Sizing a greenhouse exhaust fan

Size the fan to the structure, then size the intake to the fan. Both steps matter: a correctly sized fan behind an undersized intake will not deliver its rated airflow.

• Freestanding greenhouse or high tunnel: multiply length by width by 7 to get your target CFM.
• Gutter-connected: multiply width by length by (gutter height plus 3).
• Intake area: divide your total fan CFM by 600 to get the intake shutter area you need, in square feet.
• Worked example: a 20' x 48' house is 20 x 48 x 7 = 6,720 CFM. The 30" fan delivers 6,770 CFM, so one fan covers it almost exactly, and it needs about 11 square feet of intake shutter on the far wall.

Worth knowing about the 30". It runs at 825 RPM where the 24" runs at 1,725. A larger, slower prop moves more air for each turn, and slower tip speed is quieter. If the fan sits near a work area or a property line, the 30" is the quieter choice even though it moves 46 percent more air than the 24".

Choose your exhaust fan size

CFM is the number that matters most: it's how much air the fan moves per minute. Match it to your structure using the sizing formula below. Rough opening is the hole you need to frame, and it is larger than the nominal fan size on every model.

CFM figures are free-air ratings. Real output drops as the fan works against a dirty shutter or an undersized intake, so give yourself headroom rather than sizing to the exact number.

Specifications

Key benefits

• Shutter is built in: there is no separate shutter to buy, frame, or wire. The louvers open when the fan runs and fall closed when it stops, which holds heat in overnight and keeps wind from driving through the opening.
• One opening, one install: because the fan and shutter are a single unit, you frame one rough opening and mount one assembly. That is the practical difference from a slantwall fan, where the shutter is a separate part.
• Galvanized housing: the housing resists the corrosion that comes with constant humidity and fertilizer salts in a growing structure.
• Direct drive: the blade is driven straight off the motor, so there is no belt to slip, tension, or replace at the start of each season.
• Five useful sizes: from 970 to 6,770 CFM means you can get close to your target without buying two fans where one would do.

Common uses

These are the everyday exhaust fans for hobby greenhouses, market gardens, and small commercial houses: one fan in an end wall, intake shutters opposite, and a thermostat controlling when it runs. They also work well as a second-stage fan in a larger house, where a smaller fan handles mild days and a bigger one runs only when the house climbs past a second set point. If you're building a new high tunnel, ventilation equipment may be NRCS-eligible as part of the project; check with your local NRCS office, since what qualifies varies by state.

How these compare to the slantwall fans

These cover the range most growers need: integral shutter, direct drive on every size, up to 6,770 CFM. The Slantwall Exhaust Fans start where these end: 7,100 CFM at the small end and 21,600 CFM at the top, with an angled housing that sheds snow off the louvers and a separately-mounted shutter. If your house needs more than about 6,700 CFM, or the fan will sit exposed to Northeast winter weather, go with the slantwall. If you're under that, these are the simpler install.

Frequently asked questions

What size exhaust fan do I need for my greenhouse?

For a freestanding greenhouse or high tunnel, multiply length by width by 7 to get your target CFM. A 20' x 48' house works out to 6,720 CFM, which the 30" fan covers at 6,770. For a gutter-connected house, multiply width by length by the gutter height plus 3.

What size rough opening do I need?

Each fan needs an opening 5/8" larger than its nominal size: 12-5/8" for the 12", 16-5/8" for the 16", 20-5/8" for the 20", 24-5/8" for the 24", and 30-5/8" for the 30". Frame to those figures before the fan arrives.

Do I still need intake shutters if the fan has its own shutter?

Yes. The shutter on the fan is an exhaust shutter: it lets air out and closes when the fan stops. You still need an intake on the opposite wall so replacement air can get in. Without it the fan cannot draw its rated airflow. Size intake area at your fan CFM divided by 600, in square feet.

Should I choose this or a slantwall fan?

Go by airflow first. These top out at 6,770 CFM; the slantwall fans run from 7,100 to 21,600 CFM. Below about 6,700 CFM these are simpler and less expensive, with the shutter built in and one opening to frame. Above that, or where the fan faces heavy snow, the slantwall's angled housing and larger sizes are the better fit.

Why is the 30" motor the same horsepower as the 24"?

Both use a 1/2 hp motor, but the 30" turns a larger prop much more slowly, at 825 RPM against 1,725 for the 24". A bigger blade at lower speed moves more air for the same power, which is why the 30" delivers 6,770 CFM to the 24" model's 4,640 while drawing 6.2 amps against 4.8.

Can these run on a thermostat?

Yes. Wire the fan to a greenhouse thermostat and it runs when the house climbs past your set point. Put a Motorized Shutter Kit on the intake shutters and both sides open together, so the fan is never pulling against a closed intake.

You may also need

The rest of a working ventilation setup: