A condensing boiler is a high efficiency advanced boiler that incorporates an extra heat exchanger so that the red-hot waste gases transfer a percentage of their heat to pre-heat the mains water to be heated. When working at peak efficiency, the water gas developed in the combustion process distills back into water putting out the latent heat of vaporisation.

In earlier years boiler installers filled bigger boilers than were required. Although this meant that there was never the chance of the boiler not meeting the heat demand, without any concern for icy weather, it also meant that they were mainly running at a part load, and so working in an inefficient style. If you have insulated your household since the previous boiler was put in, you will probably find that you will be able to fit a lower-ranking capability boiler than the current one.

We urge seeking professional advice from a CORGI recorded installer before choosing a suitable substitute boiler.

Do I need to put in oversize radiators with condensing boilers??

The main efficiency gain from a condensing boiler is because it contains a bigger heat exchanger. More Prominent radiators would result in cooler return water temperatures, and so enable greater energy efficiency, but the additional savings suffer from falling returns, as you should consider the fact that that the system is is only using a small part of its overall capacity for the main heating months. That probably remains true, though the smaller heating demand for new build may well mean that homeowners would now allow over-sized radiators more willingly.

These consequences were studied during the SEDBUK initiative, during which it was mooted whether condensing boilers required to be treated differently from others. The outcome was that no new recommendations were expected, and the test results forthe condensing and the existing boilers share the same SEDBUK equation.