The condensing boiler is a high efficiency, modern boiler. It has a larger second heat exchanger, so that the hot exhaust, releases extra gas energy which is used to preheat the water in the boiler system. The term condensing applies to the technology which is used to maximise boiler efficiency, in order to use less fuel, which produces more heat? Due to the condensing boiler's efficiency - Since March 31st 2008 when installing a replacement oil or gas boiler it is now a requirement that the boiler be condensing, where practical (Section L3, Building Regulations Part L amendment – S.I. No. 847 of 2007)

The three main types of condensing boilers are regular boilers, system boilers, and combi boilers. The common feature for the different condensing boilers is the ability to extract heat from flue gasses that are not used. Due to their efficiency, condensing boilers, if used moderately, will help lower heating bills.

For installation, it is best to seek professional advice from a Registered Gas Installers (RGII) or the Oil Firing Technical Association (OFTEC), registered installers. The new installation measurements for boilers have been changed. Your old boiler, had output measurements in Thermal Units per hour (BTU/h), but the current condensing/combi boilers use the metric equivalent of kilwatts (kW). Also, older boilers required additional storage space for its copper cylinders. Typically, the "as new" efficiency of an oil or gas boiler over 15 years old would have been less than 80%. Its present efficiency today, due to wear and tear is unlikely to be greater than 70%.

The condensing boiler is strongly advocated by environmental groups and government bodies, concerned with reducing energy emissions. Condensing boilers have been used for a number of years in the Netherlands. The National Energy Foundation, in their earlier testing, found that condensing/combi boilers operated at 90% efficiency rate. Because of the condensing boiler' technology and rapid popularity, there are many manufacturers who can accommodate their customers. Condensing/combi boilers carry the Seasonal Efficiency of Domestic Boilers (SEDBUK) A rating and the Energy Saving Trust logo.

Combi boilers, is short for "combination," and are the most popular and widely used boilers today. A combi boiler is environmentally friendly. It is a very clean and efficient water heater and central heating boiler, combined in one single unit. For homeowners, combi boilers are quite compact, because they can be hung, out of the way, on a kitchen wall or can fit into cupboards, thus saving a flat, bungalow, or detached, additional space. Because of their compact size, their installation is less disruptive and less expensive than that of older boilers. A combi boiler usually only supplies hot water to one outlet, at a time, which is why they are best, suited, for smaller housing units. Combi boilers function, by water being heated, when needed, from within the unit itself, rather than a large water tank. In addition to saving space, a combi boiler can provide hot water, quite smartly - just by turning a tap on the boiler, it will fire up quickly.