How Centrifugal Exhaust Fans Work — and How to Pick the Right One
Quick Specs
| Airflow Capacity | 100–50,000+ CFM |
| Static Pressure Range | 0.5–15 in. w.g. |
| Motor Power | 1/4 HP – 50 HP |
| Voltage Options | 115V / 208V / 230V / 460V |
| Blade Types | Forward-curved, backward-curved, radial |
| Mounting Options | Roof (upblast/downblast), wall, inline duct |
| Common Wheel Diameters | 8″ – 60″ |
| Drive Types | Direct drive, belt drive |
A centrifugal exhaust fan is a piece of equipment that extracts contaminated and/or stagnant air from a building by spinning an impeller wheel inside a scroll-shaped housing. As opposed to propeller style exhaust fans that push through air in a linear fashion, a centrifugal model turns the airflow 90° and creates enough static pressure to push air through lengths of duct, water- or oil- soaked filters, and are even equipped for hoods. That static pressure advantage is why you’ll see centrifugal exhaust fans used in commercial kitchens, laboratories, and industrial ducting where resistance is a fact of life.
This page unpacks the working principles of centrifugal exhaust fans, compares the four main mounting types, and runs through a CFM calculation to size the right machine for your space. Whether you’re refurbishing a clapped out roof exhaust fan or designing a new facility, the information and spreadsheet below will allow you to scientifically select the best fan for your needs.
What Is a Centrifugal Exhaust Fan?
A centrifugal exhaust fan uses a wheel spun by a motor (that can be directly linked or “belt-driven”) to create centrifugal force. This impeller wheel draws some air into itself along its axis (i.e. parallel to the fan’s shaft), then flings it out at 90° into the housing (scroll), where it is decelerated and the rotational energy is converted into static pressure before leaving the fan body.
“Centrifugal” refers to this radial acceleration—using the rotation of the impeller wheel to “throw” the air outward. These forces allows the user to generate static pressures as high as 15+ in. of water gauge, far exceeding the capabilities of axial fans at similar sizes.
📐 Engineering Note
The Air Movement and Control Association (AMCA) Standard 210 provides the standardized means for testing all sorts of centrifugal fan specifications—including pressure, airflow, power use, and efficiency—so you can compare different fan brands apples to apples. Be sure that any CFM information you use in sizing comes from testing per AMCA210; this standard defines normal baselines and ensures equal comparisons.
How a Centrifugal Exhaust Fan Works: Blade Types, Motor and Airflow Path
Understanding the composition of centrifugal fan starts with three basic pieces: an impeller wheel with lifting blades, a propped-up motor (belt-driven or directly driven), and a housing, usually made of steel or aluminum. The motor turns the blades, the blades accelerate the air outward, and the scroll collects the exhaust and directs it. Beyond impeller styling, one important decision exists: impeller blade design – each of which has trade-offs on efficiency and the kinds of airstreams the fan can pull.
Three impeller blade styles dominate across industries:
| Blade Type | Peak Efficiency | Pressure Capability | Best Application |
|---|---|---|---|
| Forward-Curved (FC) | 60–75% | Low–Medium (up to ~5 in. w.g.) | Clean-air HVAC, residential ventilation |
| Backward-Curved (BC) | 80–85% | Medium–High (up to ~15 in. w.g.) | Industrial exhaust, high-efficiency systems |
| Radial / Paddle | 50–70% | Medium–High | Dust-laden air, material handling |
Centrifugal backwards-curved wheels use airfoil, backward-curved blades that are the most efficient type known to the fan community. Per the sources in the U.S. Department of Energy’s Improving Fan System Performance sourcebook, airfoil backward-curved impellers are the most efficient to operate—by volume, pressure, and energy. Forward-curved wheelsmove higher volumes of air at lower pressure, with lower efficiency,—so they are sensitive to dust and debris and should only be used in clean-air applications.
