If you are buying a new boiler for your home heating needs, make sure you select one that is efficient and affordable. Boiler manufacturers are required to disclose how well the boiler produces heat by displaying its AFUE or annual fuel utilization efficiency rating. The higher the rating, the more efficient the boiler in terms of how much fuel gets converted into heat. The minimum rating allowed for boilers varies depending on the type of boiler.
There are several types of boiler on the market, and they differ by the fuel and medium. Hot water boilers heat the home with hot water, while steam boilers heat the home with steam. The fuel source is usually either oil or gas.The minimum AFUE ratings for these boilers are:

Oil fired hot water boiler – 84 percent

Oil fired steam boiler – 82 percent

Gas fired hot water boiler – 82 percent

Gas fired steam boiler – 80 percent

Make sure you understand these are minimum standards, and you can obtain greater efficiency by buying a higher AFUE boiler. More efficient condensing boilers (90 percent plus AFUE) utilize heat from the condensation of water vapor in a second heat exchanger. These boilers have a much higher up-front cost, but they will reduce energy costs over the boiler’s lifetime.