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.

Click here to view our range of dehumidifier to solve your damp problem.

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.

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 a dehumidifier help with allergies?

A dehumidifier will reduce the relative humidity in the house, which will prevent dust mites and other pests from breeding. As there will be less mould, there will also be less mould spores in the air to trigger allergy attacks. In this way a dehumidifier can help with allergies. For more information on allergies and fighting dust mites please click here. Many of the Meaco dehumidifiers also have silver-nano, anti-bacteria filters and ionisers as well to help keep the air clean.

Click here to view our range of dehumidifier to solve your damp problem.

Using a dehumidifier outside of the house (or in a holiday home)

Why do unheated areas get separated out?

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. Generally speaking if the temperature is going to drop below 15°C then you should think carefully about what you are going to use and call Meaco on freecall 0500 418458 for some advice.

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. The only compressor dehumidifiers to now have hot gas defrost are the Mitsubishi Electric ones. Desiccant dehumidifiers like the Meaco DD8L are the other option.

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?



How does the desiccant dehumidifier work?

The desiccant dehumidifiers draw the air in from the room over a filter and into a desiccant wheel that is rotating slowly. The excess moisture in the air is absorbed by the wheel. The wheel is now saturated and as it rotates heat is blown over the wheel to force the moisture out and onto a metal plate where it condenses. Thus the wheel is regenerated so that it can collect more moisture from the air. The heat used is mixed with the dry air and blown back into the room providing dry warm air into the space. Whereas a compressor dehumidifier has to stop dehumidifying to defrost itself the desiccant is able to dry the air 100% of the time. This is why they are more popular in cold applications.

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