Upblast vs Downblast Exhaust Fans: Which One Belongs on Your Roof?
Upblast vs downblast exhaust fans — the difference matters for grease, code compliance, and your neighbors. Here's how to pick the right roof-mounted fan for your kitchen.
Look at the roof of any restaurant and you will see one of two fans: an upblast or a downblast exhaust fan. They look similar from the parking lot, but they do very different jobs — and using the wrong one is a code violation waiting to happen.
Here is the difference, in plain English.
Upblast Exhaust Fans (For Grease)
An upblast exhaust fan is a centrifugal roof fan that discharges air upward and away from the roof surface through a top-mounted cowl or windband. That vertical discharge is critical for commercial kitchens for three reasons:
Grease is thrown clear of the roof. The centrifugal wheel flings grease outward into a collection trough or grease box, and the discharge shoots the remaining vapor high enough that it does not settle on the roof membrane or nearby HVAC intakes.
NFPA 96 requires upward discharge for grease applications. Grease-laden exhaust must be discharged in a way that prevents recirculation and rooftop contamination (NFPA 96 §8.1).
The hinged base opens for cleaning access. Every upblast fan built for grease service hinges up so cleaners can reach the duct riser without disassembling the fan.
Upblast fans are required for any hood over grease-producing equipment — fryers, griddles, charbroilers, ranges.
Downblast Exhaust Fans (For Everything Else)
A downblast exhaust fan is typically a propeller or axial fan that discharges air downward across the roof surface through louvers or a discharge shroud on the side of the fan housing.
Downblast fans are used for:
General building exhaust (bathrooms, warehouse spaces, storage rooms)
Type 2 condensate hoods over dishwashers and steam equipment
Non-grease industrial exhaust
They are not legal for Type 1 grease hood applications in the U.S., and they will fail an NFPA 96 inspection every time.
| Feature | Upblast | Downblast |
|---|---|---|
| Airflow direction | Straight up | Down / horizontal |
| Fan type | Centrifugal | Propeller / axial |
| Grease service | Yes — required | No — code violation |
| Type 1 hood compatible | Yes | No |
| Type 2 hood compatible | Yes | Yes |
| Hinged for cleaning | Yes | Rarely |
| Rooftop grease containment | Built-in trough | None |
| Typical cost | Higher | Lower |
Sizing Both Fans
Regardless of type, exhaust fans are sized by CFM (cubic feet per minute) to match the hood's rated exhaust volume plus static pressure losses through the duct run and any pollution control unit. For a typical wall-canopy grease hood, plan on 150-300 CFM per linear foot of hood, then match a fan curve that hits that CFM at the calculated system static pressure (2021 IMC §507.5).
Undersized fan = smoke rolling into the dining room. Oversized fan = wasted make-up air, higher energy bills, and hood capture problems. Neither is good.
The Bottom Line
Grease-producing equipment → upblast fan. Always.
Steam, heat, or general exhaust → downblast fan is fine.
When in doubt, spec upblast. It is never wrong on a commercial kitchen roof.
Shop the Right Fan for Your Kitchen
Mr. Hood USA carries upblast centrifugal exhaust fans for grease applications and downblast fans for Type 2 and general exhaust, sized from small food-truck applications up through full restaurant cook lines.