In. W.C in HVAC

When measuring static pressure, the unit of measurement used is inches of water column, which is often shown as an abbreviation such as “in. wc,” “in. wg” or “in. H2O.” One key to interpreting and diagnosing static pressure is to first understand how pressures change throughout an HVAC system.


Static pressure is the amount of pressure a fan has to push and pull against to move air through a duct system. Static pressure is exerted equally on all sides of a duct system. This pressure is similar to that found by blowing up a balloon. When measuring static pressure, the unit of measurement used is inches of water column, which is often shown as an abbreviation such as “in. wc,” “in. wg” or “in. H2O.”

Supply Plenum

A plenum is an air-distribution box attached directly to the supply outlet of the HVAC equipment that heats or cools the air to make the house comfortable. The ductwork that distributes the heated or cooled air to individual rooms of the house connects to the plenum. The HVAC equipment has a blower that forces heated or cooled air into the supply plenum and from the plenum into the rooms through the supply duct system. Air-cleaning filters may be located in the supply plenum.

Return Plenum

Forced-air systems also have return ducts that connect to the HVAC equipment through an air-collection box known as a return plenum. As freshly conditioned air is blown out of the room supply ducts, “used” air from the rooms is sucked into the return ducts. The air from the rooms collects in the return plenum and is drawn back into the HVAC equipment through the return inlet for another round of heating or cooling. Air filters often are located in the return plenum.

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