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How a dehumidifier works and other FAQs about dehumidifiers and humidifiers

How does a dehumidifier work?

There are four types of dehumidifier sold to the domestic market in the UK - Compressor (the majority), desiccant, peltier and silica gel.

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.  

A desiccant dehumidifier has no coils and uses a wheel filled with a 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.

A peltier dehumidifier basically uses a cold metal surface to condensate the air on.  These should not be used below 15°C and will not control much more than a large wardrobe.

A silica gel based dehumidifier normally comes in a tub or a rechargeable cassette and should not be used to control more than a box/draw or wardrobe.

Why does water appear on my windows/cupboards/walls?

It is a basic law of physics that if the surface is cold enough and if there is enough water vapour in the air then it will condense.  This means in the case of windows whether they are double-glazed or not that if the windows are cold enough and there is enough moisture in the air then condensation will occur.  

Where does this water come from? 

We all produce water by drying clothes indoors, the boiling of vegetables, showering, rain, calor gas heating and making cups of tea all produces moisture.  It is just an on-going process that never stops.    

Can I just get rid of it by opening my windows? 

This is fine on a nice summers day but in winter opening your windows will just result in you losing the heat from your central heating and creating draughts.  This is a waste of money and if it is raining will just let more moisture in.  You might as well just throw fivers out of the window.  

Will a dehumidifier do 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.  

What about the black spots of mould on my window and bathroom sealant? 

Mould occurs around 68% humidity, condensation occurs at 100%.  As you can see from this you can have mould but still have clear windows.  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.  

Do I put it in the worst room? 

You can if you like put it in the main problem area to start with and then move it somewhere more convenient when you feel that the problem is under control.  If it is just condensation on the windows that you are worried about a dehumidifier in a central position, at for example the bottom of the stairs, will cure the problem in the whole house.

Are they expensive to run? 

A compressor based dehumidifier costs about 2-3p an hour to run.  You should take into consideration the fact that the dehumidifier will not run all the time, as it will be controlled via its humidistat.  Secondly 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.  

What is the humidistat? 

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 off and on as required without you having to worry about it.  

Should I turn the dehumidifier on for just a few hours a day? 

No just let the dehumidifier decide when to come on using its humidistat.  

Will they help with allergies? 

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. A dehumidifier that can be found in your local high street store is designed for use in your home, which is nice and warm.  In a garage, store, workshop, boat, caravan, holiday home or a conservatory the temperature will drop a lot lower.  Since a compressor based dehumidifier works by reducing the temperature internally it can easily reduce down to freezing even if the ambient temperature is 10°C.  What you do not want is for your dehumidifier to form a block of ice or for you to find a puddle on the floor.  

What happens when the temperature in the room drops down towards freezing? 

Most dehumidifiers designed for use in the home will just stop working and turn themselves off.  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.  

What is the correct type of dehumidifier to use in these applications? 

You need a machine with a function called hot gas defrost or a desiccant based machine.  If the dehumidifier does not have either of these features then do not buy it. 

How does Hot Gas Defrost work? 

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.  

So what temperature will a hot gas defrost system or a desiccant dehumidifier work down to?



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