What a Commercial Kitchen Exhaust Fan Has to Handle
A commercial kitchen exhaust fan has to move hot, grease-laden air continuously, against significant resistance, while staying cleanable and keeping its motor out of the grease stream. That is a harsher job than a general building exhaust fan, which is why kitchen fans are built and listed specifically for restaurant service.
Look for a listing to UL 762, the standard for power ventilators for restaurant exhaust appliances. A listed fan has been evaluated for the temperatures and grease exposure of kitchen exhaust, and the listing usually defines how it may be installed, including the curb, the orientation and the accessories. Your inspector will look for the label.
The fan is one part of a larger path. The duct route and discharge location are covered on our commercial kitchen exhaust system page; this page is about choosing and installing the fan itself.
Upblast, Utility Set and Inline Fans Compared
Three fan configurations cover most kitchen exhaust applications. The building and the discharge location usually decide which one fits.
| Fan type | Where it goes | Strengths | Watch for |
|---|---|---|---|
| Upblast centrifugal | On a roof curb directly above the duct | Throws exhaust vertically away from the roof; simple; easy to hinge for cleaning | Limited static pressure range compared with utility sets; needs grease containment |
| Utility set | On the roof, on a platform, or near a wall termination | Handles higher static pressure; discharge can be turned to different directions | Heavier; housing needs drain and cleanout access; more roof space |
| Inline | Within the duct run | Useful where the fan cannot sit at the end of the duct | Must be listed for the purpose and fully accessible for cleaning and service |
Upblast fans
The upblast fan is the familiar mushroom-shaped unit on restaurant roofs. A backward-inclined wheel pulls air up through the curb and discharges it straight up through a top opening, keeping grease and heat off the roof. The motor sits in a separate, ventilated compartment out of the airstream.
Utility set fans
A utility set is a centrifugal fan in a scroll housing, with the motor mounted outside the airstream. It suits long duct runs with high resistance, and its discharge can point up or sideways, which makes it a common choice where exhaust leaves through a wall.
Inline fans
Inline fans sit in the duct itself, typically when the fan has to be indoors or in a mechanical space. Because grease passes through the fan housing inside the building, access for cleaning, drainage and the clearances around the housing need the same attention as the grease duct.
Direct Drive vs. Belt Drive Kitchen Exhaust Fans
Direct drive fans mount the wheel on the motor shaft; belt drive fans connect motor and wheel through pulleys and a belt. Both are used in kitchen exhaust, and the choice affects maintenance, speed adjustment and efficiency.
| Factor | Direct drive | Belt drive |
|---|---|---|
| Maintenance | No belts to inspect, tension or replace | Belts and bearings need periodic service |
| Speed adjustment | By VFD or an electronically commutated (EC) motor | By changing pulleys, adjustable sheave or VFD |
| Size range | Common on small and mid-size fans | Covers larger fans and higher static pressures |
| Efficiency | No belt losses | Some loss through the belt |
| Failure mode | Motor failure stops the fan | A broken belt stops the fan while the motor keeps running |
A belt failure is easy to miss: the motor still hums, but no air moves. If you have belt drive fans, include belt checks in the regular maintenance plan. Direct drive with variable speed control pairs well with demand-controlled ventilation, since the drive is already there.
Sizing a Commercial Kitchen Exhaust Fan to Static Pressure
A kitchen exhaust fan is sized by two numbers: the design airflow in CFM and the total static pressure the system creates at that airflow. The selection point is where the fan curve meets the system curve. A fan chosen at the right CFM but the wrong pressure will settle at a lower airflow than the hood needs.
Selection steps
- Take the total exhaust CFM from the hood design.
- Take the total static pressure from the duct and hood calculation, including an allowance for loaded filters.
- Correct for air density if the site is at high altitude or the exhaust air is very hot.
- Choose a fan whose operating point sits in the stable, efficient part of its curve, not at the edge.
- Check motor horsepower, sound level, outlet velocity and the listing for the intended installation.
The fan laws, briefly
For a given fan and system, airflow changes in proportion to speed, static pressure with the square of speed, and power with the cube of speed. A small speed increase to recover airflow can raise motor load more than expected, so check horsepower before turning a fan up.
Selecting near the middle of the curve leaves room to adjust during system balancing without overloading the motor.
