Quick answer: A central system dries air at one plant and delivers it through ducts to the crop areas. It works best when zones share similar loads and schedules and when a duct route can reach every row evenly. It keeps equipment and service out of the crop. A distributed system puts several units in or near each zone. It works best when zones differ (crop stages, screens, irrigation times), when houses are long or divided, or when you want partial capacity if one unit fails. Decide from a zone map: where the moisture load is, when it peaks, and whether one control signal can serve all zones.
Quick answer
- Central: one plant, ducted, even distribution if designed for it; single point of failure.
- Distributed: zone-by-zone control and redundancy; more units, drains and power points.
- Decide from zone loads, schedules and air paths, not total capacity.
Comparison
| Central (ducted plant) | Distributed (units per zone) | |
|---|---|---|
| Zone control | One setpoint unless zoned with dampers | Each zone controlled separately |
| Distribution | Depends on duct design reaching all rows | Each unit covers its zone; HAF helps |
| Redundancy | Plant failure affects all zones | One failure affects one zone |
| Equipment in crop area | None (ducts only) | Units hung or placed in zones |
| Installation | Ducts, larger fans, one power and drain point | Power and drain at each unit |
| Service | One location | Several locations |
| Expansion | Limited by plant size | Add units zone by zone |
| Energy | Fan power for ducts; can be efficient at scale | Lower fan power; unit efficiency at each zone's condition |
Map the zones first
- Mark crop blocks, propagation areas, screens, doors and existing air paths.
- Record canopy-height temperature and RH in a problem zone and a comparison zone, day and night.
- Note where wetting or condensation appears.
- Separate shared duty from zone duty: same crop stage, irrigation and schedule → shared; different → zone control.
See humidity sensor placement.
What a central system has to prove
- Ducts and outlets deliver dry air to the wettest row, not just the nearest.
- One control signal represents all zones, or dampers zone the supply.
- Acceptable behaviour if the plant stops.
What a distributed system has to prove
- Each unit's capacity at its zone's condition covers that zone's load.
- Units don't fight each other or the climate computer.
- Drains and power are practical at each position.
Compare operating modes
| Mode | Question |
|---|---|
| Screens closed at night | Does each zone still get dry air? |
| Vents open on mild days | Does the system stop or stage down? |
| Irrigation in one zone | Can the system respond locally? |
| One unit or plant offline | What capacity remains, and where? |
Calculate zone loads with the greenhouse moisture load method, then count units with how many dehumidifiers.
Prove the arrangement at the crop
After installation, trend canopy-height RH and dew point in every zone for several nights, and check for wet spots at dawn; see the commissioning checklist.
Our greenhouse dehumidifiers include ceiling-mounted units for distributed layouts and ducted units for central plant.
FAQ
Frequently asked questions
Is central or distributed dehumidification better for a greenhouse?
Central suits uniform zones with a good duct route; distributed suits varied zones, long houses and redundancy needs.
Can I combine both?
Yes: a central plant for the main crop and distributed units for propagation or problem zones.
Does a central system save energy?
It can at scale, but duct fan power and running the whole plant for one zone can offset that.
Sources
- ASHRAE Handbook—HVAC Systems and Equipment, Dehumidification; Fundamentals, Psychrometrics.
