Why is my dehumidifier icing up?

Why the coil freezes

A compressor dehumidifier pumps refrigerant through a cold coil, called the evaporator. Room air passes over it, cools below its dew point and the water vapour condenses into droplets that drip into the tank. The coil normally runs many degrees colder than the room.

In a room at 20 °C the coil stays above freezing. As the room cools towards 10 to 15 °C, the coil can drop below 0 °C, so the condensed water freezes onto the fins. Frost then acts as insulation and blocks the air, so the coil gets colder still and more ice forms. That is why icing tends to get worse quickly once it starts. Our explainer on how a compressor dehumidifier works covers the refrigerant cycle.

What the defrost cycle does

Most modern compressor dehumidifiers have automatic defrost. A sensor on the coil notices it is getting too cold, the compressor stops, and the fan keeps blowing room air over the coil to melt the frost, or the machine reverses warm gas through the coil on some designs. The meltwater drains into the tank and extraction resumes.

Defrost is normal in cool rooms, but every minute spent defrosting is a minute not collecting water. In a cold garage a compressor unit can spend a large share of its time defrosting, which is why its real output falls far below the rating on the box. If you see a thick block of ice that never clears, the defrost system may have failed, or the room is simply too cold for the machine.

Fixes to try in order

Work through these checks before deciding the machine is faulty. They cover the common causes, from airflow to temperature.

1. 1

   Switch off and unplug, then let the ice melt fully at room temperature. Do not chip at it or use a hair dryer.

2. 2

   Clean the air filter as the manual describes and let it dry.

3. 3

   Move the unit so the intake and outlet have clear space, away from curtains and walls.

4. 4

   Check the room temperature. If it is below about 15 °C, warm the room or move the dehumidifier to a warmer space.

5. 5

   Restart and watch for frost over the next hour.

Note:Always unplug before removing the grille or filter, and never use sharp tools on the coil. The fins and refrigerant pipes are easy to damage.

Room temperature and the right type of dehumidifier

The single biggest factor is room temperature. Compressor machines are at their best in heated living spaces. In unheated rooms, a desiccant dehumidifier works differently: a slowly rotating zeolite wheel absorbs moisture and a small heater dries the wheel. With no cold coil there is nothing to freeze, and output stays fairly steady down to low temperatures.

Desiccant units use more power per hour than many compressor models, but in a cold space they usually collect more water for the energy used. They also warm the room slightly. Compare options in our guides to the best desiccant dehumidifiers and the best dehumidifiers for garages and cellars.

When icing points to a fault

If the coil ices up in a warm room above 18 °C with a clean filter and clear airflow, something else is wrong. A low refrigerant charge after a slow leak makes the coil run colder than designed. A failed fan or defrost sensor has the same effect. These are sealed system repairs for the maker or a qualified engineer.

Note the model number, room conditions and how quickly the ice forms, then contact the maker or retailer. If the unit is old and out of warranty, a repair may not be worth it. Our guide to the best dehumidifiers for UK homes lists current models across both types.

Compare desiccant dehumidifiers →

Frequently asked questions

Is it normal for a dehumidifier to ice up? A little frost that clears on a defrost cycle is normal in a cool room. Thick ice that never clears is not, and points to a cold room, a blocked filter or a fault.

At what temperature does a dehumidifier freeze? Many compressor models start to frost when the room drops below about 15 °C, and it becomes frequent below about 10 °C. Check the minimum operating temperature in your manual.

Can I keep using a dehumidifier that ices up? Switch it off and let it thaw, then fix the cause. Running it while iced wastes electricity and collects little water.

Do desiccant dehumidifiers ice up? Not in the same way, because they have no cold coil. That is why they are popular for garages, cellars, caravans and other unheated spaces.