The 30-second guide to condensing boilers

Figures from BRE show that a modern conventional boiler is about 75% efficient over a year, whereas a condensing boiler has an efficiency of 88% over the same period. This means lower fuel bills and a smaller contribution to global warming.

The increase in efficiency is a result of the condensing boiler's large heat exchanger.

This extracts so much heat from the flue gases that it causes the water vapour in them to condense – hence the name. The heat exchanger then recovers heat from the flue gas and from the water as it changes from a vapour into a liquid.

Condensing boilers have been criticised because they do not operate continuously in condensing mode – it only operates when the temperature of the water returning to the boiler from the radiators is less than 50°C (the point at which condensation can occur). As this temperature is below the normal return temperature of a conventional heating system, the boiler will cease condensing when the system has run for a certain time. However, condensing boilers still run very efficiently.

These units are no more complicated to install than conventional ones, but because they produce condensation, this must be collected and drained away. Consideration also needs to be given to the location of the flue because it produces a plume of visible water vapour, which neighbours could consider a nuisance.

The heat exchanger on condensing boilers must be made from a corrosion-resistant material, such as stainless steel, which is why they have tended to be more expensive than conventional boilers. However, gas condensing boilers have come down in price and are now not much dearer than conventional boilers, says Bruce Young at BRE, although oil-fired versions are still considerably more expensive.

The greater efficiency of gas-fired condensing boilers can be traded off against a building's insulation under the proposed changes to Part L of the Building Regulations, to be introduced in February 2002.