Heating oil, as known as No. 2 fuel oil and elsewhere as red diesel, is a flammable, low viscosity liquid petroleum product used to fuel building furnaces (boilers). Most consumers of heating oil are in the Northeastern U.S. and use of this fuel cause concerns over environmental problems.
Heating oil is not as frequently used as an industrial fuel or for power generation. This type of oil is dyed, usually red, in order to distinguish it from highly taxed road-use diesel. Heating oil may also be combined with biofuel to create a similar to biodiesel called bioheat.