A boiler converts the chemical energy of a fuel into heat by combustion. The released heat is used for space heating and hot water heating in buildings. The main components of a boiler are a fuel supply system, a burner unit, and a boiler body (which includes a combustion chamber, a flue gas exhaust system, and a water tank). The most common fuels for boilers servicing individual buildings (or small groups of buildings) are natural gas, light fuel oil, and solid fuels. Recent technological developments include:

• reduction of NOx and particulate emissions; and
• incorporation of condensation and low-temperature technology to increase efficiency.

For a conventional gas boiler, the average system efficiency falls off as the load decreases, from 70 or 75% to values of about 65% for loads below 25% of the design maximum. For high-efficiency boilers, the efficiency at 25% load is in the range of 76% to 80%.

Boilers designed to condense water vapor in the flue gases have a higher inherent efficiency, with peak load efficiencies of 90% and more, together with a load/efficiency characteristic which falls very little with decreasing load. This gives large gains in seasonal efficiency of about 30% compared with old boilers, and 10% compared with modern conventional boilers.

Old boilers (say 15 years old) tend to have a seasonal efficiency of about 50%. When replacement is due, installing a modern system with a peak load efficiency of about 80% will provide a seasonal efficiency of typically 70%.