Static pressure in a fan system is usually expressed in:

In fan systems, static pressure is commonly expressed in inches of water. This measurement is effective for gauging pressure differences in air systems, particularly because it aligns well with the small pressure increments typically encountered in HVAC applications, like those involving air movement and ventilation.

Using inches of water allows for a direct correlation to the characteristics and performance of fan systems, as it provides a convenient way to measure and compare pressure drops across components such as filters, ducts, and grilles. Moreover, this measurement is particularly useful given that air density remains relatively constant in typical building environments, making it a practical unit for engineers and technicians when evaluating system performance.

While Pascals and pounds per square inch are also units of pressure, they are less commonly used in HVAC systems due to their scale and the context in which they are applied. Millimeters of mercury is another pressure unit, but it is primarily used in applications involving fluids or gas measurements in a laboratory setting rather than in day-to-day HVAC practices. Thus, inches of water stands out as the standard and most operationally relevant choice for expressing static pressure in fan systems.