Testing & Balancing
Airflow measurement, static pressure, velocity pressure, manometers, air balance reports, and acoustical treatment.
- Explain the relationship between static, velocity, and total pressure
- Describe airflow measurement instruments and techniques
- Perform air balance procedures and document results
Lesson 1
Static, Velocity & Total Pressure
The Three Duct Pressures
Every HVAC duct system has three types of pressure. Understanding their relationship is essential for troubleshooting and balancing.
Static Pressure (SP)
Static pressure is the pressure that pushes equally in all directions against the duct walls. It is measured perpendicular to the airflow direction. Static pressure is positive (above atmospheric) on the supply side of the fan and negative (below atmospheric) on the return side. It is measured in inches of water gauge (in. w.g.).
Velocity Pressure (VP)
Velocity pressure is the pressure created by the movement of air. It acts only in the direction of airflow. Velocity pressure is always positive and is calculated from air velocity using the formula:
VP = (V / 4005)^2
Where V = velocity in feet per minute (fpm) and VP is in inches w.g.
The Pressure Equation
Total Pressure = Static Pressure + Velocity Pressure (TP = SP + VP). This fundamental equation governs all duct airflow. As velocity increases in a duct (smaller cross-section), VP rises and SP drops. This is Bernoulli's principle applied to ductwork.
How Pressure Changes in a Duct System
As air moves through a duct system, total pressure always decreases in the direction of airflow due to friction and turbulence losses. At a duct restriction (smaller cross-section), velocity increases, VP increases, and SP decreases. At an expansion (larger cross-section), velocity decreases, VP decreases, and SP increases (pressure recovery).
Total pressure = static pressure + velocity pressure. Static pressure pushes on duct walls in all directions. Velocity pressure acts only in the direction of airflow. Total pressure always decreases downstream due to friction losses.