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What do Dehumidifiers do?

Humid indoor air is uncomfortable, causes damage, and leads to the growth of mould and dust mites. Humidity is usually discussed in terms of relative humidity - the amount of water vapor present in the air compared to the greatest amount of water the air can hold at a given temperature. Optimal relative humidity indoors is 30 percent to 50 percent in the summer, and 30 percent to 40 percent in cool weather.
Dehumidifiers can maintain the indoor relative humidity at the desired level, controlling musty odours and protecting furniture from water damage. 
It is worth noting that a dehumidifier will slightly raise the air temperature, as opposed to an air conditioner that dehumidifies the air as it cools.

Types of Dehumidifer
1/ Compressor Dehumidifiers
A compressor based dehumidifier draws the air in from the room over a filter and passes it over some cold coils similar to the coils on a fridge. As the coils are cold, water condenses and drips into a bucket. The air is then reheated to room temperature and blown back out of the dehumidifier.  
 2/ Desiccant Dehumidifiers
A desiccant dehumidifier has no coils and uses a wheel filled with moisture absorbing desiccant material to extract the water from the air.  The air is then reheated to about 10°C above room temperature and is then blown back into the room. Desiccant dehumidifiers are smaller, lighter, quieter and even more energy efficient than compressor driven dehumidifiers. They also have the advantage of working effectively at lower temperatures than compressor based dehumidifiers.
Points to consider when purchasing a dehumidifier
Storage tank capacity / continuous drainage option – virtually all dehumidifiers have a collection tank for the condensed water. Unless you have a unit with a continuous drainage option and you have connected up a hose for continuous drainage, you will need to empty the tank periodically. If you want to use the dehumidifier in a remote location and continuous drainage isn't an option, look for a tank of 8 litres or more. Collection containers can be hooked up to a hose for continuous drainage through a floor drain or sump pump, pumped through water pipes or emptied manually. Many dehumidifiers are made to accommodate all of these options.
Adjustable humidistat – an adjustable humidistat controls the relative humidity level. Just as you have a thermostat that you set on your central heating you have a humidistat on your dehumidifier. This will allow the dehumidifier to turn on and off as required without you having to worry about it.
Automatic restart - automatic reset capability after power failure and when the storage tank has been emptied. This is a nice convenient feature if your power supply is unreliable or if you are leaving the unit unattended (eg on a boat or in a garage), or on a plug timer for long periods.
Low-temp dehumidifier - if the dehumidifier will be working in temperatures less than 65° Fahrenheit, it is advisable to purchase a model that's rated to operate down to 42º or 44° F, with an auto-defrost feature to prevent ice formation. Some models come with anti-frost sensors that automatically shut off the unit when air temperatures drop below a certain point.
Clearance around the unit – make sure you have good air circulation all around the unit. For a unit with a top exhaust, you need eight or ten inches on all sides -- so it shouldn't be placed flat against a wall. If you choose a dehumidifier that exhausts out the back, you'll need even more space.
Choosing a size
When choosing a dehumidifier, the size of the area needing moisture removal is the first consideration. Dehumidifier sizing is based on the amount of moisture extracted from the air in 24 hours, measured in litres. Large-capacity dehumidifiers can handle a room in a shorter time whereas a small-capacity model in a large room might run continuously and still not lower the humidity to an acceptable level.
Condition without dehumidification
50 square meters
100 square meters
150 square meters
200 square meters
250 square meters
Moderately damp:
Feels damp and has musty odour only in humid weather.
6 litre
8 litre
10 litre
12 litre
15 litre
Very damp:
Always feels damp and has musty odour. Damp spots show on walls and floor.
7 litre
10 litre
12 litre
12 litre
18 litre
Feels and smells wet. Walls or floor sweat, or seepage is present.
8 litre
12 litre
15 litre
18 litre
22 litre
Extremely wet:
Laundry drying, wet floor, high load conditions.
10 litre
13 litre
17 litre
21 litre
25 litre

A dehumidifier for the whole house - a dehumidifier creates a volume of dry air in as large a space as its fan can effect. After that it will be helped by the fact that damp air will always migrate to dry places. As long as you leave all of the internal doors open a correctly sized dehumidifier will prevent condensation and mould from appearing around the whole house.

Mould prevention / cure - Mould occurs around 68% humidity, condensation occurs at 100%. A dehumidifier will prevent the mould from getting any worse and once you have cleaned the mould away, it will stop it from coming back.
Dehumidifier running costs - a compressor based dehumidifier costs about 2-3p an hour to run. A desiccant wheel dehumidifier will cost less to run. However the dehumidifier will not run all the time, as it will be controlled via its humidistat. It is worth noting that it is very expensive to heat a damp house and you will see a reduction in your central heating bill when you start to use your dehumidifier.
On or Off - let the dehumidifier decide when to come on using its humidistat.
Allergy control - A dehumidifier will reduce the relative humidity in the house, which will prevent dust mites and other pests from breeding. In this way a dehumidifier can help with allergies. 
Using dehumidifiers in low temperatures areas – Most compressor driven dehumidifiers designed for use in the home will just stop working and turn themselves off (below five degrees centigrade). Others will try to work and might well suffer from a build up of ice. Those with a defrost system are likely to only extract a tiny amount of water. For low temperature usage it is best to specify either a desiccant based machine or a machine with hot gas defrost. Hot gas defrost works by reversing the coils and using the heat from the hot side of the coils to melt the ice to water. When this is done the heat is returned to the front of the machine to warm the air before it is blown back out.
Please do call for further information or advice.
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Sunbelt Rentals Limited
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Birchwood Park