Condensing Boiler - Principles of Work

Principles of Work

In a conventional boiler, fuel is burned and the hot gases produced are passed through a heat exchanger where much of their heat is transferred to water. Thus raising the water's temperature.

One of the hot gases produced in the combustion process is water vapour (steam), which arises from burning the hydrogen content of the fuel. A condensing boiler extracts additional heat from the waste gases by condensing this water vapour to liquid water, thus recovering its latent heat. A typical increase of efficiency can be as much as 10-12%. The effectiveness of this condensing process varies, it depends upon the temperature of the water returning to the boiler, but for the same conditions, it is always at least as efficient as a non-condensing boiler.

The condensate produced is slightly acidic, 3-5 pH, so the choice of materials used in the wetted areas have to be suitable. At high temperature most commonly used are aluminium alloys and stainless steel, in the low temperature areas plastics are most cost effective, for example uPVC and polypropylene. The production of condensate also requires the installation of a heat exchanger condensate drainage system. For a basic installation this is the only difference required compared to a non-condensing boiler.

For the heat exchanger within a condensing boiler to be economical to manufacture, and for the appliance to be manageable at installation, the smallest practical size for its output is preferred. This has produced heat exchangers with very high combustion side resistance and so the use of a combustion fan to move the products through narrow passageways has been adopted. This also has had the benefit of providing the energy for the flue system as the expelled combustion gases are usually below 100°C (212°F) and no longer have much natural buoyancy.