One of the most fundamental principles used in all areas of science and engineering is the first law of thermodynamics. It states that energy cannot be created or destroyed, only changed in form.
For example, when a gallon of propane, which represents about 91,800 Btu of energy in chemical form, is combusted within a boiler, the majority of the chemical energy is converted into heat and transferred to a stream of water flowing through the boiler. This is useful heat output. Another portion of the energy contained in the propane becomes heat that leaves through the boiler’s exhaust system, and thus serves no useful purpose in heating the building. Yet another portion of the fuel energy drives chemical reactions during combustion, such as when hydrogen in the propane combines with oxygen in the air to produce water vapor.