A BTU is the amount of heat required to raise the temperature of one pound of water:

A BTU, or British Thermal Unit, is defined as the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. This unit is commonly used in the heating and cooling industries, especially when discussing the energy content of fuels or the output of heating systems like furnaces and boilers in the United States.

Understanding the significance of a BTU in terms of Fahrenheit is essential because the measurement system used in the context of HVAC (Heating, Ventilation, and Air Conditioning) in the U.S. primarily revolves around Fahrenheit. By using this specific temperature scale, professionals can accurately calculate heating, cooling loads, and system efficiencies, which directly affect the design and performance of heating systems.

The other options represent temperature scales that may not typically be used in domestic heating contexts, making them less relevant to this particular question about BTUs and their application in heating systems.