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Furnace (central heating)
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<p>A furnace , referred to as a heater or boiler in <a href="/facts/British_English/ymJR5Rnd">British English</a>, is an appliance used to generate heat for all or part of a building. Furnaces are mostly used as a major component of a <a href="/facts/Central_heating/GsYBDLmn">central heating system</a>. Furnaces are permanently installed to provide heat to an interior space through intermediary fluid movement, which may be <a href="/facts/Air/hQ95Jznz">air</a>, <a href="/facts/Steam/RDGCdmzT">steam</a>, or hot <a href="/facts/Water/0lkXnJPK">water</a>. Heating appliances that use steam or hot water as the fluid are normally referred to as a residential steam boilers or residential hot water boilers. The most common <a href="/facts/Fuel/H8DIRL1j">fuel</a> source for modern furnaces in North America and much of Europe is <a href="/facts/Natural_gas/9MYQ7NAP">natural gas</a>; other common fuel sources include <a href="/facts/Liquefied_petroleum_gas/bw6trakm">LPG</a> (liquefied petroleum gas), <a href="/facts/Fuel_oil/5Qbta9f5">fuel oil</a>, <a href="/facts/Wood/z13NpHPl">wood</a> and in rare cases <a href="/facts/Coal/83kV3qxg">coal</a>. In some areas <a href="/facts/Electrical_resistance/hxSt0UAj">electrical resistance</a> 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% <a href="/facts/Thermal_efficiency/fR0ysNpr">efficient</a> and operate without a chimney, with a typical gas furnace being about 80% efficient. Waste gas and heat are mechanically ventilated through either metal <a href="/facts/Flue/XS791fhp">flue</a> pipes or <a href="/facts/Polyvinyl_chloride/9VkegJvr">polyvinyl chloride</a> (PVC) pipes that can be vented through the side or roof of the structure. Fuel efficiency in a gas furnace is measured in <a href="/facts/Annual_fuel_utilization_efficiency/Plzmjxab">AFUE</a> (Annual Fuel Utilization Efficiency). </p>