We have five guides for the different blades:
- Backward Curved Centrifugal Fan
- Forward Curved Centrifugal Fan
- What Is an Airfoil Fan?
- What Is a Radial Fan?
- Backward Curved vs Forward Curved Centrifugal FansFan Efficiency Grades (FEG), which are defined in the AMCA Standard 205-10, put a number on how good a fan is—from FEG 1 (worst) to FEG 90 (best). Most backward-curved fans perform at or above FEG 67. When energy codes call for the minimum efficiency of a fan, as ASHRAE 90.1 does, they are pointing to these FEG numbers.
Centrifugal fans’ use in HVAC applications is on the rise. Researchers have found that combining high-efficiency centrifugal fans with variable speed controls can cut the overall energy footprint of HVAC systems by between thirty and fifty percent, over equivalent systems using fixed speed fans.
Being near your fan curve’s most-efficient point also means the fan is as quiet as possible. Anything else—oversizing, damper. Even installing a smaller fan in a larger unit—causes the same issues: more drag, more energy use, more noise.
Types of Centrifugal Exhaust Fans — Upblast, Downblast, Inline and Sidewall
It is essential to identify, and know when it is appropriate to use, the different types of centrifugal exhaust fans that are mounted in different ways. Each mounting configuration exposes the fan to a different building code requirement and might need a different airflow path—placing the wrong type of fan in your system can mean failed code inspections or poor airflow performance.
| Type | Mounting | Discharge Direction | Typical CFM Range | Primary Application |
|---|---|---|---|---|
| Centrifugal Upblast | Roof curb | Vertically upward | 500–30,000 CFM | Kitchen grease exhaust, fume hood exhaust |
| Downblast | Roof curb | Downward toward roof | 300–15,000 CFM | Clean air general ventilation |
| Inline Duct | Inside ductwork | Along duct axis | 100–5,000 CFM | Bathrooms, utility rooms, retrofit projects |
| Sidewall | Exterior wall | Horizontally outward | 200–10,000 CFM | Warehouses, parking garages, storage rooms |
Upblast Exhaust Fan
Upblast centrifugal exhaust fans are designed to eject the air stream vertically away from the building. They are the only option for commercial kitchen ventilation when the recipe calls for grease to be kept away from the roof membrane. Electric motors are mounted externally to the airstream—extending motor longevity in congested, hot, greasy environments. Both belt drive and direct drive upblast units are on the market. The standard exterior shell material is spun aluminum or stainless steel construction, finished with a weather-proof coating for durable outdoor service.
Downblast Roof Exhaust Fan
Downblast fans direct the airflow downward where it enters the building’s roof space. They are only suitable for applications where the exhaust air is free of grease. Because the ductwork and sound may be directed downward—sometimes downward out of a window—occupants on the floor below benefit from the quiet operation that downblast fans provide.
Inline Centrifugal Fan
Inline centrifugal exhaust fans are located directly inside the duct itself and are therefore ideal for building renovations where there is no way to penetrative the roof. An inline EC centrifugal fan has a variable speed drive, with a 30-50% energy savings compared to a fixed speed AC fan.
Sidewall Exhaust Fan
Wall-mounted centrifugal exhaust fans are often selected where roof access is restricted or where its horizontal discharges is desirable. These units can be found in shopping malls’ parking ventilation, storage and warehouse exhausts, and general ventilation across huge, open spaces. A centrifugal blower variant—like the direct drive horizontal configuration available on some models—are a good option where high static pressure against duct runs is needed.
Important
The biggest specification mistake is accidentally installing a downblast instead of an upblast exhaust fan— or vice versa. If local fire standards demand downward exhaust flow, you must consult the building code beforehand to make sure grease is not present to avoid making the wrong choice.
