This is a design of boiler which can have an increased efficiency over the more traditional boiler. The efficiency of a typical non-condensing boiler is around 75%, whereas with condensing boilers it can be over 87%. This increased efficiency is due to the extraction of heat from the otherwise wasted flue gases. Most boilers have a single combustion chamber enclosed by the waterways of the heat exchanger through which the hot gases can pass. These gases are eventually expelled through the flue, located at the top of the boiler, at a temperature of around 180°C.
Condensing boilers, on the other hand, are designed first to allow the heat to rise upwards through the primary heat exchanger; when at the top the gases are rerouted and diverted over a secondary heat exchanger. These can reduce the flue gas temperature to about 55°C. This reduction of temperature causes the water vapour (formed during the combustion process) to condense and, as the droplets of water form, fall by gravity to collect at the base of the flue manifold. The remaining gases are expelled to the outside environment through a fan-assisted balanced flue. The condensation produced within the appliance should be drained as necessary into the waste discharge pipework or externally into a purpose-made soakaway.
It is only possible for a condensing boiler to work to these very high efficiencies if the flow and return pipework is also kept below 55°C. The flow & return temperatures need to be maintained for the heat transference to occur from the flue to the water (i.e. heat transference goes from hotter to cooler materials).
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