# Dehumidifier

> A dehumidifier removes water vapour from grow-room air, usually by condensing it on a cold coil; it is the main tool for controlling humidity in flower.

- Canonical: https://genetixbay.com/knowledge/glossary/dehumidifier.html
- Also written: dehumidifiers, dehu, dehumidification, desiccant dehumidifier, refrigerant dehumidifier
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

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](https://genetixbay.com/knowledge/glossary/transpiration.md) 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](https://genetixbay.com/knowledge/glossary/exhaust-fan.md) cannot. Capacity is rated in litres or pints per day at specific test conditions (often 30 °C / 80% [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) 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](https://genetixbay.com/knowledge/glossary/dew-point.md). Run the dehumidifier on a humidistat through the night in flower. Empty or [drain](https://genetixbay.com/knowledge/glossary/runoff.md) 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.

## Related terms

[Relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md), [VPD](https://genetixbay.com/knowledge/glossary/vpd.md), [Dew point](https://genetixbay.com/knowledge/glossary/dew-point.md), [Humidifier](https://genetixbay.com/knowledge/glossary/humidifier.md), [Absolute humidity](https://genetixbay.com/knowledge/glossary/absolute-humidity.md)

## Read more

- [Humidity by stage: RH targets from clone to cure](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)

## Sources

1. Corredor-Perilla IC, Kwon H, Park J, 2025. High relative humidity in cannabis cultivation (effects on flowering, biomass and cannabinoids). Frontiers in Plant Science 16:1678142. https://doi.org/10.3389/fpls.2025.1678142