How Does a Dehumidifier Work?

High levels of humidity in a home can lead to numerous problems. These include damage to the integrity of your property such as rotting wood and mould growing on walls, clothes and elsewhere, and health problems that can include respiratory illness.

The recommended indoor humidity is between 40% and 60%, and a dehumidifier will generally aim to maintain a level of around 55%.

The Dehumidification Process

Although different types of dehumidifiers work in different ways, the overall process is basically similar. The main difference tends to be the way the moisture is extracted from the air.

All dehumidifiers use a fan to draw moist air in through a grille at the front of the machine. The moisture is then extracted from the air and the dry air is heated, although the method of doing this varies between the different types of machine. The dry and warm air is then expelled using the same fan or a different one.

The moisture extracted from the air is generally dripped into a collecting tank, which needs to be emptied periodically. When the tank is full, the machine shuts off automatically and a warning light is displayed. Some dehumidifiers have a hose that takes the water directly to a drain, so emptying is never needed.

The Different Ways Moisture is Extracted

Compressor dehumidifier

Dehumidifiers originally had a compressor to help extract moisture, using a similar technique to refrigerators and air conditioning. This type of machine has an array of pipes through which coolant is circulated constantly by the compressor.

The moist air passes over these pipes and, as it cools, the moisture condenses to form water that drips into the tank. The dried air is then passed over a heating element before being expelled through another grille.

Desiccant dehumidifier

Another variation is a desiccant dehumidifier, where the moist air passes over a rotating wheel that has absorbent material similar to the silica gel often found with products that need to be kept dry. This material absorbs the moisture, which is then heated to re-vaporise it before ducting externally.

Thermo-electric dehumidifier

The third option is sometimes described as a thermo-electric dehumidifier, or one that has peltier technology. Here the input fan draws the moist air onto a cold heat sink, where the moisture condenses and drips into the collecting tank. The dried air then passes to a hot heat sink and is then expelled as warm and dry air.

Pros and Cons of the Different Types


Because the older condenser types use a pump to circulate the coolant and generally have larger fans, they are noisier than the other two types. The desiccant types have a rotating wheel instead of a compressor and so are quieter, while the peltier types, with few moving parts and often only one small fan, are the quietest of all. See the quietest dehumidifiers available in the UK here.


The extra noise level, however, is partly the result of increased throughput, with compressor models on average able to extract 70 pints of water a day, while desiccant versions are capable of 15-20 pints and peltiers only 0.5-1.25 pints. This can of course vary depending on the size of the machine but does suggest that the latter are suitable only for small areas and compressor types are the main workhorses.


Since they have very few moving parts, the peltier models should in theory be the most reliable and the longest lasting. However, this will depend on the quality of the build, with many being made from cheap components to keep the cost to the customer as low as possible. The most likely part to fail is the fan. However, in the peltier models at least, this is often a low powered type, the same as used in PCs, and can be replaced easily and cheaply.

Make Your Choice

In essence, the type of dehumidifier you choose will be the one that you can afford and which will do the job. The peltier types tend to be the cheapest but are limited in what they can do, while the compressor models are the most effective but are noisiest.

Other considerations are the relative size of the machines, with full size and compact models available, and the level of control you need. Some basic models have only an on/off switch while others have laundry drying options, required humidity level settings and timed operation, so it all depends on what you really want.


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