Heating Fundamentals: Heat and Work relationship

Heat is a form of energy in transit between its source and destination point as a result of temperature differences between them. Heat energy exists as such only while flowing between the source and destination.

British Thermal Unit

Heat energy is measured by British thermal unit (Btu). Each thermal unit is regarded as equivalent of one unit of heat (heat energy).
Prior to 1929, Btu was defined as the amount of heat necessary to raise the temperature of one pound of water by one degree Fahrenheit. Due to the difficulty in measuring an exact value of a Btu, it was later defined in terms of the more fundamental physical unit.
Since 1929, British thermal unit has been defined on the following basis:
1 Btu = 251.996 IT (International Steam Table) calories =778.26 foot-pounds of mechanical energy units (works) = approximately 1/3 watt-hour.

Heat and Work relationship

Work is the overcoming of resistance through a certain distance by the expenditure of energy.
Work is measured by a foot-pound. Foot-pound may be defined as the amount of work done in raising one pound a distance of one foot, or overcoming a pressure of one pound through a distance of one foot.
The relationship between heat and work is called a mechanical equivalent of heat. One unit of heat being equal to 778.26 ft-lb.
1 Btu = 778.26 ft-lb