<P> The energy source selected for a central heating system varies by region . The primary energy source is selected on the basis of cost, convenience, efficiency and reliability . The energy cost of heating is one of the main costs of operating a building in a cold climate . Some central heating plants can switch fuels for reasons of economy and convenience; for example, a home owner may install a wood - fired furnace with electrical backup for occasional unattended operation . </P> <P> Solid fuels such as wood, peat or coal can be stockpiled at the point of use, but are inconvenient to handle and difficult to automatically control . Wood fuel is still used where the supply is plentiful and the occupants of the building don't mind the work involved in hauling in fuel, removing ashes, and tending the fire . Pellet fuel systems can automatically stoke the fire, but still need manual removal of ash . Coal was once an important residential heating fuel but today is rarely found . </P> <P> Liquid fuels are petroleum products such as heating oil and kerosene . These are still widely applied where other heat sources are unavailable . Fuel oil can be automatically fired in a central heating system and requires no ash removal and little maintenance of the combustion system . However, the variable price of oil on world markets leads to erratic and high prices compared to some other energy sources . Institutional heating systems (office buildings or schools, for example) can use low - grade, inexpensive bunker fuel to run their heating plants, but capital cost is high compared to more easily managed liquid fuels . </P> <P> Natural gas is a widespread heating fuel in North America and northern Europe . Gas burners are automatically controlled and require no ash removal and little maintenance . However, not all areas have access to a natural gas distribution system . Liquified petroleum gas or propane can be stored at the point of use and periodically replenished by a truck - mounted mobile tank . </P>

Who was the first to introduce the forced-air furnace for residential heating