Quick answer: Automate greenhouse humidity on a temperature-aware signal (humidity deficit, VPD or dew-point margin), not on RH alone, and let one controller, usually the climate computer, stage every device in a fixed order: air movement first, then vent and heat when outside air is drier, then dehumidifiers, with heating to protect temperature. Give each stage a deadband (a gap between start and stop) and minimum run and off times so equipment does not short-cycle, and start moisture removal before sunset rather than after the crop is wet.
Quick answer
- Control signal: humidity deficit or VPD; alarm on dew-point margin.
- One owner of the sequence: the climate computer, not each device's own humidistat.
- Deadband example: start at 88 % RH, stop at 84 %, with minimum run and off timers.
Choose the control signal
| Signal | What it tells you | Use it for |
|---|---|---|
| RH | Water relative to saturation at the sensor temperature | Display; simple systems |
| Humidity deficit (g/m³) or VPD (kPa) | How far the air is from saturation, independent of temperature changes | Main control target for the crop |
| Dew point vs leaf or glazing temperature | Whether surfaces will get wet | Condensation alarms and pre-night actions |
| Absolute humidity inside vs outside | Whether venting can remove water | Deciding vent vs dehumidify |
An RH setpoint alone makes the climate computer chase temperature: heating lowers RH without removing water, and cooling at night raises it without adding any. See greenhouse humidity and disease for night humidity-deficit targets, and MSU Extension on why VPD predicts plant water loss better than RH (MSU Extension).
A staging sequence
| Stage | Action | Condition to move to the next stage |
|---|---|---|
| 0 | Horizontal airflow fans on continuously | — |
| 1 | Open vents slightly with heating on ("heat and vent") | Only if outside air holds less water than inside air |
| 2 | Increase vent opening and heat | Humidity deficit still below target after a set time |
| 3 | Start dehumidifier stage 1 | Outside air too humid, vents limited by screens, CO₂ or heat loss |
| 4 | Start further dehumidifiers | Target still not met after a set time |
| — | Heating holds the temperature setpoint throughout | — |
Order the stages by cost: in cold, dry weather venting is cheapest; on humid nights or with expensive heat, dehumidifiers move ahead of venting. Many climate computers let you switch the order by outside conditions.
Pre-night action: start stage 1 or 3 about an hour before sunset when the forecast night is humid. Moisture removed before the canopy cools is far cheaper than drying a wet crop.
Setting deadbands and timers
A deadband is the gap between the start and stop points of a stage. It stops small sensor fluctuations from switching equipment on and off.
| Setting | Example | Why |
|---|---|---|
| Start point | 88 % RH (or humidity deficit below 1.5 g/m³) | Below the crop's risk level |
| Stop point | 84 % RH (or deficit above 2.2 g/m³) | Enough gap to avoid cycling |
| Minimum run time | 10–15 minutes | Lets the unit remove water and drain |
| Minimum off time | 3–5 minutes, or the compressor maker's value | Protects the compressor |
| Stage delay | 10–20 minutes before adding the next stage | Gives each stage time to work |
These are starting values to tune in your house. A deadband that is too narrow causes short-cycling; one that is too wide lets humidity swing more than the crop allows. The right band is the one that keeps the crop within its approved range in a recorded test.
Interfaces to specify
- Start/stop: dry contact from the climate computer to each dehumidifier, with the unit's own humidistat set wide open or disabled so only one controller decides.
- Status back: run, fault and drain-alarm signals to the climate computer.
- Analog or digital: 0–10 V or 4–20 mA, or a fieldbus such as Modbus, where staging or monitoring needs more than on/off. Confirm what each device supports.
- Safety interlocks: stop dehumidifiers if their drain fails; stop fans when exhaust fans run, if your system requires it.
Sensor placement for automation
Use canopy-height sensors in aspirated shields, at least one per control zone, including the coldest zone. Control on the zone that needs it most, or stage dehumidifiers by zone. See greenhouse humidity sensor placement.
Test the sequence
- Trace a full night: log canopy humidity and temperature, outside conditions and every device state on one timeline.
- Check cycling: count dehumidifier starts per hour; frequent starts mean the deadband or timers are too tight.
- Check the sunset transition: humidity should not spike as vents close.
- Test failures: simulate a sensor fault, a dehumidifier trip and a drain alarm; check that the system alarms and falls back safely.
- Record the settings that passed, as the baseline for future seasons.
UConn IPM and UF/IFAS summarise practical ways to reduce greenhouse humidity and the controls involved (UConn IPM; UF/IFAS AE069).
Equipment
For our greenhouse dehumidifiers, ask us which start/stop and status signals each model provides, so they can be matched to your climate computer. See how to choose a greenhouse dehumidifier.
FAQ
Frequently asked questions
What is a humidity deadband?
The gap between the humidity at which equipment starts and the humidity at which it stops. It prevents rapid on-off cycling.
What deadband should I use for a greenhouse dehumidifier?
Start with about 3–5 RH points (for example on at 88 %, off at 84 %) plus minimum run and off times, then tune it with a recorded test.
Should the dehumidifier use its own humidistat?
Not when a climate computer controls the house. Let one controller stage all devices so they do not work against each other.
Should greenhouse humidity be controlled by RH or VPD?
VPD or humidity deficit for the crop, with dew-point alarms for condensation. RH alone changes with temperature.
Sources
- Michigan State University Extension, Vapor pressure deficit for greenhouse growers.
- UConn IPM, Reduce greenhouse humidity.
- UF/IFAS Extension, AE069.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
