Why a fan curve only tells half the story — and which of the three numbers actually belongs in a fan selection.
Every point in a duct system has three pressure numbers attached to it, not one. Two of them — static and velocity — can rise and fall in opposite directions at the very same spot in the duct without anything being wrong. The third, total pressure, is just their sum. Knowing which of the three to hand to a fan curve, and which two are mostly there to help you get to that number, is the difference between a system that hits its design airflow and one that quietly falls short after startup.
Static pressure (SP) pushes outward equally in every direction inside the duct, against the walls, the same way pressure inside a balloon does. It is the component that gets consumed pushing air throughresistance — friction dragging along every foot of duct wall, plus the dynamic losses at every elbow, damper, coil, filter, and diffuser along the way. Static pressure is what is actually "spent" overcoming the system, which is exactly why it is the number engineers calculate, sum, and hand to a fan curve when sizing a fan against a duct system's resistance.
Velocity pressure (VP) exists purely because the air is moving — it is the kinetic-energy signature of the airstream itself, not a force pushing against the duct wall. It rises with the square of velocity, so a duct that narrows and speeds the air up shows a disproportionately larger jump in velocity pressure than the velocity increase alone might suggest.
Total pressure (TP) is simply TP = SP + VP at that same point — the complete mechanical energy the air is carrying there, pushing and moving combined. It is a genuinely different quantity from either one alone, and it behaves differently too: TP only ever falls as air travels downstream, consumed by friction and fitting losses, while SP and VP can trade back and forth locally without TP itself changing much at all.
None of this is a malfunction. A fan does work on the air to raise its total pressure at the fan outlet; from there, as the air travels the duct system, its velocity — and therefore its velocity pressure — changes at every size transition, while static pressure and total pressure each respond differently. Total pressure is the quantity that is genuinely conserved (minus whatever small loss each fitting actually causes); static pressure is the quantity that is genuinely spent doing the work of pushing air through friction and fitting resistance. Reading a single gauge without knowing which of the three it reports is how a perfectly normal narrowing gets mistaken for a leak, a blockage, or a design error.
A fan's job, in energy terms, is to add total pressure to the airstream at its outlet. But once that air is on its way through the duct system, engineers don't track total pressure fitting-by-fitting when sizing the fan — they track static pressure loss, because static pressure is the specific component that is consumed doing the work of pushing air through friction and resistance. Duct friction charts and the equivalent-length method exist specifically to add up these static losses, component by component, straight run by straight run, until they reach the terminal device. That running total — the system's static pressure requirement — is the number plotted against a fan's performance curve to find the fan's operating point at the design airflow. Velocity pressure still matters enormously along the way: it is exactly what a Pitot-tube traverse measures to back-calculate airflow, and it is a required input to the loss coefficients used at elbows, takeoffs, and transitions. It just is not, itself, the number a fan is sized against.
Incomplete, and a real, common calculation error. Total pressure genuinely does represent the complete energy state of the air at a given point — but fan selection is specifically based on the system's static pressure requirement: the resistance the fan has to overcome pushing the design airflow through duct friction and every fitting, filter, coil, and damper in its path. Velocity pressure changes at every single duct-size transition — even ones where the actual loss, and the static pressure drop, are both genuinely small — so treating velocity pressure (or total pressure generally) as the primary fan-sizing reference, instead of calculated static pressure loss specifically, routinely produces a fan selected against the wrong number. Velocity pressure remains essential elsewhere — fitting-loss calculations and Pitot-tube airflow measurement both depend on it — it simply is not the quantity a fan curve is read against.
Explains the three pressure quantities present at every point in a duct system — static pressure (the potential-energy component that does the work of overcoming duct and fitting resistance), velocity pressure (the kinetic-energy component that exists only because the air is moving), and total pressure (their sum) — and clarifies why fan selection is specifically based on the system's static pressure requirement rather than velocity or total pressure alone.
