زيت الوقود
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.