Greenhouse Ventilation Fan Calculator

How it works

Flow = volume × air changes per minute (0.75–1.5 by climate) + 3.5 m³/min per kW of lighting; CFM = m³/min × 35.31; intake area = flow ÷ 2.5 m/s; pad area = flow ÷ 1.27 m/s

A closed greenhouse in sun gains heat faster than any glass can lose it, and the standard cure is to move the entire volume of air through it about once a minute — more in hot, bright or high-altitude sites, less in cloudy climates. Fans are rated in cubic feet per minute, so the volume in cubic metres is multiplied by 35.3, and any grow lights add their heat load on top. The exhaust fan must be matched by enough intake opening at the far end (roughly 1.25 to 1.5 times the fan’s area) or it will simply stall, and placing the intake low and the fan high uses buoyancy. Where summers are dry, an evaporative pad on the intake wall can drop the incoming air by 10 °C or more at the cost of water; its area is set by a face velocity of about 1.3 m/s.

Worked example

A 6 × 3 × 2.4 m hobby greenhouse (43 m³) in a warm sunny climate: 43 m³/min ≈ 1,530 CFM — two 750 CFM fans, with about 0.29 m² of intake vents at the opposite end.

Frequently asked questions