Dehumidifiers: why do they freeze over?

The internal coils of compressor-type dehumidifiers freeze over when the device is used in temperatures below 60 degrees F (15 °C). The colder it gets, the quicker the coils freeze. A clogged air filter can lead to more frequent freezing. Malfunctioning sensors can be another reason for freezing.

Why does my dehumidifier freeze over?

There are a few reasons your dehumidifier can freeze. The temperature of the room can be too low to operate the device, its filter is clogged with dust, or the device is malfunctioning.

The room temperature is too low

The most common type of household dehumidifier is a compressor dehumidifier. These dehumidifiers work by cooling down the air that flows through the device. Because the air gets cooled, its water vapor starts to condensate. These water droplets normally drip into a container to be disposed of.

However, when the air temperature is below 60 degrees F (15 °C), the coils cool down to freezing temperatures to get the air to condensate. Therefore, the water droplets that form on the cooling coils start to freeze. This stops the dehumidifier from functioning and can damage the coils.

The dust filter is clogged

For the device to function properly, airflow is required. If the dust filter is severely clogged, airflow can be greatly reduced. This can lead to the freezing of the cold coils because they get colder and colder when they’re unable to cool incoming warm air.

Normally, there is an exchange of energy. The cool coils take up warmth from the passing air, cooling the air and causing condensation. In exchange, the coils heat a little, preventing them from freezing over.

Air filter full of dust, partly cleaned
Scraped-off dust from an air filter

The dehumidifier is malfunctioning

A malfunction of the device can lead to it freezing over. There are two main malfunctions that could be the cause:

The fan is broken
A broken or stuck fan means that there is hardly any airflow through the machine. Similar to the abovementioned severely clogged filter, reduced airflow can lead to the freezing of the cooling coils. When airflow stops, the coils will continue to get cold and eventually freeze.

The anti-freezing mode is malfunctioning
When the anti-freezing mode does not function properly, or the frost sensor is broken, the device will not stop cooling even when it starts freezing due to operating in low temperatures. This will only happen when the device is running in temperatures below 60 degrees F (15 °C). At temperatures higher than 60 °F, the anti-freezing mode should not turn on.

Why a dehumidifier can’t continuously run in defrost mode

A dehumidifier cannot function in defrost mode because it won’t remove any water vapor from the air. Normally, a (compressor-type) dehumidifier works by cooling coils along which air is flowing. This leads to cold air, causing condensation and therefore dehumidification.

However, in defrost mode, the cooling mechanism is shut down and only the fan is running. This helps defrost the coils because the relatively warm air running along the ice causes it to warm up and turn to water again. In this way, the frozen water on the coils is removed.

Because the cooling mechanism is shut down, no more water vapor is turned to condensation. therefore, running the device in defrost mode does not help dehumidify the air.

When the temperature is so low that the defrost mode is constantly turning on, you might as well shut down the dehumidifier. If you have this problem, I highly recommend looking at a desiccant-type dehumidifier instead. I own both a compressor and a desiccant dehumidifier so I can run one regardless of the temperature.

I highly recommend reading my article on the differences between compressor and desiccant dehumidifiers, as well as my guide on what to look for in a dehumidifier.

At what temperatures can a dehumdifier run?

The most common dehumidifiers, compressors, can run at temperatures above 60 °F (15 °C). Their dehumidification efficiency is measured between 60 °F (15 °C) and 95 °F (35 °C). They function optimally at 95 °F (35 °C). The efficiency of compressor-type dehumidifiers drops with the temperature.

A desiccant dehumidifier’s efficiency is not dependent on the room’s temperature.

Is there an upper limit for dehumidifier operating temperature?

No, there is no upper limit of operating temperature. Dehumidifiers can function in extremely hot weather. In fact, their efficiency goes up as long as there is an excess of humidity. However, temperatures above 95 °F (35 °C) are uncommon and the relative humidity at these temperature is normally so low that dehumidification is not required.

For more in-depth information about optimal operating conditions, I recommend reading my article about the differences between compressor and desiccant dehumidifiers. It includes operating principles, how the conditions of the room affect their performance, and their energy use and running costs. You can find it here:

How to prevent the dehumidifier from freezing

To prevent freezing of your dehumidifier, make sure to check which of the abovementioned options may be the cause.

Do not run the (compressor) dehumidifier at temperatures below 60 degrees F (15 °C).

Make sure the filter is clean. You can take it out and simply wash and rinse it.

Contact customer support or an expert to look at your device for malfunctions.

A clogged filter next to the dehumidifier
Removing the clogged dehumidifier filter