Why Does Dehumidifier Freeze Up and How to Fix It?

Dehumidifier Freeze Up

Have you ever had a dehumidifier freeze up? A dehumidifier that is frosted up will no longer do its job, and might have you pulling your hair out as to what to do.

Before we look at some reasons why your dehumidifier might have iced up, let’s look at how a dehumidifier is supposed to work and that will make it easier to understand why it doesn’t and what you can do about it.

How a Dehumidifier Is Supposed to Work

The unit is supposed to remove humidity without making the air cooler.

To do that, a dehumidifier works in the same way as an air conditioner but with one big difference. With an air conditioner, a compressor circulates a refrigerant through the system in a continuous loop. After being compressed, the refrigerant is very hot and circulates through condenser coils, where a fan blows outside air over those hot coils to transfer heat from the refrigerant into the outside air.

From there the refrigerant goes through a special valve where it is changed into a very cold liquid/vapor mix and enters the evaporator coils. The hot condenser coils and the cold evaporator coils are separated. In a central air system, one is outside and the other is inside. In a window AC unit, they are separated by an insulated partition within the unit. Keep this point in mind.

When the warm room air passes over these very cold coils, the air is cooled and then circulated by the air handler back into your rooms. At the same time, moisture in the air is removed, making the air in your home both cooler and drier. The refrigerant is then sent back to the compressor where it is compressed again, and the process repeats.

The one big difference with the dehumidifier is that there is virtually no separation between the very hot condenser coils and the very cold evaporator coils. The entire purpose of the dehumidifier is to remove moisture from the air but NOT to cool the air. Just as soon as the air passes over the cold evaporator coils it passes over the hot condenser coils. If anything, the air comes out warmer than it went in.

But when it does that, the main objective of the dehumidifier is accomplished. The air from your room hitting the very cold coils of the evaporator causes the coils to sweat. Moisture from the air condenses on them. It’s just like the sweat that forms on a cold beverage glass or bottle sitting on your table. The sweat runs down the evaporator coils and drips in a tub in the bottom of the dehumidifier. That’s how a dehumidifier is supposed to work. Get rid of the humidity – without making it colder in your home.

If you wanted the air both cooler and drier, you’d buy a room AC such as a portable air conditioner or a window air conditioner.

But Why Is the Dehumidifier Icing Up?

Now, what would cause the dehumidifier to freeze up? Here are several reasons this might happen.

1. Dirty Air Filter

Probably the most common reason for a frosted dehumidifier is that there is a lot of dust and dirt on the air filter. This will prevent the proper circulation of air passing over the evaporator coils and the moisture (sweat) that forms on them will turn to ice. Before too long, it can become a solid mass of frosty ice.

Many people who purchase a dehumidifier don’t even realize that their unit has an air filter. Unfortunately, some units don’t, so make sure you get one that does. The air filter is very important to the life of your unit by filtering out dust before it gets into the internal components where it can hold onto moisture long enough for it to freeze due to the very cold internal coil. Then, it frosts up.

The air filter should be right behind the grille of your dehumidifier so that it will trap any dust that is drawn through by the fan. Consult your User’s Manual for how to remove the filter and clean it regularly, especially if you live in a dusty climate or have pets in the house. Pet hair will clog an air filter in a hurry.

2. Dirty/Clogged Condenser Coils

As mentioned above, the air entering your dehumidifier passes through the air filter, over the evaporator coils and then immediately over the condenser coils. In other words, the condenser coils are directly in line with the flow of air. Even with regular cleaning of the air filter, over time the condenser coils will accumulate dust, dirt and pet hair that got through or around the filter. A dirty condenser coil will also result in the dehumidifier freezing up because the air cannot circulate properly.

To clean the condenser coils, the cover of the dehumidifier will have to be carefully removed. If you decide to do this, be sure to unplug the unit first. Usually just a few screws will hold the cover to the chassis, and if there is a control knob, it will also have to be removed. On newer models with digital touchpad controls, great care should be exercised when lifting the cabinet off so as to not damage the touch pad or the wires connected to it.

The condenser coils have many very thin and fragile fins that will have collected the dust. They must be cleaned very carefully to prevent bending and/or folding them over. A vacuum cleaner with a long snout and a soft, thin brush will help remove the dust without damage to the coils. In fact, caring for those fins is so essential to the functioning of the dehumidifier or an AC coil that special tools are made to repair/straighten them.

3. Room Temperature Too Cool

Most dehumidifiers work best in relatively warm temperatures – between 70-90°F. If you run a dehumidifier in a room that is cooler than 60-65°F, you are likely to cause the unit to freeze up. The reason is that the evaporator coils will get too cold, and before the water droplets can drip into the collection tub, they will freeze on the coils. Pretty soon, they have built up to a solid mass, which stops the flow of air through the dehumidifier.

You can prevent freezing up by simply not using a dehumidifier in a room where the temperature is below 65°F or so. If you have a room – let’s say in the basement – that is quite cool and feels damp, you might be better served to try using a small floor-mounted heater to take the chill off and make it feel drier.

4. Mechanical and/or Electrical Failures

This category of reasons why a dehumidifier is icing up probably contains issues that you will not be able to fix by yourself. They would include the following:

  • Faulty Humidistat – an air conditioner has a thermostat that you set for the temperature you want. A dehumidifier has a humidistat that you set for the humidity you want. If this device is not functioning properly, the result could be an iced up unit.
  • Defective Fan Motor – the fan motor turns the fan blades to circulate the air through the dehumidifier to keep warm air always on the coil, reducing the chance of dehumidifier freezing up. If the motor is defective there will be no air movement, which can cause frosting of the unit.
  • Broken/Loose Fan Blades – this will have the same result as a bad fan motor. No or very poor air movement equals icing on the evaporator coils.
  • Faulty Frost Detection Switch – many units have this device to shut the dehumidifier off or to switch it into Defrost Mode if it begins to ice up. Once the unit has defrosted, it will then allow it to start up again. If this switch is bad, it can result in an iced up dehumidifier.
  • Electronic Control Board Malfunction – on newer dehumidifiers the unit may be controlled by a touch pad connected to electronic circuitry. Various problems in this area can result in a dehumidifier frosting up.

If the unit is clean – both the filter, coil on back and any other parts that you can get to – and dehumidifier ice up is still occurring, it is likely mechanical. This is especially true if the area in which it is being used is reasonably warm.

Different models will have other electro/mechanical components that, if malfunctioning, could mean a dehumidifier that freezes up. Unless you are familiar working with these devices, you should contact a reputable and competent service technician for assistance.

Finally, if you think you might be in the market for a new unit, see the Pick HVAC Dehumidifier Buying Guide for full details and assistance. 

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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