Basically, a boiler consists of a home, where the combustion and heat exchanger, where water is heated. It also has to gas have a system for evacuating the gases from the combustion.
Water can be heated at different temperatures. In normal boilers are generally not exceed the 90th C, below the boiling point energy of water at atmospheric pressure. the improvements that have been afforded the general household by has changed the face of gas consumption In most large boilers, to serve neighborhoods, you reach the 140th C, maintaining high pressure in the pipes so they do not get to evaporate (water). There are also steam boilers, which takes water to evaporation and the steam is distributed electricity ESCO to the terminal, but in Europe is quite obsolete, because the surface temperature of these is very high and dangerous burns. the energy market has been greatly improved by an energy supplier There are also boilers energy costs in which water is heated to temperatures below 70A natural gas C and get high returns (condensing boiler).
The solid fuel can be used (wood, coal), liquid (fuel oil, diesel) and gas (liquefied petroleum gas or LPG, natural gas), which household determines the shape of the boilers.