22 Pressure Terms

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Static pressure: The force required to move air through various components, such as filters, dampers and diffusers. This represents a resistance to air flow.

Static pressure is the outward force of the air in the ducts and represents a waste or loss of available pressure. Anything that adds air resistance, such as corners in ducting and filters will increase the static pressure.

Velocity pressure: The force exerted by a moving air stream. It is produced in the direction of the moving air. This represents the useful value of pressure that moves air through the ductwork.

Total pressure: The sum of static and velocity pressure inside an HVAC system; the total opposition to airflow in a ducting system.

An air-handling unit will have a maximum value of pressure that it can produce. If static pressure is too high relative to velocity pressure, the fan will be unable to circulate air throughout the ducting system. Most air handling units will have a maximum pressure rating that they are designed to deliver.

In testing HVAC systems, it is sometimes necessary to test the pressure at various places. Direct measurements can give values for static or total pressure, from which the velocity pressure can then be derived.