The boiler of a heating system has the function of generating heat. If the fluid being heated is air, the heat exchange device is called a furnace, not a boiler.

Boilers may be grouped into classes based on working pressure and temperature, fuel used, shape and size, and steam or water. Most classifications are of little importance to a specifying engineer, exept as they affect performance and efficiency. From environmental point of view, high-efficiency, low-No_x boilers should be applied.

Until recently, boilers were designed to operate without condensing flue gasses in the boiler. This precaution was necessary to prevent corrosion of cast-iron or steel parts. Hot-water units were often operated at 60°C minimum return temperature to prevent rusting when natural gas was used. Because higher boiler efficiencies can be achieved with lower water temperatures, condensing boilers purposely allow the flue gas water vapor in the boiler to condensate and drain.