Heating oil, or oilheat, also known in the United States as No. 2 fuel oil and elsewhere as "red diesel", is a low viscosity, flammable liquid petroleum product used to fuel building furnaces or ("boilers"). It is commonly delivered by tank truck to residential, commercial and municipal buildings and stored in above-ground storage tanks ("ASTs") located in the basement or adjacent to the building. It is sometimes stored in underground storage tanks (or "USTs"), but less often than ASTs.