A furnace, referred to as a heater or boiler in British English, is a heating unit used to heat up an entire building. Furnaces are mostly used as a major component of a central heating system. The name derives from Latin word fornax, which means oven. Furnaces are permanently installed to provide heat to an interior space through intermediary fluid movement, which may be air, steam, or hot water. Heating appliances that use steam or hot water as the fluid are normally referred to as a residential steam boiler or residential hot water boiler. The most common fuel source for modern furnaces in North America and much of Europe is natural gas; other common fuel sources include LPG (liquefied petroleum gas), fuel oil and in rare cases coal or wood. In some areas electrical resistance heating is used, especially where the cost of electricity is low or the primary purpose is for air conditioning. Modern high-efficiency furnaces can be up to 98% efficient and operate without a chimney, with a typical gas furnace being about 80% efficient. Waste gas and heat are mechanically ventilated through PVC pipes that can be vented through the side or roof of the house. Fuel efficiency in a gas furnace is measured in AFUE (Annual Fuel Utilization Efficiency). Furnaces primarily run on natural gas or electricity. Furnaces that are used to boil water are called boilers.
The term furnace also refers to the various types of metallurgical furnaces, used for smelting and other metalworks, as well as industrial furnaces, which are used in various industrial applications such as chemical plants and providing heat to chemical reactions.