Enrichment · Cultural Studies · Gardening and food

Enough fan
to keep the plants
from cooking.

Type the greenhouse’s size and site. The calculator starts from the standard 8 cubic feet a minute per square foot of floor, adjusts for elevation, light and how much warming you accept, and counts the fans.

Size the fansWhat you get

The greenhouse ventilation calculator

The worked example is a made-up 24 by 48 ft school greenhouse at 1,000 ft. Nothing you type leaves this browser.

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What you get

Enrichment · Cultural StudiesFree

What you finish with
Summer fan capacity, air changes a minute, number of fans, inlet area and winter rate, with the design factors. It prints as a summary and downloads as a spreadsheet.
Who it is for
School garden and agriculture teachers, horticulture and FFA students, and hobby growers.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The Cultural Studies space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Measure the floor and estimate the average height.
  2. Type the site’s elevation.
  3. Keep 5,000 footcandles and 7 °F unless you have reasons to change them.
  4. Type a fan’s rated cfm at 0.10 in static pressure.
  5. Read the fans and inlet area.

Size the fans

How the fans are sized

Floor area, three factors, then fans.

The worked example: a 24 by 48 ft school greenhouseFloor1,152 sq ft24 × 48 ftBase rate9,216 cfm× 8 cfm per sq ftFactors× 1.041,000 ft elevationFans2 × 5,0009,585 cfm neededThe worked example: a 24 by 48 ft school greenhouseFloor1,152 sq ft24 × 48 ftBase rate9,216 cfm× 8 cfm per sq ftFactors× 1.041,000 ft elevationFans2 × 5,0009,585 cfm needed
Ventilation is sized by floor area, because the sun heats the floor; volume only tells you how often the air turns over.

Why floor area

On a summer day the sun pours roughly the same heat onto each square foot of a greenhouse floor, whatever the roof height. The standard design rate removes it with 8 cubic feet of air a minute per square foot, which holds the rise from inlet to fan to about 7 °F at sea level under 5,000 footcandles.

The factors

At higher elevation air is thinner and carries less heat per cubic foot, so you need about 4 percent more air per 1,000 ft. Brighter light means more heat; a smaller allowed temperature rise means more air. Shade cloth or whitewash lowers the light factor, which is often cheaper than another fan.

Sources

  1. National Greenhouse Manufacturers Association. Standards for ventilating and cooling greenhouses (ngma.com).
  2. Penn State Extension. Greenhouse ventilation and cooling (extension.psu.edu).
  3. Rutgers Cooperative Extension. Bartok, J. W. Greenhouse engineering: fan ventilation (njaes.rutgers.edu).
  4. USDA Natural Resources Conservation Service. Conservation practice standard 325, high tunnel system (nrcs.usda.gov).

The worked example is made up. Check the fan maker’s data at your static pressure before buying. Your entries stay in this browser. Support: team@caveatacademic.com.