Quick answer: To size greenhouse dehumidification, a supplier needs more than floor area. The essentials are the layout (area, heights, compartments, screens), the crop, stage and water use, the day and night targets, the ventilation, heating, screen and CO2 strategy, 7–14 days of trend data covering the humid nights (temperature, RH, dew point, vent and screen positions, irrigation times), site pressure or altitude, the air-distribution layout, and power, drainage and controls. State which inputs are measured, which are estimated and which are unknown, so the recommendation can be checked.
Quick answer
- Floor area alone can't size a greenhouse dehumidifier.
- Send water use, night targets, ventilation and screen strategy, trend data.
- Mark each input measured, estimated or unknown.
Minimum data set
| Input | Example | Why it matters |
|---|---|---|
| Layout | Area, eaves/ridge height, spans, compartments | Controlled volume and zoning |
| Crop and stage | Tomato, mature canopy | Transpiration and disease risk |
| Water use | Daily irrigation and drainage | Estimate of moisture into the air |
| Targets | Day and night T and RH (or dew point, VPD) | The condition to hold |
| Ventilation | Vent type, fans, CO2 limits | Whether venting can dry |
| Screens | Thermal, blackout, shade schedule | Moisture trapped below screens |
| Heating | Pipe rail, crop pipes, air heaters | Leaf temperature, heat budget |
| Air distribution | HAF fans, ducts, tubes | Whether dry air reaches the canopy |
| Controls | Climate computer, Modbus, relays | Staging and alarms |
| Power and drains | Voltage, phases, drain points | Installation |
| Site | Altitude or pressure, climate | Humidity ratio and outdoor exchange |
Separate the operating periods
| Period | Design question |
|---|---|
| Day | Is venting enough with sun, transpiration and CO2 strategy? |
| Dusk | Does RH spike when air cools and screens close? |
| Night | Can the crop stay dry without venting heat away? |
| Screen or blackout closed | Is moisture trapped below the screen? |
| After irrigation or misting | How long do leaves and trays stay wet? |
The period with the largest remaining load sets the size; usually a mild, humid night with screens closed.
Size for the largest remaining moisture load under the planned ventilation and screen strategy.
Trend data that helps most
- 7–14 days, ideally in the humid season.
- Temperature, RH and dew point at canopy height, with sensor positions.
- Vent position, screen status, heating, irrigation times on the same timeline.
- Outdoor temperature and RH.
Moisture-load evidence
- Irrigation minus drainage approximates crop water use, but storage in the substrate and evaporation from wet surfaces must be accounted for.
- Leakage or air-exchange tests if available; vent opening percentage is not airflow.
- See the greenhouse moisture load calculation and psychrometric calculation.
RFQ checklist
- [ ] Layout drawing with spans, heights, screens, aisles
- [ ] Crop, cultivar, stage, planting density
- [ ] Daily water use and drainage
- [ ] Day and night targets
- [ ] Ventilation, heating, CO2 and screen strategy
- [ ] Trend data (7–14 days)
- [ ] Site altitude and climate
- [ ] Air distribution (HAF, ducts) and space for units
- [ ] Climate computer and interface type
- [ ] Power supply and drain points
- [ ] Each input marked measured / estimated / unknown
Send the package for a greenhouse dehumidifier review; see also how many dehumidifiers and the calla lily case study.
FAQ
Frequently asked questions
What information is needed to size a greenhouse dehumidifier?
Layout, crop and water use, day and night targets, ventilation and screen strategy, trend data, distribution and controls.
Can I size from floor area?
No; two greenhouses of the same area can have very different night moisture loads.
Why is altitude needed?
Humidity ratio depends on air pressure; moisture balances at altitude use local pressure.
Sources
- ASHRAE Handbook—Fundamentals, Psychrometrics.
