If you’ve ever stood next to a running centrifugal fan in a warehouse, HVAC unit, or industrial process line, you might’ve noticed a low, constant hum and a steady flow of air moving through the ductwork. As someone who’s spent the last 12 years selling centrifugal fans for a living—working side-by-side with plant managers, HVAC technicians, and design engineers—one question comes up more than any other: “What’s the total pressure of a centrifugal fan, and why does it matter?” It’s not a trick question, but you’d be surprised how many people mix up static pressure, velocity pressure, and total pressure, which ends up leading to fans that are too small, underperforming, or even broken within months. Let’s break this down like I do when I’m sitting across from a customer at their facility, not just throwing engineering jargon at them. Centrifugal Fan
First, let’s start with the basics: pressure, in fan terms, is just a measure of force per unit area that air exerts as it moves through a system. Every part of that moving air has two types of pressure, and total pressure is just the combination of both. The first is static pressure. That’s the pressure you’d measure if you held a small sensor so it’s parallel to the airflow—like sticking a straight tube into a duct wall. Static pressure is the force that pushes air against the sides of ducts, filters, coils, or any other component in your system. If you’ve ever felt a duct warm or cool when a fan’s running, that static pressure at work is why. The second is velocity pressure, which is the pressure from the actual movement of air. It’s measured with the sensor facing directly into the airflow, catching the speed of the moving air. Think of it like sticking your hand out of a car window on the highway—the push you feel is velocity pressure, not the static pressure of the air around you. Total pressure is the sum of these two, plain and simple: Total Pressure = Static Pressure + Velocity Pressure. That’s the core formula, but the context of what that number means for your operation is where the real value comes in, especially when you’re sizing a centrifugal fan.
I learned this the hard way early in my career. Back in 2012, I sold a small 24-inch centrifugal fan to a food processing plant in the Midwest. The customer needed to pull stale air out of their mixing room and push it through a 50-foot duct to an outdoor exhaust. They told me they needed “2 inches of static pressure,” which is a common way people reference pressure, so I sized the fan accordingly. A month later, they called me furious—their fan was making a racket, the air flow was barely half what they expected, and they thought I’d sold them the wrong unit. When I drove out there to check, I pulled a manometer (the tool we use to measure pressure) and tested the system. The static pressure was indeed 2 inches, but when I checked the velocity pressure at the fan outlet, it was another 0.8 inches. That meant the total pressure the fan had to deliver was 2.8 inches, not the 2 inches the customer had quoted me. The fan I sold only maxed out at 2.2 inches of total pressure, so it was working way too hard and couldn’t keep up. We swapped it for a slightly larger model that delivered 3.2 inches of total pressure, and they’ve been happy customers ever since. That story stuck with me—total pressure isn’t just an engineering term; it’s what makes or breaks a fan that works for you or one that becomes a costly headache.
Now, why is total pressure the right number to focus on when selecting a centrifugal fan, instead of just static pressure? Let’s go back to that formula. When air moves through a system, every component adds resistance. Filters add drag, coils slow air down, bends in ductwork create turbulence, and the length of the duct itself adds friction. All of that resistance shows up as static pressure you have to overcome, and then you also have to account for the pressure required to move the air at the speed your system needs. If you ignore velocity pressure, you end up with a fan that can’t generate enough total pressure to push air all the way through your system, which is exactly what happened with that food plant. Centrifugal fans are designed to generate specific total pressure curves—this is the graph that shows how much pressure a fan can deliver at a given airflow rate, usually measured in cubic feet per minute (CFM). For example, a forward-curved centrifugal fan (common in HVAC systems) will have a different total pressure curve than a backward-inclined or airfoil centrifugal fan (used for high-volume industrial applications). Forward-curved fans deliver lower total pressure, but they’re quieter and more efficient for low-pressure HVAC loads, while airfoil fans deliver high total pressure for heavy industrial tasks like conveying dust or moving large volumes of air through long duct runs. The key is matching your required total pressure at your desired CFM to the fan’s performance curve.
Let’s talk about how we measure total pressure in real-world settings, because that’s what I do every week when I’m sizing a fan. To get accurate numbers, you need a pitot tube and a manometer—two tools that every good HVAC technician or process engineer should have in their truck. The pitot tube has two openings: one facing into the airflow to capture velocity pressure, and one parallel to the airflow to capture static pressure. When you connect both ends of the manometer to the pitot tube, it automatically calculates the total pressure by adding those two together. The best place to measure total pressure is at the fan outlet, right after the air leaves the impeller, because that’s where the fan is delivering its full pressure. You can also measure at the fan inlet, but you have to account for any pressure loss from the inlet ductwork, so the outlet reading is more reliable for sizing.
