Auto A/C Terms - Glossary
Heat Quantity

Heat is measured in different ways. When you use a thermometer, you are measuring the intensity of heat and that is expressed in degrees Fahrenheit or degrees Celsius. The quantity of heat is measured by using BTUs or British Thermal Units.

Technically, a BTU is the amount of heat that is required to raise one pound of water one degree Fahrenheit. Therefore, in order to increase one pound of water from freezing to boiling or from 32 degrees F to 212 degrees F, you would require 180 BTUs of heat. 212 minus 32 equals 180.

What is really important to understand is that although it takes 180 BTUs to increase the temperature of one pound of water from freezing to boiling, it actually takes 5 times that much heat to cause that one pound of boiling water to change state to one pound of vapor or steam. It actually takes about 980 BTUs of heat to cause the one pound of boiling water at 212 degrees F to change state and become one pound of steam at 212 degrees F. Understanding that is a really important point to understand because the liquid refrigerant is actually boiling and changing state to vapor refrigerant in the A/C system, and that is the point where all of the heat is absorbed or transferred.