A guide to dehumidifier basics

Practical Dehumidification

One of the things that we have noticed over the many years that we have been selling dehumidifiers is that customers will often come on the phone with problems and questions which they feel are unique to them, but we have heard it all 100s of times before. When we point this out customers are relieved to know that they are not alone.

Below are answers to the sorts of questions that we are often asked by people looking to buy their first dehumidifier. We have divided this section into two, dehumidifiers for the home and dehumidifiers for unheated or colder areas (i.e. storage, garages, workshops, boats, caravans, holiday homes, conservatories etc.).

How does a dehumidifier work?

There are four types of dehumidifier sold to the domestic market in the UK - Compressor, desiccant, peltier and absorbent salt based (calcium chloride or 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 draws the air from the room in over a filter in the same manner as a compressor dehumidifier but unlike the compressor dehumidifier it has no compressor and no cooling coils. Instead it 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.

An absorbent salt 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.

Using a dehumidifier in the house.

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. This is why we get more condensation in the winter, because the windows are colder.

Where does this water come from?

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

Can I just get rid of the condensation 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?

These black spots are mould. 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 the dehumidifier 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, for example the bottom of the stairs, will cure the problem in the whole house.

Are dehumidifiers expensive to run?

A compressor based dehumidifier costs about 2-3p an hour to run depending on your electricity tariff and whether you leave the windows open or not. 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.