Unless you're half human/half orchid, you probably don't like humidity. Whether in a hot humid summer or a cold wet winter, damp indoor air can be a problem. It's uncomfortable, it encourages mould and dust mites, and condensation can stain or even ruin paintwork. If you've got damp air, a dehumidifier could be the answer.
Renovate, move... or dehumidify?
A cheaper, easier and less time-consuming effort than renovating your house to rid yourself of damp, a dehumidifier can keep the status quo during the humid seasons and save you having to get down into the foundations. It's also particularly useful if you're renting a damp house.
Which type of dehumidifier is best?
Most dehumidifiers, including most of the models we test, work by refrigeration. They draw air in, filter it, cool it to condense the moisture, then reheat it and push the warmer, dryer air back into the room. This warms the room somewhat, but not as much as an actual heater. Refrigeration models operate best in hot, humid conditions. The hotter the air, the easier it is to cool it by a large amount and extract moisture from it. They don't work so well in cold air as it's harder to cool already cold air in order to condense water from it.
Desiccant rotor dehumidifiers are becoming more readily available. These models use a slowly rotating disc or belt of material which absorbs moisture from the air; the damp section then moves through a drying process (typically a heating element), which evaporates the water and collects it in the tank.
Cold and damp, or hot and humid?
The best type of dehumidifier for your home can depend on whether you mainly need to dehumidify in cold damp conditions, or hot humid conditions.
Because they don't use cooling to extract the moisture, desiccant rotor models can be much more effective than refrigeration models in cold climates; in fact, their performance should be largely unaffected by the air temperature. We tested a good dessicant and a good refrigeration model in identical hot humid conditions (30°C and 80% relative humidity) and found that both performed to their rated water removal rates. The dessicant model performed much the same as in it did in cold conditions. The refrigeration model, which had performed relatively poorly in cold conditions, performed very well in hot conditions; on average, it extracted more than four times as much water in hot conditions as it did in cold conditions.
This shows that both desiccant and refrigeration dehumidifiers can do the job in hot humid conditions. But desiccant models rarely exceed a rated extraction rate of 10L/day, while you can find refrigeration models rated up to 30L/day or more. So for heavy-duty dehumidification in hot conditions, a refrigeration model would be the best option. The better-performing refrigeration models in our tests should perform very well in hot conditions.
Our article on how we test dehumidifiers explains more about our tests and test conditions.
But note that the dehumidification process can also result in the air being heated up to some extent, so if you just need moderate dehumidification in hot conditions, an air conditioner might be a better option, as these dry the air as they cool it.
What to look for
The humidistat lets you set a target relative humidity for the room (basically, that's the amount of water in the air for a given temperature). Depending on the model, this might be a precise number setting, or it might just offer a few basic settings such as low, medium and high. Rotary-dial humidistats with no precise markings make it hard to find the actual humidity level. Ideal relative humidity for human comfort is in the 40–60% range, but you might try as low as 30% if you prefer a dryer feeling, or up to 70% for more humidity.
Low temperature effectiveness
Excessive moisture in the air can occur at high and low temperature. Manufacturers might give water removal rates at conditions which make their machines look the best – typically 30°C and 80% relative humidity. Performance is often lower at colder temperatures; if you want to deal with winter condensation, ask for figures at 10°C and 15°C (although they will not always be available).
Wheels, castors and handles are useful, as dehumidifiers can be heavy. Cord storage is handy when moving the unit or packing it away.
A small water-collection tank makes a dehumidifier more compact and easier to carry around – but if the tank is too small, you'll have to empty it several times a day. Models with larger tanks won't need to be emptied as often, but can be bulkier. The tank should be easy to remove, and big enough to hold at least four litres of water.
Useful if the dehumidifier will be run for long periods in the same location, and some come with an adaptor hose to pour the collected water straight down a drain. For other models you'll need to supply your own hose.
Controls and labels
Should be easy to see, understand and use.
All the tested models have an air filter for trapping dust, and some models have a UV light filter for killing airborne bacteria.
Are dehumidifiers noisy?
Most dehumidifiers make upward of 50 decibels (dB) of noise – enough to be annoying if you're trying to watch TV, hold a conversation or sleep. By comparison, most fridges are about 30–40dB.
As with any appliance, there's also the quality of the noise to consider; a quiet, high-pitched whine may be more irritating than a louder but low-pitched noise. If you can, get the retailer to turn the unit on so you can hear the noise before you buy.
Note that the decibel is a logarithmic unit; an increase of 3dB is approximately equivalent to a doubling in loudness. In other words, if model A is measured at 50dB, then model B at 53dB is approximately twice as loud as A, and model C at 56dB is approximately four times louder than A.
How do dehumidifiers compare to air conditioners or heaters?
In winter you can use an electric heater and dehumidifier together. Each will boost the effect of the other. However, an unflued gas heater won't help, as it releases moisture into the room.
An air conditioner can be an alternative to a dehumidifier, as it can dry the air as well as cool or heat, and may be a better option in hot, humid climates where a dehumidifier's heating effect could be undesirable.