Quick answer: In a sealed indoor grow room, almost all the water you irrigate, minus runoff and what the plants keep, ends up in the air, and the dehumidifier has to remove it. Size from daily irrigation water, split into lights-on and lights-off periods, and choose capacity at the room condition, not the 30 °C / 80 % RH rating. Then allow for the heat a dehumidifier adds to the room, and place units so dry air passes through the canopy rather than straight back into the unit.
Quick answer
- Moisture load ≈ irrigation − runoff − water stored in plants and media.
- A refrigerant dehumidifier adds heat: about 0.68 kWh per litre condensed, plus its electrical input.
- Several smaller units, or ducted units, cover a room more evenly than one large unit in a corner.
Step 1: Estimate the moisture load from water use
For a sealed room, the water balance gives the evapotranspiration, the same method used in irrigation science (FAO Irrigation and Drainage Paper 56):
water into the air ≈ irrigation − drainage/runoff − increase in water stored in media and plants
Example: a room with 100 m² of canopy receives 300 L of irrigation a day, with 15 % runoff. The plants and media hold back a little as they grow. Roughly 250 L/day goes into the air.
| Period | Share of transpiration (example) | Load |
|---|---|---|
| Lights on (12 h) | about 80 % | about 200 L in 12 h ≈ 17 L/h |
| Lights off (12 h) | about 20 % | about 50 L in 12 h ≈ 4 L/h |
Lights-on transpiration is much higher, but lights-off is often the harder period: the air conditioning stops removing moisture as it cycles off, the room cools, and RH rises fast. Size for the peak hourly load, and check both periods.
If the room has intentional fresh air or leakage, add or subtract the water carried by that air.
Step 2: Pick capacity at the room condition
Catalogue ratings at 30 °C / 80 % RH overstate removal in a grow room at, for example, 26 °C / 60 % RH by day or 22 °C / 55 % RH at night. Ask the supplier for capacity at your day and night conditions. See how much water a dehumidifier collects in a day.
Air conditioning removes some moisture while its coil is cold and running, but it is controlled on temperature. When the cooling load drops, so does its moisture removal. Do not count on it for the lights-off period.
Step 3: Allow for the heat the dehumidifier adds
A refrigerant dehumidifier releases into the room:
- the latent heat of the water it condenses, about 0.68 kWh per kg of water;
- its own electrical input.
Example: removing 17 L/h at lights-on releases about 17 × 0.68 ≈ 11.6 kW of heat, plus the unit's electrical input. The cooling system must handle that, or the room warms. At lights-off this heat is often welcome.
Step 4: Place units so the air passes through the crop
| Arrangement | Good for | Check |
|---|---|---|
| Floor units in the room | Small rooms, easy service, near drains | Aisle space, irrigation spray, blocked intake |
| Ceiling-mounted units | Rooms where floor space is used for crop | Support, service access, drain route, clearance to lights |
| Ducted units outside the room | Keeping service out of the grow room; even supply through ducts | Duct pressure within the unit's external static pressure; supply and return routes |
| Several units by zone | Long rooms, multi-tier racks, partitions | Sensor per zone; avoid units fighting each other |
Avoid short circuits: if the dry discharge flows straight back to the inlet, the controller reaches its setpoint while the far side of the room stays humid. Put the return on the humid side and the supply across the canopy.
Example of installation data to check: our ceiling-mounted RYDZ-90DM measures 900 × 570 × 450 mm, weighs 49 kg, moves 1,000 m³/h at 50–80 Pa external static pressure, and is rated at 90 L/day at 30 °C / 80 % RH. Use such figures to plan supports, service access and duct resistance; the support design and clearances come from the installation manual.
Step 5: Controls and sensors
- Place sensors at canopy height in each zone, away from the dehumidifier discharge and lights.
- Control on RH or VPD with temperature measured at the same point.
- Stage dehumidifiers with the air conditioning, especially at the lights-off transition.
- Alarm on high humidity, drain faults and unit failure.
Drainage and service
- Gravity drain with a trap and fall, or a condensate pump with overflow cut-out.
- Access to filters, coils and drain pans; clean filters on a schedule, since grow rooms produce dust and plant debris.
- Condensate is low in minerals; test it before reusing it for irrigation.
Which units
| Room type | Start with |
|---|---|
| Single rooms, temporary or growing operations | Portable commercial dehumidifiers (60–138 L/day) |
| Rooms with crop on the floor or racks | Ceiling-mounted and ducted units (26–960 L/day) |
| Large rooms with plant-room space | Floor-standing industrial dehumidifiers with ducting |
Related: portable commercial dehumidifiers, ceiling-mounted and ducted units, industrial dehumidifiers.
All ratings at 30 °C / 80 % RH. Related: lettuce and leafy greens humidity and hydroponic greenhouse humidity.
FAQ
Frequently asked questions
How do I size a dehumidifier for a grow room?
Start from daily irrigation minus runoff; that water ends up in the air. Split it into lights-on and lights-off loads, and choose capacity at the room condition.
How many dehumidifiers does a grow room need?
Enough to cover the peak hourly load at room conditions, spread so every part of the canopy gets dry air. Two or more units also give redundancy.
Where should a dehumidifier go in a grow room?
Where it draws humid air from the crop and sends dry air back across the canopy, with a drain and service access. Avoid placing the supply right next to the return.
Does a dehumidifier heat a grow room?
Yes. It adds the latent heat of the water it condenses plus its electrical input. Include it in the cooling design.
Is air conditioning enough to dehumidify a grow room?
Usually not at lights-off, when cooling demand falls and moisture removal falls with it.
Sources
- FAO Irrigation and Drainage Paper 56, Crop evapotranspiration.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
- Product data: Yakeclimate RYDZ ceiling-mounted dehumidifier specifications.
