Design Weather
Use local summer dry-bulb, humidity, solar and altitude inputs appropriate to the crop and operating season.
Climate-Control Sizing
Greenhouse Exhaust Fan Sizing Needs More Than Floor Area
Use house volume, design weather, crop and solar load, inlet pressure and staged controls. A fan count from floor area alone is not a defensible greenhouse design.
GeometryVolume + Length
LoadClimate + Crop
InletArea + Pressure
ControlStages + Backup
Direct Answer
How Do You Size Greenhouse Exhaust Fans?
Establish the design outdoor condition and crop target, calculate a preliminary whole-house flow from a recognized greenhouse method, then verify inlet velocity/pressure, fan performance at installed resistance and the staged control sequence.
Floor area misses ridge height and total air volume; one ACH value misses solar gain, crop transpiration, altitude, shade, evaporative cooling and allowable inside-to-outside temperature rise. Long houses also need a credible path from inlet to exhaust without dead rows or short circuits.
Natural vents may carry mild-weather operation while mechanical exhaust handles hotter or windless conditions. The transition, fan staging, inlet control and failure response belong in the design together.
Engineering scope and sources reviewed 4 October 2026 · Project conditions still govern final selection
Decision Framework
Four Inputs That Common Size Calculators Hide
A calculator output is only as useful as its climate and system assumptions.
Crop And House
Crop density, evapotranspiration, glazing, shade, ridge height and acceptable temperature rise change the load.
Inlet System
Louver, pad, screen and opening area determine pressure and distribution; an undersized inlet starves the fans.
Staged Control
Fans, vents, shade and evaporative cooling must stage in a stable sequence with alarms and failure provisions.
Field Inputs
Greenhouse Inputs To Collect Before Fan Count
Use measured dimensions and a named climate basis rather than a generic online size label.
| Input | Record | Design Consequence |
|---|---|---|
| House geometry | Length, width, gutter/ridge height, bays and partitions | Sets volume, travel length and zones. |
| Envelope and solar | Cover material, shade percentage, orientation and leakage | Changes heat gain and the natural/mechanical balance. |
| Crop condition | Crop, stage, density, target temperature and humidity | Defines acceptable environment and latent load. |
| Outdoor design | Temperature, humidity, altitude and wind basis | Sets fan correction, evaporative potential and design day. |
| Inlet path | Vent/louver/pad area, screen, pressure and distribution | Determines whether rated fan flow can enter and reach each row. |
| Controls/reliability | Stages, sensors, hysteresis, alarms, backup power/venting | Prevents simultaneous instability and protects living inventory. |
Boundary: Greenhouse flow methods are climate and production specific. Confirm current extension guidance, local practice and equipment performance at the installed static pressure.
Practical Sequence
Size The Path From Outdoor Air To Discharge
- Set the crop target.Define acceptable temperature and humidity by crop, growth stage, day/night and season.
- Choose design weather.Document outdoor temperature, humidity, solar, altitude and the desired inside-to-outside difference.
- Calculate preliminary flow.Use a recognized greenhouse method and state every assumption rather than copying one universal ACH.
- Size the inlet.Check opening/pad/screen area, velocity, pressure and distribution across the full house.
- Select installed performance.Use fan data at the real guard, shutter, inlet and system resistance, including altitude/density correction where applicable.
- Stage and verify.Commission sensors, vents, shade, cooling and fan stages; map temperature and humidity across representative crop rows.
Connected Decisions
Use The Next Page Only When It Answers The Next Question
These links continue the same project workflow rather than sending the reader to loosely related content.
Calculate A Preliminary Exhaust Flow
Use the general math only after the greenhouse-specific design basis is chosen.
Understand Make-Up Air
Apply the same full air-balance logic to inlet opening and pressure.
Compare Exhaust Platforms
Review current products while keeping shutter, corrosion, climate and installed pressure in scope.
Review AXE-SQ
Treat the current square exhaust platform as a candidate only after installed duty is defined.
Questions Buyers Actually Ask
Greenhouse Exhaust Sizing Questions
Can I size greenhouse fans by square feet?
Floor area alone omits house height, volume, climate, solar gain, crop, inlet system and acceptable temperature rise. Use a greenhouse-specific method with complete inputs.
What ACH should a greenhouse use?
There is no universal ACH for every greenhouse and season. Use current regional guidance and state the weather, crop, house and cooling assumptions behind the flow.
How large should the air inlet be?
It must pass the design flow at acceptable pressure and velocity through the actual louver, vent, pad and screen. Size it from tested pressure loss and distribution, not opening area alone.
Should fans be installed on one end wall?
End-wall exhaust with opposite inlets is common, but length, obstructions and zones determine whether the path stays uniform. Very long or divided houses need additional analysis.
Can fans cool below outdoor temperature?
Ventilation alone approaches outdoor conditions; it does not normally cool below outdoor dry-bulb. Evaporative or mechanical cooling and humidity limits are separate decisions.
What redundancy is needed?
Living crops make alarms, staged capacity, spare components and an emergency natural-vent or power strategy important. Define the acceptable failure condition with the grower and designer.
Source Boundary
References Used For This Guide
Sources define methods and safety boundaries. They do not certify a proposed AirMoveX model or replace the adopted code, OEM instructions or a qualified project designer.
UMass Extension
Fan Ventilation for Greenhouses
Extension guidance covers mechanical ventilation, inlet sizing and greenhouse operating considerations.
Open sourceUniversity of Georgia Extension
Greenhouse Ventilation
Regional extension material illustrates why climate, inlet and cooling assumptions govern sizing.
Open sourceAMCA International
Fan Performance and System Effect
Supports selecting installed performance at the real system resistance.
Open sourceProject Review
Send The Greenhouse And Climate Inputs
Share geometry, crop and stage, outdoor design condition, cover/shade, inlet or pad, required temperature/humidity, electrical supply and control stages. AirMoveX can review a fan candidate without inventing a universal count.