Condensing boilers are the most energy efficient Central Heating boilers on the market today. They achieve this high level of efficiency by removing the heat from the fuel it is burning and also cooling the products of combustion, which are normally wasted up the flue, so much so that the water vapour in them turns into liquid. This happens at around 55°C. For a boiler to cool the flue products this much, it has to have a larger, more efficient heat exchanger than traditional boilers.
So why aren't all manufactured boilers condensing?
Well, firstly, because the liquid water, the condensate, has to be safely removed from the boiler and to include this into the design requires the use of materials not normally used in traditional boilers.
Secondly, the natural buoyancy of the flues products is lost when the temperature is brought this low so it is necessary to have a fan to remove them.
Finally, larger heat exchangers cost more to make so the boilers will be higher in price and it takes very clever designs to ensure that the user will be able to save more in reduced fuel bills than they spend on the extra cost of Condensing Boiler.
There is also another benefit obtained by the water vapour turning into liquid, it gives up energy, which is called latent heat. This adds to the rest to make the overall efficiency of a condensing boiler maybe 15% greater than a modern non-condensing one. The more efficient the boiler is, the less fuel it uses, the less carbon dioxide it produces and the less it costs to run.