Which unit is used to measure static pressure in a central HVAC duct system?

Which unit is used to measure static pressure in a central HVAC duct system?

• A. PSI
• B. Inches of water column
• C. Pascal
• D. Torr

Answer: (B)

In a central HVAC duct system, the pressure that resists airflow is measured in inches of water column because the differential pressures involved are very small. This unit is purpose-built for duct work, making it easy to relate readings to how hard it is to move air through long runs and fittings. For example, 1 inch of water column equals about 0.036 psi, so readings like 0.25 or 0.5 in. w.c. are common and meaningful for describing how well air is circulating. When measuring, technicians use a pressure tap connected to a manometer or digital gauge to read static pressure in inches of water column. It’s helpful to keep in mind that static pressure is the pressure in the duct walls due to airflow, separate from velocity pressure (which comes from air speed) and total pressure (the sum of static and velocity pressures). Other units such as psi, pascal, or Torr exist, but they’re less convenient for the small, differential pressures typical in duct systems, which is why inches of water column is the standard.

In a central HVAC duct system, the pressure that resists airflow is measured in inches of water column because the differential pressures involved are very small. This unit is purpose-built for duct work, making it easy to relate readings to how hard it is to move air through long runs and fittings. For example, 1 inch of water column equals about 0.036 psi, so readings like 0.25 or 0.5 in. w.c. are common and meaningful for describing how well air is circulating. When measuring, technicians use a pressure tap connected to a manometer or digital gauge to read static pressure in inches of water column. It’s helpful to keep in mind that static pressure is the pressure in the duct walls due to airflow, separate from velocity pressure (which comes from air speed) and total pressure (the sum of static and velocity pressures). Other units such as psi, pascal, or Torr exist, but they’re less convenient for the small, differential pressures typical in duct systems, which is why inches of water column is the standard.