A dehumidifier pulls water vapour out of the air. Refrigerant (compressor) units blow air over a cold coil where water condenses, then reheat the air slightly over the hot coil; they work best above about 18 °C (64 °F). Desiccant units absorb water on a drying wheel and work better in cold spaces. Both add heat to the room: refrigerant units release the latent heat of the water they condense plus their own power.
In practice
Plants transpire most of the water you give them. A flower tent watered with 10 L (2.6 US gal) a day releases close to that into the air, so the dehumidifier must remove a similar amount when the exhaust cannot. Capacity is rated in litres or pints per day at specific test conditions (often 30 °C / 80% RH or 27 °C / 60% RH); real capacity at 25 °C and 50% RH is much lower. Size with margin.
The critical time is lights-off, when temperature drops and RH climbs towards the dew point. Run the dehumidifier on a humidistat through the night in flower. Empty or drain it continuously; a full tank stops it.
Don't confuse with
- Exhaust fan: removes humidity only if outside air is drier.
- Air conditioner: dehumidifies as a side effect while cooling, often not enough on its own.
