A semi-closed greenhouse can recirculate crop-zone air, admit controlled outside air, or mix the two before treatment and supply. Those states do not remove the same amount of moisture. Before accepting an equipment proposal, draw the actual intake, return, treatment, supply and exhaust paths for each operating period. Count water carried by air and water collected by equipment across one declared boundary, without counting the same removal twice. Assign any remaining duty to equipment with capacity evidence at its actual entering condition.
“Semi-closed” describes an operating concept, not a fixed leakage rate or a promise that every vent stays shut. The grower and greenhouse controls designer must set the crop objective, permitted exchange and sequence. A dehumidification-equipment review can then check the water-removal duty, heat path, treated-air delivery and signals at its boundary. The conceptual diagram accompanying this article is an illustration, not a Yakeclimate installation or an approved project design.
Name the mode before calculating a moisture-removal duty
Write the state of the outside-air intake, return-air path, roof vents or other exhaust, screens and treatment equipment for the period being reviewed. A command labelled “semi-closed” is insufficient: an intake damper can be commanded open while actual flow, mixing and discharge remain unknown. Weather, solar load, crop stage, CO₂ strategy and screen position may cause the same facility to operate in several states during one day.
| Period to document | Air path to confirm | Possible water-removal route | Do not assume |
|---|---|---|---|
| Recirculation with no planned outside-air drying | Crop-zone return → treatment → supply; justified leakage and any safety purge recorded separately | Condensate or applicable regeneration discharge from operating equipment | Air movement alone removes water |
| Treated outside air | Measured or justified intake → treatment → crop zone → permitted exhaust | Flow-weighted water entering in treated supply versus leaving in exhaust, across the declared crop-zone boundary | An open damper proves useful drying, or pretreatment condensate can be added a second time |
| Mixed return and outside air | Both inlets, their actual flow shares, mixed-air condition, treatment, supply and discharge | Flow-weighted water carried by each crossing stream; treatment discharge belongs in the wider facility account if that boundary is used | Catalog capacity measured at a different inlet state applies, or one stream represents the whole mix |
| Vent or safety override | The actual vent/exhaust state and equipment response | A separately evaluated route for the override period | The original semi-closed duty or crop-zone result still represents this period |
The greenhouse ventilation-versus-dehumidification guide explains how to choose a moisture route from outdoor conditions, heating and CO₂ constraints. This page starts after a semi-closed arrangement has been proposed and asks what each part of that arrangement must prove. A research review of semi-closed greenhouses describes recirculated, outside and mixed-air treatment as distinct modes. Its particular arrangements are examples, not equipment instructions for every site.
Keep the water account on one boundary and time basis
For each mode, state the controlled volume, the start and end time, and the indoor and outdoor air conditions. List crop transpiration, wet surfaces or substrate, irrigation events and any other justified moisture input. Record the water collected or discharged by treatment equipment, together with the period it actually operated. Surface condensation and wet leaves should be logged as unresolved moisture storage or local risk, not credited as a successful designed drain.
Choose the boundary before assigning credits. For the growing volume, compare moisture carried out in exhaust and return-to-treatment with moisture carried in by the treated supply and any other intake, using each stream's dry-air flow and humidity ratio. Water condensed before that supply enters is already reflected in its lower humidity ratio; do not add the same condensate again to this growing-volume account. For a wider boundary that includes the air-treatment equipment, use the untreated outside intake, the exhaust and the actual condensate or regeneration discharge instead. Do not then add a second treated-supply credit. In a mixed-air mode, the return and outside streams need their own flow and moisture conditions; one indoor–outdoor reading cannot represent the mix.
Untreated outside air that is drier than the greenhouse air can give an exchange-only route a drying direction, but this does not establish useful removal after air treatment and mixing. Actual dry-air flow, conditioned supply, exhaust and the permitted operating period determine the crop-zone balance. If these paths are unmeasured or the flow balance is uncertain, ask the greenhouse designer for a justified boundary rather than inserting a guessed ventilation credit.
Use one time unit when comparing crop-air moisture input, net exchange and mechanical removal. A daily water total cannot establish whether a short night or transition period will pass. Nor is irrigation minus drain automatically transpiration: crop uptake, substrate storage and other evaporation can change that difference. The existing greenhouse design-data checklist owns the wider load-input method. Here, the question is which portion of that load each actual semi-closed mode can carry. An unmeasured flow or incomplete load remains an open design input; it is not repaired by a generic safety factor.
Check what air enters the equipment and where heat and water leave
A treatment unit may see greenhouse return air, outside air or a mixture. These streams can differ in temperature, humidity ratio and flow. Ask for the selected equipment's removal capacity and operating envelope at the expected inlet conditions of the relevant mode, including any defrost, staging or runtime limitation. A label rating at 30 °C and 80% RH, for example, is a rating condition, not proof of duty at a colder or drier mixed-air inlet. The same caution applies when an existing cooling coil is credited with latent removal: its actual condensation and operating period matter.
Trace the heat path at the same time. An indoor recirculating dehumidifier can return condenser heat to its space; an externally cooled air-treatment arrangement or regeneration exhaust has another heat balance. If outside air is admitted, its heating or cooling requirement and any CO₂ discharge must be reviewed by the greenhouse designer. Draw where condensate drains or regeneration moisture exits. Do not count water collected at a coil while ignoring a crop zone that remains humid because treated air missed it.
