What is auto defrost and why is my dehumidifier defrosting so frequently?

by Anon

I have a Dimplex DXDH10N dehumidifier and the defrost light keeps coming on even when it's warm in the house and it seems to have stopped extracting any water as the tank doesn't have any water in it.

What does the defrost setting do?


Hello and thank you for your question.

Let me start by explaining what "defrost" means in relation to dehumidifiers and why it's necessary.

Basically your dehumidifier is a refrigeration system plus a fan. The refrigeration system is just like the one in your kitchen fridge. It has three main parts, a compressor, an evaporator and a condenser. A fluid, called a "refrigerant", is contained in a sealed system to which these three parts are all connected.

The compressor applies pressure to the refrigerant which then travels to the evaporator. The pressure is instantly released and the refrigerant evaporates in a small fraction of a second. As it evaporates it absorbs heat and cools the surface of the evaporator.

The fan draws air into the dehumidifier and it passes across the cold surface of the evaporator. Water vapour in the air condenses onto the cold evaporator and drips into the water bucket. The air flow continues across the condenser where the refrigerant is returned to a liquid state. The condensing process warms the surface of the condenser which is why the air that comes out of a dehumidifier is warmer than when it went in.

When the air in your room falls below a certain temperature, about 18C, some of the water condensing on to the evaporator will turn to ice before it has time to drip into the bucket. If you were to look at the evaporator when this is happening you would see the ice forming a "frost" on the coils.

As the frost builds up it insulates the compressor and reduces the amount of water vapour condensing on it. If this went on unchecked the dehumidifier would eventually be damaged.

To protect the dehumidifier and to enable it to remove more water from the air the dehumidifier must stop cooling the evaporator coil to allow the ice to melt. A sensor detects the formation of ice, triggers a switch which turns off the compressor and the evaporator is no longer cooled. The fan continues to run so that the flow of air from the room can pass over the evaporator and assist with the thawing process.

This is called "auto-defrost" and when the defrost light shows on the dehumidifier this means that the defrosting cycle has been activated and is proceeding.

Incidentally, you may find my reference to 18C confusing given that the manufacturer states that the DXDH10N has a temperature operating range of 5-35C. The same, or a very similar, range is quoted by all manufacturers and refers to the temperature range in which the unit will operate without risk of damage, not to the range in which it will be effective.

If the temperature in your home is usually close to 18C or higher, and the dehumidifier is functioning normally, the defrost light should only come on occasionally and the defrost cycles should be fairly short. From your question is does not appear that low room temperature is the cause of your problem.

There are two other possible explanations.

First, the air flow through the dehumidifier is too low. This could be caused by one of three things.

  1. The dehumidifier does not have sufficient clearance around it from walls and furniture to allow a free flow of air. I think this is unlikely.

  2. The fan is faulty and is not running fast enough. This is possible but also fairly unlikely.

  3. The air filter inside the dehumidifier is clogged with dust and other debris. The filter should be cleaned every two weeks and this simple task is often overlooked. This is the most common cause of insufficient air flow.

Instructions for removing and cleaning the filter can be found in the user's manual. If you don't have a manual you can down-load one from:


Just copy and past into your web browser's navigation bar to obtain the manual.

Second, there could be a fault in the refrigeration system, most probably a leak of refrigerant. When there is too little refrigerant this causes ice to form on the coils at any temperature and this would trigger the excessive defrosting that you describe.

This is, I'm afraid, a serious fault. If your dehumidifier is still under warranty you may obtain a replacement or repair. If it is not you will almost certainly find it difficult to get the unit repaired and, even if you can, the job may be too expensive to make the repair worthwhile.

If you are able to see the evaporator coil in your dehumidifier the "normal" frost formation will appear as a thin, even layer of frost across the coils. A refrigeration fault is normally indicated if the frost is more concentrated on some areas and less dense or absent on others.

I hope that your problem is just a clogged filter and, whatever the cause, that you have found my comments helpful.

Tom (Webmaster)

Comments for What is auto defrost and why is my dehumidifier defrosting so frequently?

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Dec 29, 2012
by: Anonymous

Great explanation, Tom. Thanks!

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