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Boiler - Types

Boilers work by burning gas which gives off heat, this energy passes through the heat exchanger and transfers heat to the water contained within. How this is done only differs slightly from boiler to boiler. In this section we will cover some of the diiferences between boilers without being too confusing.

There are two main types:


  • System Boiler

  • Combinational Boiler


With a system boiler the hot water storage vessel (cylinder) is located separately from the boiler. Generally this is true for the pump and diverter valve also.


While a combinational boiler (or combi for short) has a small hot water storage vessel, pump and diverter valve all inside the casing.


There are also differences in how the heat exchangers transfer the heat but we will cover that at a later date.

When positioning a boiler you have only three options:


  • Back boiler unit (bbu for short) - located in the fire opening behind the fire.


  • Free standing - these stand on the floor.


  • Wall mounted - these are hung on the wall.

Condensing boilers


Condensing boilers work by retaining some of the heat lost in the flue gases. The gases are forced through a restricted path so it transfers more of its heat. Scientists discovered that when boiling water no matter how fierce the heat, the water got no hotter. So where did the extra energy go? Simple it was escaping in the form of steam. Similar a lot of energy produced from burning gas went straight out of the flue in the form of water vapour.

The result of taking this energy back is that the flue gases are cooled down extremley, to dew point - around 55 degrees C ( they are usually well into the hundreds). At this temperature the flues gases loss their natural buoyancy so have to be extracted by a fan. Also the water vapour produced rolls back down the flue, through the boiler chamber, collects in a trap and is discharged to and appropriate point.

Take Note:


From 1 April 2005, Part L1 of the Building Regulations requires gas and oil boilers installed in new and existing premises to be condensing types, with a SEDBUK efficiency in band A or B, unless there are exceptional circumstances that make this impractical and/or too costly. In those cases where it is considered difficult to install a condensing boiler, an assessment procedure is used to demonstrate that a non-condensing boiler is justified.

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