Semi-closed greenhouses in research use both centralized air-treatment corridors with supply ducts and distributed units. The Wageningen climatic evaluation measured temperature and RH at multiple crop heights and registered the treatment state because layout can change local conditions. That observation supports checking crop-zone delivery; it does not prescribe one duct layout, device position, sensor count or acceptable gradient for another greenhouse. Use the existing sensor-placement guide when selecting representative measurement positions.
Define the equipment and greenhouse-control handoff
The greenhouse controls designer owns the mode sequence and the crop-temperature, humidity and CO₂ objectives. The equipment supplier should state what the selected unit can do at each approved inlet condition and which commands, feedback and protections it actually supports. Agree on a point list before treating a line on a controls drawing as an implemented interface.
| Interface question | Record needed for the semi-closed review |
|---|---|
| Mode authority | Which controller requests recirculated, outside or mixed air; which states permit the dehumidifier; which safety override takes priority |
| Flow evidence | Intake, return and exhaust measurement or justified design basis, damper/vent feedback and the time each state was active |
| Equipment proof | Inlet condition, stage or speed, actual run feedback, available removal-capacity basis, condensate or regeneration discharge evidence |
| Heat and water discharge | Heat-rejection destination, drain or exhaust route, full-drain/fault response and service constraints |
| Failure response | Who sees a sensor, damper, communication or unit fault; approved fallback and restart behavior |
A request to open a damper does not prove exchange flow, just as an enable signal does not prove the dehumidifier ran. A controller RH trend does not establish a crop-zone condition if its sensor is unrepresentative. The smart-greenhouse control guide owns the wider authority and commissioning sequence. No universal protocol, fail state or transition timer is assumed here.
Verify separate periods and the transitions between them
Capture at least one relevant recirculation period and one period with intended outside-air or mixed-air treatment if both are part of the approved concept. Include a transition or override when it governs the risk. Align clocks and trend indoor/outdoor air state, relevant crop-zone points, intake/return/exhaust or justified flow proxies, vent and damper states, screen positions, equipment requests and independent run feedback, alarms, condensate or other removal evidence, and active heating/cooling. Label each interval with its actual mode, not just the scheduled one.
Check three questions against the same time window. First, did an available water-removal route exist and operate? Second, did treated air reach the intended zones without an overlooked screen or bypass? Third, were temperature, CO₂ and safety constraints respected under the greenhouse designer's approved sequence? A trace from a closed interval cannot validate the mixed-air interval, and an apparent RH improvement may reflect temperature change or unplanned venting rather than device removal. Record those possibilities before attributing a result.
If a mode switch causes a wet crop zone, investigate the actual air path, local measurements and equipment state before increasing machine capacity. If outdoor air was expected to dry the space but did not, recheck its humidity ratio, mass flow and exhaust path during that event. If the unit was commanded but not running, compare feedback, protection and the approved delay or lockout. These are diagnostic questions, not a substitute for project commissioning.
Send an equipment brief that can be checked
Provide a mode schedule and annotated air-path drawing; crop, stage and zone boundaries; targets and allowable excursions from the crop owner; indoor/outdoor time-series conditions; irrigation, drainage and moisture-load basis; screen and CO₂ states; intake and exhaust basis; existing cooling/dehumidification evidence; proposed equipment inlet conditions for each mode; heat and drain constraints; control point list; and the acceptance periods to be trended. Mark each input measured, supported assumption or open.
Yakeclimate can review dehumidification-equipment duty, operating conditions and interfaces against a project brief. A selected model, its performance at the stated conditions and the finished greenhouse sequence still require project-specific engineering confirmation. That is the useful decision point for a semi-closed concept: a reviewer can tell which moisture duty belongs to the equipment and which remains with the greenhouse air path.
FAQ
Frequently asked questions
Does a semi-closed greenhouse always need a dehumidifier?
It needs a verified route for the moisture entering each operating period. Controlled outside-air exchange may carry some duty when its condition, actual flow and permitted exhaust support it; mechanical removal may carry another portion. The mode schedule and period load must be checked before deciding equipment necessity or size.
Can a partly open intake be counted as moisture removal?
The damper position alone is insufficient. For the crop-zone balance, check conditioned supply and exhaust moisture states, each justified dry-air flow and the permitted period. Untreated outdoor conditions help determine the treatment duty. Keep equipment condensate and treated-supply effects on one declared boundary so the same water is not credited twice.
Does the equipment rating stay the same when outside air is mixed with return air?
Do not assume so. The unit sees the actual mixed-air temperature, moisture condition and airflow. Request capacity evidence for those inlet conditions, and check heat rejection, runtime and drain or regeneration limits for that same mode.
What proves the semi-closed mode worked?
A synchronized record of the actual mode and air path, unit request and run feedback, removal evidence, indoor/outdoor conditions and representative crop-zone response during the agreed period. The greenhouse controls designer must compare it with the approved temperature, humidity, CO₂ and safety criteria.
Technical sources
- Sapounas et al., Design, Control, and Performance Aspects of Semi-Closed Greenhouses — research review of air-treatment arrangements and modes; no savings figure is transferred to this article.
- Wageningen University & Research, Climatic evaluation of semi-closed greenhouses — measured treatment states and crop-height conditions for specific installations.
- Wageningen University & Research, Crop management in semi-closed greenhouses — research context for reduced vent opening and interacting climate factors.
