Condensing boilers are more advanced than conventional boilers. The incorporation of an additional heat exchanger improves efficiency by using the warm exhaust gases that would otherwise be lost, to preheat the cold water as it enters the boiler.  When working at peak efficiency the water vapour produced by the burning of gas or oil in the boiler condenses back into liquid thus giving the boiler its name-Condensing Boiler. The design has its greatest advantages with gas-fired boilers, although it is also viable with oil.


A condensing boiler can achieve over 90% efficiency of fuel conversion to heat, compared to 70%-80% with a conventional design. The water that condenses out tends to be acidic because of the impurities in the fuel, chiefly sulphur and nitrogen, so it has to be piped away; also, the relevant parts of the boiler have to be constructed of materials that will withstand this acidity. Also the exhaust from a condensing boiler is much cooler than that produced from a standard boiler, so to expel it, a fan is normally required.

Condensing boilers are now largely replacing earlier traditional designs in domestic central heating systems.  In England and Wales since April 2005, all new gas central-heating boilers must be high-efficiency condensing boilers unless there are exceptional reasons and the same will apply to oil-fired boilers from  April 2007.


At current uk prices the extra cost of fitting a condensing boiler should be recovered in around 3 years through lower fuel use; although this figure can vary depending on the boiler purchased and fuel price changes and to a certain extent, the weather conditions.