Static and Velocity: Understanding Pressure in a Duct
In a hurry? Skip straight to the numbers.
Open the Duct Static Pressure Calculator →A blower does not simply push air — it fights a resistance measured as static pressure, and the way pressure behaves inside a duct follows a principle discovered centuries ago by Daniel Bernoulli. Grasping the difference between static and velocity pressure demystifies why a system delivers its rated airflow only when the blower can overcome what the ductwork demands.
Two Kinds of Pressure
Moving air carries its energy in two forms. Static pressure is the outward push the air exerts on the duct walls in all directions, while velocity pressure is the energy of the air's motion in the direction of flow. Together they make up the total pressure, and a moving airstream constantly holds both at once.
Bernoulli's Trade
Bernoulli's principle describes how these forms trade off: as a fluid speeds up, its static pressure falls, and as it slows, static pressure rises. Where a duct narrows, air accelerates and static pressure drops; where it widens, air slows and static pressure recovers. The total energy is conserved, merely shifting between the pressure of stillness and the pressure of motion.
| Type | Represents |
|---|---|
| Static pressure | Push against the duct walls |
| Velocity pressure | Energy of the air's motion |
| Total pressure | The sum of both |
The Resistance the Blower Fights
Every duct, elbow, filter, and coil resists the flow, and overcoming that resistance consumes static pressure. The blower must generate enough static pressure to push the required air through all of it; if the total resistance exceeds the blower's rated capacity, airflow falls short no matter how large the equipment's nominal tonnage. This is why a dirty filter or an undersized duct quietly starves a system of air.
Measured in Inches of Water
These pressures are tiny compared with the atmosphere, so they are measured in inches of water column — the height a column of water would rise under that pressure. A residential system commonly operates against a fraction of an inch of total external static pressure, a reminder that ventilation deals in gentle pressures where small added resistances matter a great deal.
Totaling the Pressure
To estimate total external static pressure from duct, fittings, and components, use the Duct Static Pressure Calculator. Standardize the friction figure with the Friction Rate Calculator, and size ducts to keep pressure in check with the HVAC Duct Sizer Calculator.
Ready to Put This Into Practice?
Now that you understand how it works, plug in your own numbers and get an instant, accurate result.
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