Dehumidifier FAQs

What causes humidity?

Humidity in buildings has a variety of causes. The following list gives some indication.

• Building related causes - Rising damp, leaks, porous walls,

• Drying washing

• Kettles

• Baths and showers

• Wet clothes

• Wet pets

• Ambient high humidity outside

What are the effects of humidity?

Humidity is noticeable by its symptoms. The following gives some idea of what to look for

• Damp or wet walls (may indicate water ingress via walls)

• Mould

• Peeling and blistering wallpaper

• Breakdown of plasterwork

• Mildew

• ill fitting doors and windows (too large for frames)

• Swelling wooden furniture (joints showing proud)

• Health deterioration (coughs, asthma, etc. triggered by mites and mould spores)

• Clothing damaged by mould

• Window condensation

How do dehumidifiers work?

Dehumidifiers work by forcing condensation on cold surfaces. Humid air is drawn over cooled fins that makes moisture  condense on the large cool surface. The condensation is allowed to drain from the fins into a collection tank, or is piped to a suitable drain. The fins are kept cool in the same way a fridge or an air conditioner keeps cool.  In many respects a dehumidifier is similar to an air conditioner, with similar components (fan, condenser, cooling fins, refrigerant and pump) Mechanical dehumidifiers should not be confused with hydroscopic granules or crystals which are attract moisture from the air to the material.

What are the benefits of dehumidifiers?

Many of the benefits of of reducing the humidity in your home or work place are quite obvious, namely;

• Protection of building from damp and mould

• Protection of contents of building from mould and damp damage

• Protection of items in long term storage

• Reduction in condensation

• Quicker drying of washing

But there are other less obvious benefits such as;

• Health improvement. Reduction in mould spores and house mites will have a marked effect on the respiratory health of the building occupants. If the application is in the home then the benefit is clear, but if it is a workplace the benefit is financial through reducing staff absence through sickness.

• More efficient heating. Wet air is takes more energy to heat than dry air as the the air mass is greater. The efficiency bonus can be significant and will vary from building to building, however on the same heat setting, a rise in 2 to 3°C should be possible with a dehumidifier.

• Where buildings are left unoccupied for long periods and the heating is left on a frost protection mode, the addition of a dehumidifier can both protect the contents and walls from damp damage and also ensure that the heating is running as efficiency as possible. For this type of use it makes sense to have permanent drain connection.

How do I choose a dehumidifier?

Dehumidifiers have a  number of elements by which to compare models, the most important of which is the water extraction rate. This is a value normally expressed in litres per day at a certain %RH value and temperature. A typical values for small unit would be 10 litres/day @ 30°C and 80%RH, whilst medium range ones can be up to 18 litres/day and larger ones in 20 - 40 litre/day range. The figures quoted are typically 'idealised' and in reality extraction rates will seldom be as large as those quoted as the air may not be as humid as 80% or as warm as 30°C.

Unless you can easily connect your dehumidifier to a gravity drain, then the tank size will be a consideration. Small tanks need empting more frequently, and all the dehumidifiers will have an auto shut off when the tank is full. The effect of this is that units with smaller tanks spend more time inactive because the tank is full. Although most, if not all will have some visual indication of a full tank and therefore inactive, nobody has yet come up with a tank that takes itself out and pours it's the contents down the sink!

Airflow is not really a consideration in choosing a unit although larger units will have greater airflow. The point is that dry air and humid air mix outside the unit anyway and the units airflow will be sufficient to enable it to achieve its design water extraction rate.

All machines that move air will create some noise; where some noise data is available it will be stated, and can be compared with others. Also consider if you need a hot gas defrost feature and if you need duel voltage which enables the unit to be connected to 220-240 V supply or a 110V supply.

What factors affect their performance?

The performance of any one dehumidifier is mainly dependant on the ambient humidity and temperature. Condensation on the cooling surface is enhanced when there is a greater temperature inversion on the surface (i.e. greater difference between the temperature of the air and that of the surface) The greater the ambient temperature, the more water they extract, and the higher the relative humidity then the more water that the unit will extract.

The performance is also affected by dust. All dehumidifiers will have some form of dust filter to protect the coils, however these only protect them for a period of time and they do need to be maintained. Disposable filters require changing and washable types require occasional cleaning. Some dust will penetrate the filter stage and accumulate on the condensation fins, which will have the effect of slightly reducing the performance. Performance can also be affected by the filter blocking therefore preventing enough humid air flow into the unit.

Frost build up on the condensation fins can also affect performance. See the section 'what is hot gas defrost?'

What is a humidistat?

In addition to too much humidity being a bad thing, too little is also undesirable. For this reason a humidistat is a feature on most dehumidifiers, and its function is to provide a shut off should the humidity be reduced below a certain level. If the humidistat is adjustable this means that the auto shut off relative humidity is adjustable to your preference.

What is hot gas defrost?

The condensation fins can over time get frosted up which has the effect of restricting airflow through the unit and means that the units efficiency is restricted. For this reason there is usually a defrost feature that ensures that frost build up is cleared. This can be either be done by 'reversing' the cooling mechanism so that the fins heat up rather than cool, so-called hot gas defrost, or the condensation fins can be heated electrically.

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