Heating oil, also known in the United States as No. 2 heating oil, is a low viscosity, flammable liquid petroleum product used to fuel building furnaces or boilers. In the U.S., it must conform to ASTM standard D396. Diesel and kerosene, while often confused as being similar or identical, must conform to their own respective ASTM standards.
Heating oil is commonly delivered by tank truck to residential, commercial and municipal buildings and stored in aboveground storage tanks located in the basements, garages, or outside adjacent to the building. It is sometimes stored in underground storage tanks but less often than ASTs. ASTs are used for smaller installations due to the lower cost factor. Heating oil is less commonly used as an industrial fuel or for power generation.
Heating oil is widely used in parts of the United States and Canada where natural gas or propane is frequently not available. Where other fuels are not available, it is sometimes referred to as the unit cost per unit, and can be less than other fuels. Boiler and "forced air" furnace manufacturers have perfected "retention head oil-fired burners" and "triple-pass flue" boilers that have increased theoretical oil burner efficiency to over 93 percent. To reach that level of efficiency, however, would require a lower flue gas temperature that most oil burners cannot produce. Therefore causing condensation that most oil-fired furnaces cannot handle without damage to the heat exchanger, venting pipes or outside casing of the appliance. Practical efficiency is typically around 86 percent.
Red dyes are usually added, resulting in its "red diesel" name in countries like United Kingdom. Solvent Yellow 124 is added as a "Euromarker" since 2002 in European Union.