What is a desiccant dehumidifier and how does it work?

What desiccant means

A desiccant is any material that attracts and holds water vapour. The little sachets of silica gel in new shoes are a familiar example. Domestic desiccant dehumidifiers almost always use zeolite, a porous mineral material with a huge internal surface area that captures water molecules from the air passing through it.

In the dehumidifier, the zeolite is formed into a honeycomb wheel, so air can flow through it easily while touching as much desiccant as possible. Unlike a single sachet, the wheel is dried out continuously inside the machine, so it never becomes saturated.

How the wheel and heater work

The wheel turns slowly, often only a few revolutions per hour. Most of it sits in the main airflow, called the process air. Room air passes through, the zeolite captures water vapour, and drier air comes out the other side.

A smaller sector of the wheel passes through a separate, heated air loop, called regeneration. Hot air drives the trapped water out of the zeolite. That hot, very humid air then passes over a small condenser cooled by room air, the water condenses and drips into the tank, and the loop repeats. Because the wheel keeps turning, each part of it alternates between soaking up water and being dried.

1. 1

   Fan draws room air through the main section of the zeolite wheel.

2. 2

   Zeolite captures water vapour, and drier air leaves the machine.

3. 3

   The wheel turns slowly into the heated regeneration section.

4. 4

   Heat releases the water from the zeolite into a small air loop.

5. 5

   That warm, wet air is cooled, and water drips into the tank.

Why it works in the cold

A compressor dehumidifier needs the room to be much warmer than its cold coil to condense water, and the coil can freeze in a cool room. A desiccant dehumidifier relies on the zeolite capturing water vapour instead, which carries on largely regardless of air temperature. Its extraction therefore stays much steadier from warm rooms right down to near freezing.

That is why desiccant models are the usual choice for unheated spaces. Our guide to the best dehumidifiers for garages and cellars covers machines for those spaces, and our explainer on why dehumidifiers ice up covers the compressor side.

Why the room warms up

The heater that dries the wheel adds heat to the machine, and most of that heat ends up in the room through the dry air outlet. Desiccant models typically warm the air coming out by several degrees, and a small closed room can warm a little overall. In a cold garage or a chilly spare room that is a bonus. In a warm living room in summer it is not.

This warm, dry air is also why desiccant models are popular for drying laundry. Many include a laundry setting that runs the heater and fan at full power. See the best dehumidifiers for drying clothes.

The trade-offs

Desiccant dehumidifiers draw more power per hour than many compressor models of a similar size, because of the heater. In a warm room a compressor model usually collects more water per kWh. In a cold room the balance shifts, because the compressor model collects so little. Our dehumidifier running costs article shows how to compare them in kWh.

Desiccant ratings are often lower on paper, typically 6 to 10 litres a day for home models, but they are much closer to what you get in a real cold room. Most also have no compressor, so they tend to be lighter and have a softer fan noise. Compare current models in our guide to the best desiccant dehumidifiers and our desiccant vs compressor comparison.

See the best desiccant dehumidifiers →

Frequently asked questions

Is a desiccant dehumidifier better than a compressor one? In cold rooms below about 15 °C, usually yes. In warm, heated rooms a compressor model is usually more efficient.

Do desiccant dehumidifiers use a lot of electricity? They draw more watts than many compressor models, often 300 to 650 W depending on the setting. Check the stated wattage and work out kWh from the hours you expect to run it.

Does the zeolite need replacing? Not as a routine job. The wheel is dried continuously inside the machine. Keep the filter clean as the manual describes.

Are desiccant dehumidifiers good for caravans? They are a common choice for caravans and boats because they work in the cold and are light to carry. Check the minimum temperature in the manual.