THE ORIGIN OF THE BRITISH THERMAL UNIT The British thermal unit (BTU or Btu) is a traditional Imperial unit of energy. It is the amount of energy needed to cool or heat one pound of water through one degree Fahrenheit. . In North America the heat value (energy content) of fuels is expressed in BTUs. Heating Load calculations as we know them today were only in their infancy in the 1890’s. Prior to that time various rules of thumb had been developed from experience for sizing the radiation to be used in a building. William James Baldwin one of the most respected steam heating engineers of his time, provided the following example, apparently in use at that time. “The ordinary rule of thumb way of the average pipe fitter, is to multiply the length by the width of a room, and the result by the height, then cut off two figures, from the right hand side, and call the remainder square feet of heating surface, with an addition of from 15 to 30 per cent for exposed or corner rooms” (Baldwin 1883) Baldwin then goes on to promote a more scientific method that converts various wall construction types to an equivalent of glass, which is then added to the actual window area of the room. A formula taking into account the room, outdoor and steam temperatures is then used to determine the required heating surface in square feet (Baldwin) At the time and for many years after, hot water or steam appliances were measured in square feet of radiating surface. The practice of using British thermal Units (Btu) for heat load calculations or for rating of heating appliances was not in use at the time. In fact, the literature of the 1880’s does not seem to use the term Btu at all, but simply refers to “heat unit” or “the unit of heat”. By the 1900’s the term “heat unit” was still used, but the use of Btu was becoming more common.