How The Condensing Boilers Works

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Condensing boilers are special design which give higher efficiency as compared to ordinary boilers. Whereas, the efficiency of regular boilers could be around 75%, with a condensing boiler it may reach over 87%. In a normal incinerator, the wasted flue gas carries away a large amount of thermal energy at 180 degree C. The special design extracts this, and the gases now leave at a cooler temperature of around 55 degree C.

Most boilers are a single combustion chamber affair, enclosed by the heat exchanger through that hot vapors pass. These ultimately leave by the flue, situated at the top of the heating system. Due to the single chamber, the thermal exchange is not sufficient. The expelled gases still retain a large amount of useful warmth, which is ultimately wasted. This results in the efficiency of these boilers to remain on the low side.

On the other hand, a condensing boiler has two chambers. The warmth first rises upwards through the elementary exchanger. At the top, the vapors are rerouted through a secondary exchanger. This extracts the extra warmth from the flue gases and reduces its temperature ...
... to nearly 55 degree C.

This reduction in temperature causes condensation of water vapor. As vapor condenses, it forms droplets of water. These fall by gravity to trickle down and collect at the bottom of the flue manifold. The remaining gases are expelled from the top by the assistance of exhaust fans.

The condensate from the appliance has to be drained into a waste discharge pipe. The flow and return pipe work should be kept at a temperature below 55 degree C, for the appliance to work. This allows for the warmth from the flue gases to be transferred to the water.

Energy extraction from the combustion products is done in two distinct phases. In the first phase, warmth is extracted as sensible heat. This is the energy transferred from a hotter body to a cooler one. This takes place in the first thermal exchanger in the heating system. In the second phase, latent heat of evaporation is used.

This is the thermal energy which went into converting the water into vapor when it was boiled in the heating system into forming steam, and is released when the vapor condenses back. This happens in the second heat exchanger. After transferring the latent heat, vapor condenses to water. It is this dual transfer of thermal energy, which gives the condensing boiler an increased efficiency over the regular one.
Northern Gas Heating - the UK's local supplier of new Central Heating Systems offer Boilers and Condensing Boilers.