Greenhouse Cooling Load.
Greenhouse Cooling Load Calculator
Solar heat gain + outdoor + transpiration → cooling load. BTU and ventilation requirements for greenhouse climate control.
Greenhouse cooling strategies
| Strategy | Best for | Limits |
|---|---|---|
| Roof / sidewall vents (passive) | Spring/fall mild climates | Limited cooling capacity at outdoor temps over 80°F |
| Active exhaust fans | Most greenhouses; 10-20°F over outdoor | Only as cool as outdoor air; high airflow needed |
| Evaporative wet-pad cooling | Dry climates (low outdoor RH) | Adds humidity; ineffective in already-humid air |
| Misting / fogging | Most climates as supplement | Adds humidity; nozzle maintenance; canopy wetness |
| Shade cloth | Solar-dominant load reduction | Reduces light to plants; can't fix temp alone |
| Active refrigerated AC | Tightly-controlled crops, greenhouse-as-indoor | Expensive; greenhouses leak; oversized requirement |
| Reflective roof films | Reduce solar absorption | Per-product variation; affects light transmission |
The math, briefly
Greenhouse cooling load combines three sources:
- Solar heat gain: roughly 250-400 BTU/h per ft² of floor area at peak summer sun, modified by glazing transmission and shade cloth.
- Outdoor heat infiltration: ΔT × surface area × U-factor. Greenhouses are leaky; approximated as ~1 BTU/h per ft² per °F differential.
- Plant transpiration (latent load): typically small in greenhouse vs sensible heat; often ignored at sizing-time.
Cooling fans are sized to "1 to 2 air changes per minute" (ACM) — meaning the entire greenhouse volume is exchanged once or twice per minute. Higher ACM gives more cooling but requires bigger fans; 1.5 ACM is a common target.
required_CFM = greenhouse_volume_ft³ × ACM_target
For a 30×20×10 average-height greenhouse (6,000 ft³) at 1.5 ACM: 9,000 CFM exhaust capacity needed. That's roughly 2× 5,000-CFM exhaust fans on a thermostat.
Evaporative cooling math
Wet-pad evaporative cooling drops outdoor air temperature toward the wet-bulb temperature. The maximum theoretical drop:
ΔT_max ≈ (T_dry_bulb − T_wet_bulb) × pad_efficiency where pad efficiency is typically 0.65-0.85.
In dry climates (Arizona, summer dewpoint ~50°F), evaporative cooling can drop incoming air 25-35°F. In humid climates (Florida summer dewpoint 75°F), the same system drops air only 5-10°F because there's nowhere for the wet-bulb to drop to.
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