Quick answer: The number of greenhouse dehumidifiers is N = design moisture load ÷ effective capacity per unit, rounded up. Both numbers have to be at the greenhouse's own condition and period. The design load is the water to remove in the worst relevant period, typically a mild, humid night with vents closed. Effective capacity is what one unit removes at the greenhouse condition (for example 16 °C / 75 % RH), not at its rating condition, and that is usually much less. Floor area alone can't give the number. After the count, check layout: units must be spread so every zone gets dry air, and redundancy is a separate decision.
Quick answer
- N = ceil(design load kg/h ÷ capacity per unit kg/h at your condition).
- Load from a moisture balance for the worst period, not floor area.
- Capacity from the unit's curve at your temperature and RH.
Step 1: the design load
Calculate the water to remove in the worst relevant period: crop transpiration and evaporation, minus what air exchange carries out. Use the signed difference in humidity ratio between inside and outside. See greenhouse moisture load calculation.
Step 2: capacity at the greenhouse condition
A unit rated at 30 °C / 80 % RH removes much less at 16 °C / 75 % RH. Ask for the capacity curve, and read the value at your night setpoint. Account for defrost at low temperature once, in the curve or in run time, not twice.
A daily rating divided by 24 (for example 480 L/day ÷ 24 = 20 L/h) is just arithmetic on the rating. It is not capacity at your condition.
Step 3: the count
N = ceil(D ÷ C)
Illustrative arithmetic only: design load D = 8.5 kg/h, capacity at the greenhouse condition C = 2 kg/h → 8.5 ÷ 2 = 4.25 → 5 units.
Step 4: layout and redundancy
| Check | Why |
|---|---|
| Zone coverage | Long houses need units spread along the length |
| Air path | Dry air must reach the canopy, not just the roof |
| Redundancy | Decide if the house must hold the target with one unit down |
| Power and drains | Each unit needs supply and a drain point |
| Controls | Staging with heating, screens and vents |
See central vs distributed dehumidification and ducted vs freestanding.
Project record: why area alone doesn't decide the count
On a pre-engineering review for a tomato glasshouse in Armenia, the client provided this data:
| Parameter | Client-provided value |
|---|---|
| Area | About 5 ha (20 spans × 12.8 m × 195 m) |
| Height | Eaves about 6 m, ridge about 10 m |
| Night RH peak | Up to 98 % with vents closed |
| Target | 16 °C / 75 % RH |
| Heating | Pipe-rail and crop pipes, upper pipes, snow-melt |
From this alone, the count stays open. It still needs the night transpiration, the leakage, and each unit's capacity at 16 °C / 75 % RH. Two greenhouses with the same area can need very different numbers of units.
What to send for a count
- Area, spans, heights, screens
- Crop and stage; night ET if known
- Night setpoints and current trend data
- Outdoor humidity-season data
- Heating and venting strategy
- Power, drains, layout constraints
Our greenhouse dehumidifiers include ceiling-mounted and ducted units; we size from the load at your condition.
FAQ
Frequently asked questions
How many dehumidifiers do I need per hectare of greenhouse?
There is no per-hectare number; it depends on the night moisture load and each unit's capacity at your condition.
Why is capacity at 16 °C lower than the rating?
Cooler, drier air holds less water for the coil to condense; ratings are at warm, humid test conditions.
Should I add a spare unit?
Only if the crop must hold the target with one unit out; check the remaining capacity meets that duty.
Sources
- ASHRAE Handbook—Fundamentals, Psychrometrics; HVAC Systems and Equipment, Dehumidification.
