Heating & Cooling / Fan calculator

Ventilation sizing

Greenhouse fan calculator.

Q: How much intake shutter area do exhaust fans need?

Plan around 1.7 square feet of intake shutter opening per 1,000 CFM of exhaust, placed on the wall opposite the fans. Undersized intakes choke airflow and make fans work against a vacuum. The calculator converts your exhaust CFM into shutter area and counts of each shutter vent size.

Q: What about circulation (HAF) fans?

Horizontal airflow circulation is sized differently from exhaust: a commonly used rule from extension engineering references is airflow equal to about 25 percent of the house volume per minute, arranged in a racetrack pattern down and back. Circulation evens out temperature and humidity between heating and venting cycles.

Q: Do I need automatic vent openers?

For houses that vent passively, solar or automatic openers lift roof and sidewall vents without power, and motorized shutter kits automate intake shutters on powered systems. Which one fits depends on whether your ventilation is passive or fan-driven; both are linked from this page.

Exhaust CFM, intake shutter area, and circulation airflow from your house dimensions. The same sizing our sales team quotes with.

Exhaust

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Intake shutters

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Circulation

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Volume

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Exhaust fans that cover this CFM

Fan	Rated CFM	Number needed

One full air exchange every 1.5 minutes. Number needed is rounded up; growers usually split large CFM across two or more fans for staging.

Intake shutters for those fans

Shutter vent	Opening ft²	Number needed

1.7 ft² of intake opening per 1,000 CFM of exhaust, on the wall opposite the fans.

Circulation (HAF) fans

Circulator	Rated CFM	Number needed

How the sizing works.

Summer exhaust is sized to change the full air volume of the house every 1.5 minutes: required CFM equals width times peak height times length, divided by 1.5. That target intentionally errs large, because a house that exchanges air fast enough on a July afternoon handles everything milder.

Exhaust fans only move what the intakes let in. Plan 1.7 square feet of shutter opening per 1,000 CFM on the wall opposite the fans, and the fans pull air the length of the house instead of fighting a vacuum.

Circulation is a different job: horizontal airflow (HAF) fans keep air moving when the house is closed up, evening out temperature and humidity. A commonly used rule from extension engineering references sizes HAF at about 25 percent of house volume per minute, laid out in a racetrack pattern, one row down each side.

Common questions

About greenhouse ventilation.

What size exhaust fan does my greenhouse need?

Size exhaust to one full air exchange every 1.5 minutes: required CFM = width × peak height × length ÷ 1.5. A 30 × 96 gothic house works out to roughly 26,600 CFM, which growers usually split across two or more fans on one end wall. The calculator above runs the math for your dimensions.

How much intake shutter area do exhaust fans need?

What about circulation (HAF) fans?

Do I need automatic vent openers?

Size it

Your house

Width peak height comes from the width

Length ft

Crop density affects HAF layout note

Required exhaust

–CFM

Get a ventilation quote

One air exchange per 1.5 minutes · shutters 1.7 ft² per 1,000 CFM

Send us your sizing.

We'll confirm the airflow, match fans, shutters, and controls, and walk through placement. Your numbers come along with the form.

(860) 341-8707 · info@ctgreenhouse.com