Centrifugal vs Axial Exhaust Fans
Centrifugal or axial — which exhaust fan fits your project? It comes down to resistance—how much work does the fan have to do to force air through this system? If ductwork, filters, hoods, or dampers are part of the design, a centrifugal model will be more efficient. If moving large amounts of air out into a big open place, with nothing in the way, deal with simple—go axial. Read more on the differences between centrifugal and axial fans.
| Parameter | Centrifugal Exhaust Fan | Axial Exhaust Fan |
|---|---|---|
| Static Pressure | 0.5–15 in. w.g. | 0.1–1.5 in. w.g. |
| Airflow per Unit Size | Moderate (pressure-optimized) | High (volume-optimized, up to 5,000+ CFM) |
| Noise at Design Point | 55–75 dB(A) (quieter operation at peak efficiency) | 60–85 dB(A) (rises against back-pressure) |
| Duct Compatibility | Designed for ducted systems | Best for free-air or short duct runs |
| Initial Cost (comparable CFM) | $300–$5,000+ | $100–$2,000 |
| Energy Efficiency (FEG) | FEG 50–85 (BC highest) | FEG 40–65 |
✔ Centrifugal Advantages
- Handles high static pressure (ducted systems, filters, hoods)
- Quieter at design-point operation than axial fans under back-pressure
- Available with grease-rated housing for kitchen exhaust
- Motor can sit outside the airstream (longer motor life)
⚠ Centrifugal Limitations
- Higher initial cost than axial fans of comparable CFM
- Bigger physical footprint – may not squeeze into tight ceiling plenums
- Belt drive models need to be checked for tension quarterly, and belts changed on a regular basis
- Heavier weight demands reinforced mounting (roof curb or structural steel)
Bottom line: if your system exceeds 1 in. w.g. static pressure, centrifugal exhaust fan is the wise choice. If not, an axial fan can save up to 50% on equipment costs.
Where Centrifugal Exhaust Fans Are Used
Centrifugal exhaust fans are used whenever contaminated, heated, or greasy air needs to be drawn through a duct system. The following are the common applications, which all have code-driven air change rate requirements that make use of them.
| Application | Required ACH | Fan Type Typically Used | Governing Code / Standard |
|---|---|---|---|
| Commercial Kitchen | 15–20 ACH | Centrifugal upblast (grease-rated) | IMC Section 507, NFPA 96 |
| Laboratory / Fume Hood | 6–12 ACH | Centrifugal upblast or inline | ANSI Z9.5 |
| Parking Garage | 6–8 ACH | Sidewall centrifugal or inline | IMC Section 404 |
| Industrial Paint Booth | 50–100+ ACH | Centrifugal with explosion-proof motor | NFPA 33, OSHA 29 CFR 1910.94 |
| General Office HVAC | 4–6 ACH | Inline centrifugal or downblast | ASHRAE 62.1 |
Commercial kitchen installations account for the largest share of centrifugal exhaust fan deployments. Upblast centrifugal exhaust fans exhaust grease-laden vapors vertically away from the building, and must have a UL 762 listing for grease duct systems. Typical restaurant kitchen exhaust fans push 1,500 to 2,500 CFM for one hood line, and larger restaurants require systems exceeding 10,000 CFM.
Another expanding application area for centrifugal fans is in fan cooling for industrial control panels. Mobile trucks use small centrifugal fans in their electrical panels to keep components cool to prevent heat-related failures.
In commercial kitchens, make-up air should be about 90% of the exhaust airflow. The other 10% comes from around the shop; this slight negative pressure in the kitchen prevents cooking odors from migrating out to the seating area.
How to Select the Right Centrifugal Exhaust Fan — CFM, Pressure and Sizing
Fan size is a matter of two numbers: airflow required, and static pressure of the system. Use the wrong number and your fan won’t be adequate or you’ll overspend.
Step 1: Calculate Required CFM
Start with this formula:
Engineering Note
CFM = Room Size (ft) x Required ACH 60
Calculation: if a restaurant measures 40 feet by 30 feet with a 10 foot ceiling, the volume is 12,000. Using an automatic kitchen ACH of 15, the air handling requirement is 12,000 x 15/60, or 3,000 CFM.