Hinge Kits and Cleaning Access
Kitchen exhaust fans are cleaned along with the duct, and the cleaning crew has to reach the wheel and the duct below the fan. On upblast fans that access comes from a hinge kit, which lets the whole fan tilt back off the curb. NFPA 96 calls for upblast fans to be hinged, with a flexible weatherproof electrical connection and a way to hold the fan open safely.
- Hinge kit: mounted between fan base and curb so the fan swings open on one side.
- Hold-open restraint: keeps the fan from falling back while a technician works under it.
- Flexible wiring: long enough that the fan opens fully without straining connections; the disconnect switch is within reach.
- Clean-out port: on some fans, an access port in the wind band or housing lets the wheel be cleaned without fully opening the fan.
Utility sets and inline fans rely on access panels or removable housings instead. Whatever the type, confirm before installation that the cleaning company can reach every grease-wetted surface. Our hood service and maintenance team sees too many fans that can only be cleaned by disconnecting them.
Grease Containment Around the Fan
Some grease always reaches the exhaust fan, and the roof around it needs protection. Without containment, grease runs down the fan housing and curb, soaks into roofing and becomes both a fire hazard and a cause of roof failure.
Fans usually have a grease drain spout that empties into a collection device. Options include a small grease box mounted to the fan, or a larger rooftop containment system with absorbent filter media around the curb. Containment needs regular service; a full grease box is only marginally better than none.
- Size containment to the cooking volume and the cleaning interval.
- Keep the drain path clear so grease does not back up into the fan.
- Inspect and replace absorbent media as part of each exhaust cleaning.
- Check roofing material compatibility with grease when planning repairs near the fan.
Mounting, Curbs and Electrical Details
A roof mounted kitchen exhaust fan sits on a curb sized to the fan base and built to raise the discharge to the required height. For upblast fans, the outlet is commonly required to be at least 40 inches above the roof surface, which may call for a taller curb or a curb extension.
Curbs are typically ventilated, so heat from the duct does not build up inside the curb walls. Structural support under the curb has to carry the fan weight plus snow and wind loads; utility sets in particular can be heavy enough to need dunnage or framing reinforcement.
Electrical work includes a disconnect at the fan, interlocks with the make-up air unit and fire suppression controls, and any VFD or EC motor control wiring. Fan location relative to air intakes and property lines is set during system design and reviewed by the authority having jurisdiction. When you are ready to price fans or a full system, request a quote with your exhaust CFM, duct route and roof plan.
Kitchen Exhaust Fan Questions
- Why is my kitchen exhaust fan running but the hood is still smoky?
- Common causes are a broken or slipping belt on belt drive fans, filters heavily loaded with grease, a wheel caked with grease, or a fan that was undersized for the actual static pressure. Lack of make-up air can also starve the fan. Check the belt and filters first, then have the airflow measured.
- Can I use a regular roof exhaust fan for my kitchen hood?
- No. A fan serving a Type I grease hood should be listed for restaurant exhaust, typically under UL 762. General building exhaust fans are not built for grease-laden air at kitchen temperatures and do not provide the motor isolation, hinge provisions and grease drainage that kitchen exhaust needs. Inspectors look for the listing label.
- Is a direct drive or belt drive fan better for a restaurant?
- For small and mid-size systems, direct drive with variable speed control is often preferred for its low maintenance and simple speed adjustment. Belt drive remains common on larger fans and high static pressure systems. Either works well when it is selected correctly and maintained on schedule.
- How long does a kitchen exhaust fan last?
- Service life depends on run hours, cleaning frequency, grease load and maintenance of bearings, belts and motor. Fans that are cleaned on the NFPA 96 schedule and have bearings serviced regularly last far longer than fans left to accumulate grease. We do not give a fixed number because conditions vary so widely.
- Does the exhaust fan shut off when the fire suppression system goes off?
- It depends on the suppression system listing, the system design and local requirements. Many designs keep the exhaust fan running during discharge to help remove smoke, while make-up air shuts off. The fan control sequence should be confirmed with the suppression contractor and the AHJ, and documented on the drawings.
- What is a hinge kit and do I need one?
- A hinge kit lets an upblast fan tilt open on its curb so the wheel and the duct below can be cleaned and inspected. NFPA 96 calls for upblast fans to be hinged, with flexible wiring and a hold-open device. If your existing fan has no hinge, one can usually be retrofitted.