A single duct gauge reading, or a single number quoted for a system's "pressure," hides which of three genuinely different quantities is actually being reported. Static pressure pushes outward against the duct walls and is consumed overcoming resistance; velocity pressure exists purely because the air is moving and rises with velocity squared; total pressure is their sum and represents the complete mechanical energy of the air at that point. At any duct-size transition, static and velocity pressure trade off against each other locally — a narrowing raises velocity pressure and lowers static pressure at that same spot — while total pressure changes only slightly, by whatever the fitting's own loss actually is. Mistaking that local trade-off for an error, or assuming total pressure is the correct number for fan sizing because "it accounts for everything," are both common sources of confusion.
Total pressure equals static pressure plus velocity pressure at any point: TP = SP + VP. Velocity pressure relates to air velocity by VP = (V / 4005)² for standard air, with V in feet per minute and VP in inches of water gauge — so it rises with the square of velocity, not linearly. As air accelerates through a narrowing duct, continuity of mass flow forces velocity — and therefore VP — to rise; since total pressure can only fall or hold roughly steady (apart from the small loss the fitting itself causes), static pressure must give up energy to velocity pressure at that same location. As the duct widens back out, some of that velocity pressure converts back to static pressure, though never completely, because the transition dissipates a portion of total pressure as turbulence.
Fan selection works from a different running total: the system's static pressure requirement, built up component by component using duct friction charts (for straight duct, based on airflow, duct size, and material roughness) and the equivalent-length or loss-coefficient method (for elbows, takeoffs, dampers, filters, and coils). That cumulative static pressure loss, from fan outlet to the final terminal device, is the number plotted against a fan's SP-vs-CFM performance curve to find the fan's actual operating point.
This distinction matters every time a fan or air handler is selected against a calculated duct system: engineers sum static pressure losses specifically, not velocity or total pressure, to arrive at the number compared against the manufacturer's fan curve at the design CFM. Velocity pressure keeps its own, separate role throughout the same design process — it's the input used to calculate velocity-dependent losses at fittings, and it's what a Pitot-tube traverse actually measures in the field to back-calculate real installed airflow when a duct-mounted flow station isn't available. Conflating the three, or assuming total pressure alone is sufficient for fan selection, is a genuinely common design and field-diagnosis error, not just a terminology nitpick.
Static pressure. Engineers calculate the system's total static pressure loss — friction along every straight run plus the dynamic loss at every fitting, filter, coil, and damper — and read that number against the fan's SP-vs-CFM performance curve at the design airflow. Velocity and total pressure are calculated along the way but are not themselves the fan-sizing reference.
Because the same volume of air must move through a smaller cross-sectional area, forcing its velocity — and therefore its velocity pressure, which rises with velocity squared — to increase at that point. Since total pressure only falls gradually along the system (mostly from friction and real fitting losses), static pressure gives up energy to velocity pressure locally to make up the difference. It is a normal energy trade-off, not a leak or a sizing error.
Total pressure is the correct quantity when analyzing the complete energy balance across a fan or a specific component (fan total pressure rise, for instance), but it is not the standard reference for sizing a fan against a duct system's resistance. That job specifically uses the system's static pressure requirement, because static pressure is what the fan has to overcome to push air through the ductwork's friction and fitting losses.
Two main things: calculating the velocity-dependent portion of losses at fittings (elbows, takeoffs, transitions) using loss coefficients, and converting a Pitot-tube traverse reading into an actual airflow rate in the field — VP = (V/4005)² is solved for velocity, then multiplied by duct cross-sectional area to get CFM.
A duct friction chart (or friction loss calculator) gives the static pressure drop per 100 feet of straight duct for a given airflow, duct size, and material. Combined with the equivalent-length method for fittings — which converts each fitting's dynamic loss into an equivalent length of straight duct — this is exactly how the cumulative static pressure requirement is built up for the system, section by section, from fan outlet to terminal device.
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