Guidelines: ASHRAE 62.1 offers minimum requirements; IMC Section 507 is the code that applies to specific ACHs in kitchens.
Step 2: Determine System Static Pressure
Static pressure includes all friction losses through ductwork, filters, hoods, and fittings. According to SMACNA duct construction standards, pure galvanized steel duct (8 foot runs) has a friction loss of about 0.08 in. w.g.., per 100 feet. Obtain static pressure increases due to elbows (0.05-0.15 in. w.g., per elbow), filters (0.1-0.5 in. w.g.), and grease hoods (0.3-0.8 in. w.g.).
Step 3: Select Blade Type
Blade geometry should be selected to match the desired air quality: backward-curved for high-efficiency systems that deliver clean air, radial for dusty or material-handling systems, forward-curved for HVAC systems with low static pressures. All three types are available for the AC centrifugal fan, which has a wide variety of voltages and sizes.
Step 4: Choose Drive Configuration
Direct drive fans mount the wheel directly to the motor shaft, which means fewer moving parts and cheap maintenance, and in most cases, a fixed speed because they are not tuned to a variable frequency drive (VFD). Belt drive fans use pulleys and V-belts that are capable of changing the pulley ratios to tune the airhandling speed, and consequently do need quarterly belt tension checks and periodic belt changes.
Step 5: Verify Motor and Voltage
Select motor horsepower equal to the brake HP of the fan at the operating point. The typical voltages are 115 V (single phase, small fans), 208 and 230 V (single or three phase), and 460 V (three phase, large industrial fans). Fan with VFD, when operating with variable speed control, can decrease energy consumption by 20-50% at part load operation.
Step 6: Confirm Code Compliance
Check the fan has the correct listings:UL 762 for grease-laden exhaust, UL 705 for power ventilators, and any local fire code requirements for spark-resistant construction (AMCA Type A, B, or C).
- ✔
Calculate CFM using room volume × ACH ÷ 60 - ✔
Total static pressure losses (dynamics + fittings + filters + hoods) - ✔
Choose blade type for desired airquality and efficiency. - ✔
Determine the number of direct drives and belt drives you’d need depending on cost of maintenance and desired speed - ✔
Check motor HP, voltage, and phase against your electrical service - ✔
Recommend UL listing and AMCA spark resistant class for your application.
Oversizing a centrifugal exhaust fan by 30% of more above the CFM calculation increases annual energy costs 15-25% with no benefit to ventilation effectiveness. ASHRAE 90.1 energy code penalizes systems that run well off design point. Always size to calculated load, never “just in case”.
Installation, Maintenance and Troubleshooting
Whether or not a centrifugal exhaust fan will run for 15 years or quit after 5 depends on the installation and scheduled maintenance. The three most common field failures (bearing seizure, belt slippage, and motor overload) are all readily preventable with minimal maintenance.
Installation Fundamentals
Place the fan on an even, solid footing. Reinforced concrete makes the ideal foundation, although structural steel frames are suitable with the fan is mounted on the roof. When shimming the fan base remove all voids in the interface between the foundation and the base plate—any “daylight” in the interface causes vibration stress to the bearings and limit shaft life.
Couple the ductwork to the inlet and discharge of the fan with flexible connections to decouple the fan from the duct system.
Maintenance Schedule
| Task | Frequency | Why It Matters |
|---|---|---|
| Belt tension check (belt drive only) | 2 weeks after install, then quarterly | Over-tension wears bearings; under-tension causes slippage and uneven belt wear |
| Bearing lubrication | Every 2 months | Prevents seizure; over-lubrication causes oil leakage |
| Vibration analysis | Semi-annually | Detects bearing wear, imbalance, or misalignment before failure |
| Wheel and housing cleaning | Semi-annually (monthly for grease exhaust) | Buildup on blades causes imbalance and reduces airflow |
| Motor amp draw check | Annually | Rising amps indicate increased load from buildup or mechanical drag |
Troubleshooting Common Issues
When troubleshooting a centrifugal exhaust fan, start with these checks:
- Low airflow: Inspect for blocked duct, clogged fan filter and Damper (closed). Ensure the CFM by conducting a pitot tube traverse of the fan inlet.
