [kerosene])
Renewable energy (Ground source and air source heat pumps)

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Open fire with a high-output
back boiler
Room
heaters
Gravity feed
boiler
Batch feed
boiler
Solid fuel
Solid fuel appliances are still used in rural areas of the UK where
access to piped fuel supply is difficult.

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Gas boilers
Gas central heating boilers are the most popular of all central heating
appliances. Over the years there have been many different types, from
large multi-sectional cast-iron domestic boilers to small, low-water
content condensing types. Both natural gas (those that burn a methane
based gas) and LPG (those that burn propane) types are available.
Central heating boilers can be categorised as:
•traditional boilers (non-condensing)
•cast-iron heat exchangers
•low-water-content heat exchangers
•combination boilers (non-condensing)
•condensing boilers
•condensing system boilers
•condensing combination boilers.

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Types of boilers
Floor-standing boiler
Back boiler
System boiler
Combination boiler
Condensing

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Traditional (non-condensing)
boilers use a single heat
exchanger to heat up the
primary water, allowing the flue
gases and latent heat to be
emitted to atmosphere.

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Condensing boilers increase in efficiency is achieved by the flue
gases passing over a second heat exchanger allowing the gases to
cool to 55ºC and condensate the latent heat which is given off.
Because of this condensation, ‘pluming’ occurs from the flue and
provision has to be made to drain the slightly acidic condensate from
the boiler.

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Typical flue arrangements
All heating appliances need a flue to remove the products of combustion
safely to the outside. The basic concept is to produce an up-draught,
whether by natural means or by the use of a fan, to eject the fumes
away from the building. There are two main types of flue:
•open flues
•room-sealed (balanced) flues.
The open flue is the simplest of
all flues. Because heat rises, it
relies on the heat of the flue
gases to create an up draught.
There are two different types:
•natural draught
•forced draught.

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Room-sealed (balanced) flues
This boiler type draws its air for combustion directly from the outside
through the flue assembly used to discharge the flue products. It is
inherently safer than an open-flue type, since there is no direct route for
flue products to spill back into the room.
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.

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Fan-assisted room-sealed appliances
work in the same way as the natural
draught flue, with the products of the
combustion outlet being positioned in the
same place (generally) as the combustion
air intake. But, there are two distinct
differences:
•The process is aided by a fan, which
ensures the positive and safe evacuation
of all combustion products and any
unburned gas that may escape.
•The flue terminal is circular, much smaller
and can be positioned in many more
places than a natural draught flue.

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Typical safety controls for gas boilers
Multi-functional valve Flame rectification device Air-pressure switch

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Any questions?