Greenhouse Ventilation – CFM per m²

Summer ventilation replaces the whole air volume every minute (60 ACH); winter humidity control needs only 2–4 ACH. Volume is floor × average height — for a 9 × 30 m tunnel with 4 m peak and 2.5 m walls, average height ≈ 3.2 m: volume = 864 m³. At 60 ACH: 51,840 m³/h ≈ 30,500 CFM — 10.5 CFM per ft² of floor, right for warm climates. Match inlet area to 1.5× fan outlet area and stage fans with thermostats 2°C apart. Worked check: a single 50-inch box fan (17,500 CFM) plus two 36-inch (2 × 10,700) gives 38,900 CFM for this tunnel at high stage.

1. 1

   Step 1

   Enter dimensions; the tool computes volume with average height.

2. 2

   Step 2

   Pick ACH: 60 summer cooling, 30 shoulder season, 2–4 winter humidity.

3. 3

   Step 3

   Read total CFM and m³/h plus CFM per m² of floor.

4. 4

   Step 4

   Verify inlet area at 1.5× fan area.

• 1

  60 ACH = one air change per minute for summer

• 2

  Inlets ≥ 1.5× fan outlet area, spread along the wall

• 3

  Stage fans 2°C apart to avoid on-off shock

• 4

  HAF fans mix; exhaust fans exchange — you need both

• Computing volume with wall height instead of average height

• Insect mesh on undersized inlets doubling static pressure

• One huge fan instead of staged smaller fans — no low-speed option

• No circulation fans: air stratifies even with correct exchange