Condensing Boilers are a relatively recent innovation in the domestic boiler market.  This type of boiler is extremely economical to run, offering efficiency rates of up to 95% (which means 95% of the fuel used is turned into heat).  There was an Energy Saving Trust  government grant available to encourage the use of this type of boiler.  Known as "Green" boilers, Condensing Boilers are environmentally the friendliest of all boilers with the lowest emission rates and highest efficiency.

With most boilers approximately 20% of the heat produced is wasted through the flue.  The figure is much higher with older boilers.  Condensing Boilers achieve much higher efficiencies by allowing the flu gasses to condense and re-using the heat given off.