Central heating systems provide heat from a single point to multiple rooms, typically with a furnace or boiler and vents, pipes, or radiators. Unlike local heating systems in which heat is generated in a fireplace or stove and limited to a single room or small area, central heating systems are designed to warm larger areas or entire structures. The most common method of central heat generation requires the combustion of fossil fuels such as oil, gas or coal. Wood-burning furnaces and electric heating systems are also available. With forced air central heating systems, the generated heat is distributed by forcing hot air through ductwork, vents and plenums. Central heating systems that circulate water or feed steam through pipes are also used in residential and commercial construction. Central heating systems that burn fossil fuels for hot water circulation require supply lines, a boiler or heat exchanger, heat pumps, and pipes and/or radiators. With gas heat, the supply lines may run from a propane tank located outside the home, building, or structure. Natural gas pipes and propane pipes should comply with building codes and be installed by a professional plumber. Boilers and heat exchangers are available in various configurations. Typically, heat exchangers are categorized as parallel-flow or counter-flow. Specific types include shell and tube, plate, and regenerative. Heat pumps are used to circulate the water used in central heating systems. Radiators are wall or floor-mounted panels through which the heated water passes before releasing heat into rooms. Like other central heating systems, hydronic heating systems use the hot water from a boiler or hot water heater as the medium for heat transfer. Radiators are common, but radiant loops are also used. Aluminum-finned copper tubing and a copper or cast iron baseboard runs in a continuous loop between rooms. By using a mono-flow T fitting, a single pipe can feed the entire loop. Radiant floor heating systems are also available. These central heating systems include pipes that can be embedded in a concrete slab or attached to the underside of a subfloor. To radiate heat downward from a ceiling, copper pipes may be attached to the ceiling joists.