Another common question I get: do I need to account for atmospheric pressure when calculating total pressure? The short answer is yes, especially for large industrial fans operating at high altitudes. Atmospheric pressure decreases as you go up in elevation—for example, at 5,000 feet above sea level, atmospheric pressure is about 15% lower than at sea level. Air at higher altitudes is less dense, so it exerts less pressure, which means your centrifugal fan has to work a little harder to move the same volume of air, and the total pressure it delivers will be lower at the same CFM. I had a customer in Denver a few years back who was installing a dust collection system for a metal stamping plant, and he tried to use sea-level total pressure specs from a fan manufacturer. He called me in a panic because the fan wasn’t moving enough air, and it turned out he needed a fan rated for high altitude to make up for the lower atmospheric pressure. That’s a detail that trips up a lot of people who don’t account for where their fan will operate, not just what it’ll be moving.
Let’s also clear up a myth I hear all the time: “Higher total pressure means a better fan.” That’s not true at all. A fan that’s designed to deliver 10 inches of total pressure is overkill for a system that only needs 2 inches. Running a fan at a pressure higher than your system requires is a waste of energy—fans use a lot of electricity, and operating at the wrong point on the performance curve can reduce efficiency by 20% or more. That’s why we never just sell a fan based on its max total pressure; we work with the customer to map out their exact system requirements: how much air they need (CFM), how much resistance their ductwork and components create (static pressure), and the altitude and air density at their location. All of that goes into calculating the exact total pressure the fan needs to deliver to run efficiently and reliably.
Let’s use a real example to make this concrete. Suppose you have a warehouse that’s 10,000 square feet, and you need to replace the stale air 6 times an hour to keep the air quality safe. That works out to about 60,000 CFM of air flow. Your duct system has a total of 100 feet of straight duct, two 90-degree elbows, a MERV 13 filter, and a heating coil. When you calculate the static pressure loss from all those components, it adds up to 1.8 inches of static pressure. The velocity pressure at the fan outlet will be about 0.4 inches, so your total required pressure is 1.8 + 0.4 = 2.2 inches of total pressure. A backward-inclined centrifugal fan with a performance curve that delivers 60,000 CFM at 2.2 inches of total pressure is the right fit. If you bought a fan that maxed out at 3 inches of total pressure, you’d be paying more for the fan and more in electricity bills every month, even if it works. If you bought a fan that only maxed out at 1.9 inches of total pressure, it would never move enough air, leading to poor air quality and even moisture buildup in the warehouse.
So, why does all this matter for you, whether you’re a plant manager, HVAC technician, or business owner? Getting the total pressure right for your centrifugal fan means lower energy bills, fewer breakdowns, longer fan life, and better performance for your operation. A fan that’s sized incorrectly because of a miscalculation in total pressure can cost thousands of dollars in repairs, lost productivity, and wasted energy over its lifespan. I’ve seen customers skip this step to save a few bucks upfront, only to spend 10 times that amount in the next two years replacing a fan that was the wrong size. That’s why when I work with a new customer, I don’t just pull a fan spec sheet and send it their way. I ask questions: How long is your duct run? What components are in your system? Where is your facility located? How much air do you need to move? I’ll even offer to come out and measure their existing system if they have an old fan, so I can get accurate total pressure numbers before recommending a new unit.
I also want to mention that total pressure is different for different types of centrifugal fans, depending on their design. Forward-curved impeller fans are smaller for the same CFM, but their total pressure curve peaks at a lower point, and they’re more prone to overloading if you run them at too high a pressure. Backward-inclined and airfoil impeller fans have flatter total pressure curves, meaning they deliver more consistent performance over a range of airflow rates, and they’re more efficient for higher pressure applications. That’s why I usually recommend airfoil fans for industrial dust collection or large HVAC systems, and forward-curved fans for smaller, low-pressure applications like rooftop units. The impeller design directly impacts how much total pressure the fan can generate, so that’s another key factor to consider when selecting a centrifugal fan.
Axial Fan At the end of the day, total pressure is the backbone of centrifugal fan performance. It’s the number that connects the fan’s design to your system’s needs, and getting it right is the difference between a fan that works for you and one that becomes a headache. I’ve been in this business long enough to know that buying a fan isn’t just about picking the cheapest unit you can find. It’s about understanding what your system needs, and making sure the fan can deliver the right total pressure to get the job done efficiently. If you’re in the market for a new centrifugal fan, or you’re not sure if your current fan is delivering the right total pressure, I’d be happy to talk through your requirements with you. We can work together to measure your system, size the right fan, and make sure you get a unit that’s reliable, efficient, and fits your exact needs.
References
- ASHRAE Handbook – HVAC Systems and Equipment, American Society of Heating, Refrigerating and Air-Conditioning Engineers
- Centrifugal Fan Application and Selection Guide, Air Movement and Control Association (AMCA) International
- Fan Engineering: Design, Selection, and Installation, The Fairmont Press
- Industrial Ventilation: A Manual of Recommended Practice, American Conference of Governmental Industrial Hygienists (ACGIH)
- Pressure Measurement in Fluid Flow Systems, ISO 5167:2003
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