Example data table
| Scenario | Exhaust | Supply | Infiltration | Recommended makeup | Intake area |
|---|---|---|---|---|---|
| Small tunnel, two fans | 3,600 CFM | 0 CFM | 10% | 3,420 CFM | 6.84 ft² |
| Greenhouse with heater supply | 5,000 CFM | 1,000 CFM | 8% | 3,834 CFM | 7.67 ft² |
| High odor exhaust, slight positive | 8,000 CFM | 0 CFM | 5% | 8,400 CFM | 16.80 ft² |
Formula used
How to use this calculator
- Choose your flow unit and exhaust entry mode.
- Enter total exhaust or fan count and each fan airflow.
- Add any existing supply airflow from heaters or intake fans.
- Set infiltration credit for typical leakage in your structure.
- Use pressure offset for slight positive or negative control.
- Click calculate, then download CSV or PDF if needed.
Air balance and plant performance
When exhaust fans remove humid air, equal replacement air must enter to prevent backdrafts and uneven CO₂ levels. A 20 ft × 48 ft tunnel is about 960 ft²; at 4 air changes per minute it can need roughly 3,800–4,200 CFM, depending on fan curve and static pressure. In hot weather, some houses target 0.75–1.0 CFM per ft².
Estimating total exhaust accurately
Rated fan airflow can drop 10–25% once shutters, screens, and duct bends add resistance. If you have two 1,800 CFM fans, your label total is 3,600 CFM, but 0.05–0.10 in. w.g. can reduce delivered flow. Dirty blades add further loss, so seasonal checks matter. Using measured flow or conservative ratings improves makeup sizing.
Infiltration credit and pressure control
Greenhouses are leaky by design, but leakage varies with wind direction and door use. Many growers start with 5–15% infiltration credit for plastic structures and 2–8% for tighter glazed houses. Add a +2% to +5% pressure offset when you want odor or drift contained, and use negative offsets for exhausting contaminants from potting areas.
Sizing intake openings with face velocity
Intake free area links airflow to comfort: Area(ft²)=Q/V. At 500 ft/min, 4,000 CFM needs about 8.0 ft² of free area. Dropping to 350 ft/min raises area to 11.4 ft², reducing drafts near benches. Louvers and insect screens may only provide 40–60% free area, so gross opening size must be larger.
Safety factor and practical checks
A 5–10% safety factor covers dirty filters, winter winds, and aging belts. After installing, confirm door swing and burner stability; both indicate whether the space is going negative. Track temperature differences across the house; more than 3–5°F suggests short-circuiting between intake and exhaust. If plants show edge burn or cold streaks, distribute intake points, increase free area, or lower fan speed to smooth mixing.
FAQs
1) What is “makeup air” in a greenhouse?
Makeup air is the replacement air that enters when exhaust fans remove air. Without enough makeup air, doors slam, heaters can backdraft, and airflow patterns become weak or uneven.
2) Should I enter rated fan airflow or measured airflow?
Use measured airflow when available. If not, use rated airflow at realistic pressure, then apply a small safety factor. Screens, louvers, and dirty blades often reduce delivered airflow.
3) How do I choose infiltration credit?
Start with 5–15% for plastic tunnels and 2–8% for tighter houses, then adjust after observing door behavior and draftiness. Wind exposure and frequent openings increase effective infiltration.
4) What does pressure offset do?
Positive offset adds extra intake to keep slight positive pressure, which can help contain odors or drift. Negative offset allows slight negative pressure when you want stronger contaminant capture.
5) What face velocity is reasonable for intake louvers?
Many designs use 300–700 ft/min. Lower values reduce drafts and noise but require more opening area. If staff feel cold streaks, reduce velocity or distribute intake locations.
6) Why does the calculator show a free-area requirement?
Free area reflects the open portion after blades, screens, and frames. If a louver is 50% free area, you may need about double the gross opening size to meet airflow targets.