How is static pressure described in supply and return ducts?

Static pressure in supply and return ducts is a critical aspect of understanding HVAC systems' operation. In a well-balanced system, the supply ducts carry conditioned air from the air handler to the spaces within a building, creating a positive static pressure relative to atmospheric pressure. This means that the air inside the supply ducts is under a pressure that is greater than the surrounding atmospheric pressure, helping ensure that air flows effectively into the intended areas.

Conversely, in the return ducts, air is pulled back from the occupied spaces to the air handling unit to be reconditioned. This system relies on creating a slightly lower pressure, referred to as negative static pressure, within the return ducts. The negative pressure facilitates the return airflow, as air naturally moves from areas of higher pressure to lower pressure, effectively directing used air back to the air handler.

Thus, the combination of positive static pressure in the supply ducts and negative static pressure in the return ducts works together to create a balanced flow within an HVAC system. This is essential for maintaining the desired indoor air quality and comfort levels in conditioned spaces.