In HVAC terminology, how is pressure expressed in relation to atmospheric pressure?

In HVAC terminology, pressure is expressed in terms of psig (pounds per square inch gauge) or psia (pounds per square inch absolute). This dual representation is essential in the field because it distinguishes between the pressure measured relative to atmospheric pressure and the absolute pressure that includes the atmospheric pressure.

Absolute pressure (psia) takes atmospheric pressure into account as part of the measurement. For example, if you have a gauge reading of 30 psi, that refers to the pressure above the atmospheric level, hence it is classified as gauge pressure (psig). Conversely, when converted to absolute pressure, you must add the atmospheric pressure (approximately 14.7 psi at sea level) to the gauge reading, resulting in an overall pressure of 44.7 psia in that example.

This differentiation is critical in HVAC applications where varying pressure conditions influence system performance, operation, and safety. Understanding how pressures are expressed helps HVAC technicians accurately diagnose issues and determine the appropriate operational parameters for heating and cooling systems.