British Thermal Unit

BTU

The amount of heat required to change the temperature of one pound of water, one degree Fahrenheit.

Heat Flow

Heat always flows from a warmer to a cooler substance. What happens is the faster moving atoms give up some of their energy to the slower moving atoms. Therefore, the faster moving atom slows down a little and the slower one moves a little faster.

LAWS OF THERMODYNAMICS

FIRST LAW OF THERMODYNAMICS is a formula for the conversion of heat into work or work into heat. The formula is: 778 foot-pounds of work is equivalent to the heat energy of 1 Btu.

SECOND LAW OF THERMODYNAMICS states that heat will only flow from a body at a certain temperature to another body, which is at a lower temperature. (Heat flows from a warmer to cooler)

So if the outdoor temperature is (example) 40F and the outdoor coil is 10F, heat will be absorbed into the refrigerant in the outdoor coil, then taken inside to reject heat to the conditioned space. This applies to a Heat Pump. In the heating mode.