Not all central heating systems require purchased energy. A few buildings are served by local geothermal heat, using hot water or steam from a local well to provide building heat. Such areas are uncommon. A passive solar system requires no purchased fuel but needs to be carefully designed for the site. Calculating output of heater required Heater outputs are measured in kilowatts or BTUs per hour. For placement in a house, the heater, and the level of output required for the house, needs to be calculated. This calculation is achieved by recording a variety of factors – namely, what is above and below the room you wish to heat, how many windows there are, the type of external walls in the property and a variety of other factors that will determine the level of heat output that is required to adequately heat the space. This calculation is called a heat loss calculation and can be done with a BTU Calculator. Depending on the outcome of this calculation, the heater can be exactly matched to the house. Billing Heat output can be measured by Heat cost allocators, so that each unit can be individually billed even though there is only one centralized system. Types of central heating Water heating Circulating hot water can be used for central heating. Sometimes these systems are called hydronic heating systems. Common components of a central heating system using water-circulation include: A supply of fuel, electric power or district heating supply lines A boiler (or a heat exchanger for district heating) which heats water in the system Pump to circulate the water Radiators through which the heated water passes in order to release heat into rooms. The circulating water systems use a closed loop; the same water is heated and then reheated. A sealed system provides a form of central heating in which the water used for heating circulates independently of the building's normal water supply. An expansion tank contains compressed gas, separated from the sealed-system water by a diaphragm. This allows for normal variations of pressure in the system. A safety valve allows water to escape from the system when pressure becomes too high, and a valve can open to replenish water from the normal water supply if the pressure drops too low. Sealed systems offer an alternative to open-vent systems, in which steam can escape from the system, and gets replaced from the building's