- Too much vibration: check the wheel balance, bearing condition, and foundation bolts. Use a vibration analyzer and compare the results with the manufacturer baseline data.
- High noise: Confirm the fan is operating near its design point on the fan curve. Off-design operation — especially at high static pressure with a forward-curved wheel — increases noise by 10–15 dB(A).
- Motor tripping overload: Measure amp draw. If above nameplate rating, check for wheel buildup, blocked discharge, or incorrect belt tension.
Contractors who skip the static pressure test after installation risk a 10–20% airflow shortfall. Always verify actual system static pressure with a manometer reading at the fan inlet before signing off on the job.
If you are still comparing this fan type with other factory cooling, cabinet cooling, HVAC, dust collection, or fume extraction options, see our cooling fan application selection guide.
Frequently Asked Questions
Q: What is a centrifugal exhaust fan?
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Q: What are the disadvantages of centrifugal fans?
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Q: Is a centrifugal fan better than an axial fan?
View Answer
Neither type is better – it just depends on the system resistance. Centrifugal fans maintain constant flow rate at between 1-15 in. w.g. of static pressure, which makes them the right choice for ducted systems with filters or hoods and longer duct runs. Axial fans are less expensive and high-volume fans for low static pressure, hence are appropriate for open space ventilation, free-air rooftop exhaust, and short duct runs (under 1 in. w.g.).
For a duct system producing over 1 in. w.g. static pressure total, go centrifugal.
Q: Do centrifugal exhaust fans make noise?
View Answer
Yes. Noise levels differ depending on blade type and operating point. Backward-curved fans usually generate 55-65 dB(A) at the design point.
Forward-curved are in the range 60-75 dB(A). Using a fan away from its best efficiency point (by oversizing or throttling) will add another 10-15 d B(A). For noise-sensitive applications specify backward-curved blades and operate the fan near the middle of the performance curve.
Q: How do I choose the right size centrifugal exhaust fan?
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Q: Why is my centrifugal exhaust fan not working?
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Begin at the beginning with all the basics: is the power getting to the motor, is the disconnect switch turned on, and is the overload relay tripped?
If the motor operates but airflow is inadequate, check for sections of duct work that are blocked, filters that are plugged, dampers that are closed, or if the rotation has reversed directions. For units powered by belts, look for a belt that has broken or slipped. If a fan is vibrating badly, the fan wheel may be out of balance, accumulated grease or debris on the blades. clean the blades and check the operation again.
Need Help Selecting a Centrifugal Exhaust Fan?
Proventofan engineers can select the correct fan type, blade geometry and motor configuration to suit your ventilation application. Receive application specific recommendations based on your CFM and static pressure requirements.
About This Analysis
This guide was developed by the Proventofan technical team, which works on centrifugal and axial fans for HVAC, industrial ventilation, and cabinet cooling applications. The CFM calculations and static pressure references are grounded in published standards of ASHRAE, AMCA, and SMACNA – not proprietary testing – so you can double check for yourself.
References & Sources
- AMCA Standard 210/214 — Laboratory Methods of Testing Fans — Air Movement and Control Association International
- Improving Fan System Performance: A Sourcebook for Industry — U.S. Department of Energy / NREL
- AMCA Standard 205-10: Fan Efficiency Grades — Air Movement and Control Association International
- ASHRAE Standard 62.2 — Ventilation and Acceptable Indoor Air Quality — American Society of Heating, Refrigerating and Air-Conditioning Engineers
- OSHA Ventilation Standards (29 CFR 1910.94) — U.S. Department of Labor
- Fan and System Curves with Fan Energy Index — Air Movement and Control Association International
