boilers used in domestic central heating systems

Level 3 6035 Diploma in Plumbing Studies

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Boilers used in domestic central heating systems

Unit 306: Central heating



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)



Open fire with a high-output back boiler

Room heaters

Gravity feed boiler

Batch feed boiler

Solid fuelSolid fuel appliances are still used in rural areas of the UK where access to piped fuel supply is difficult.



Gas boilers

Gas central heating boilers are the most popular of all central heatingappliances. 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.



Types of boilers

Floor-standing boilerBack boilerSystem boilerCombination boilerCondensing



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.



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.



Typical flue arrangementsAll 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.



Room-sealed (balanced) fluesThis 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 principleis 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.



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.



Typical safety controls for gas boilers

Multi-functional valve Flame rectification device Air-pressure switch



Any questions?

boilers used in domestic central heating systems

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