Excess humidity causes problem for health, buildings, property and general comfort in a room. Excess humidity (above 50% relative humidity) will cause these problems of building damage, and general unpleasantness in a room.
Dehumidifiers operate on a humidistat to keep a rooms humidity at a desired level. This will stop the problem of excessive humidity in a room but will also prevent the often ignored problem of over-dehumidification, where the relative humidity is below 30%, which can also make the room uncomfortable and seem too dry.
There are two different types of dehumidifier, compressor and desiccant dehumidifiers.
Compressor Dehumidifiers work on the same principal as an air conditioning unit, with a compressor inside them producing a cold copper coil inside the unit which water condenses on and drains into a collection bucket. This air then passes over a warm coil and is brought back up to temperature as it leaves the dehumidifier
Desiccant Dehumidifiers work by packets of desiccant material, similar stuff to silica gel, rotating on a wheel and absorbing moisture from the air, these packets of desiccant material are then heated to about 10 degrees above room temperature. Which takes the moisture from the packets of desiccant material.
Storage tank capacity - An important thing to consider if you do not plan on using the continual drain option. Almost all dehumidifiers have a storage tank, which can be connected to a continual drain if required. If you think about how often you will be in the house, the collection capacity of the dehumidifier and the storage capacity of the water tank, you should be able to work out if the dehumidifier is suitable for you or if you need to get one that you can use the continual drain function for.
Adjustable Humidistat - All systems should come with an adjustable humidistat, but it is worth checking first off, or you may find that is is set to a setting that you don't find comfortable.
Automatic Restart - This is an essential feature for systems which are being left on automatic drain and you have very little intention of going back to the area on a regular basis, e.g. a dehumidifier in your boat or in a storage unit for a classic car. If there is no automatic restart on the unit then if there is a power cut, the unit will not come back on when the power comes back on and moisture will build up in the air again.
Temperature - If the dehumidifier is going to be used in a house, or somewhere where there is a constant source of heat then a compressor dehumidifier would be better as this will drain more water, however if the dehumidifier is going to be used somewhere where there is not continual heat a desiccant dehumidifier would be better as this warms the air up as well as removing the moisture from it.
Choosing the right size of dehumidifier is quite important, if the wrong size is chosen then it won't do the job properly and you may find your dehumidifier is on all of the time and not bringing the moisture down to the correct level.
Luckily because moisture will find its own equilibrium within an enclosed space its not that easy to get wrong, generally speaking if you go for a 12-14L a day Compressor model for a 2-3 bedroom house and a 20L compressor model for a 4-5 Bedroom house. 7-8L Desiccant models are fine for a 5 bedroom house and 10L models are fine for any situation, oversizing the unit won't hurt at all so there is no need to worry about that.