Boilers are categorized by the type of fuel they use, how the water they store is heated and how the heat is distributed. Some types of boilers can be converted to another type.
Coal and wood boilers have largely been supplanted by gas and oil boilers, and many boilers in use are converted units. Most boilers distribute either hot water or steam through a home, and most of these boilers cannot easily be converted. Condensing boilers are popular; their ability to reuse the heat in water not warm enough to circulate enables greater efficiency. Other types of boilers typically cannot be converted into condensing boilers, although their greater efficiency may make a replacement a sound investment.