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Dehumidifiers - Frequently Asked Questions

Date posted: 30 January 2017

  1. ​What does a dehumidifier do, and how does it work?
  2. What is relative humidity?
  3. Where does the water come from and where does it go?
  4. What is the ideal humidity level for my home?
  5. Can I use the water that collects in the bucket?
  6. What do 'pints per day' and 'CFM' mean?
  7. Do all dehumidifiers have a water container?
  8. How do I know how big a dehumidifier I need?
  9. How does a dehumidifier prevent mould and mildew?
  10. Could a dehumidifier dry the air too much?
  11. How does a dehumidifier help to reduce allergic reactions?
  12. Where is the best place to site my dehumidifier?
  13. What causes condensation?
  14. What are the running costs?
  15. What is “continuous drainage”?
     

 What does a dehumidifier do, and how does it work?

A dehumidifier removes excess moisture from a room, helping to protect the building's structure and contents from the damaging effects of mould and mildew, and protecting the health of any people living or spending time there. 

Most households tend to have a refrigerant dehumidifier, which draws damp air into the unit, which then cools and condenses. The drier, dehumidified air is then pushed back out into your room using a fan. This creates a healthier, more comfortable environment in which to live or work.

Desiccant dehumidifiers work in a slightly different way. They incorporate an adsorbent material that extracts water from the air. The material is then heated so that the moisture drips into a water tank. This type of unit sometimes costs more to run, and works most effectively in lower temperatures such as in a conservatory or garage. It’s often claimed that desiccant dehumidifiers tend to use more energy than refrigerant dehumidifiers because of the way they use heat to warm the adsorbent material. 


 What is relative humidity?

Relative humidity is the amount of water present in air expressed as a percentage of the amount needed for saturation at the same temperature.

When relative humidity reaches 100% it is at the "dew point" – that is, the air is saturated or "full" of water. At that point the water in the air will condense onto any surface it encounters. Warm air retains more water than cool air, so as air cools its relative humidity rises, even though the physical quantity remains the same. For each drop in temperature of 10 degrees C, the capacity of the air to hold water is halved. 


 Where does the water come from and where does it go?

All air contains water, whether it's inside or outside your home. The amount of water found in the air inside your home is likely to be increased, however, by many everyday activities such as washing clothes, showering, cooking and boiling the kettle - as well as breathing! Uncovered volumes of water such as a fish tank will also increase the amount of water in the air.

Usually the water that is collected from the air by your dehumidifier collects in a tank inside the unit. Many small domestic dehumidifiers need to be emptied manually once the tank is full – simply by removing the tank and emptying the water down a drain, or onto house plants. Other dehumidifiers come with an internal condensate pump, which pumps the water through a tube to your chosen location, such as a drain -- even pumping it vertically, where necessary, such as from a basement or cellar. Other models can support external condensate pumps, which are a handy solution when you are using a dehumidifier in a remote space because they automate the water removal process, to some extent, which means you don't have to be present to manually empty the tank on a regular basis. 


 What is the ideal humidity level for my home?

Most spaces benefit from maintaining a healthy 45-50% relative humidity level. 

Once relative humidity levels reach over 50% the environment becomes the ideal breeding ground for mould spores, dust mites, bacteria, and even cockroaches and other harmful pests. You may also experience unpleasant odours and damage to your home's structure and interior, such as rotting wooden window frames, and mould or damp patches spreading on walls or furnishings.

Equally, a low relative humidity level of below 30% can also be damaging. It can create an environment in which viruses such as cold and flu can thrive, as well as irritating our skin and respiratory passages and causing structural damage such as ceilings cracks and separated wooden floors boards.


 Can I use the water that collects in the bucket?

The water that collects in the dehumidifier's tank is called condensate, and this can be used to water plants or even in your iron. It should not, however, be consumed by humans or animals because it may contain traces of substances that could cause illness.


 What do 'pints per day' and 'CFM' mean?

A "30 pint" (or 14.2 litre) dehumidifier is capable of extracting 30 pints of water from the air each day  (“pints per day”). This figure is established under specified conditions of temperature and relative humidity – and it is designed to help consumers choose a unit that offers a suitable level of performance that will meet their requirements. In drier and/or cooler conditions the actual performance will be less than this quoted figure, but the system still allows us to compare different models “like-for-like” – for example, we would know that a 60 pint unit can always remove twice as much water as a 30 pint unit under identical conditions. 

CFM stands for cubic feet (of air) per minute. A dehumidifier's power depends a great deal on the quantity of air it can process in a given time. So the higher the CFM rating, the more powerful the dehumidifier is, and when you need your unit to work harder you choose a higher fan speed. 


 Do all dehumidifiers have a water container?

Most portable dehumidifiers used in a single room do incorporate a water container. However, some other types of dehumidifiers, such as those designed for basements, whole houses and crawl spaces, get rid of water by continuous drainage. And, mainly in industrial environments, the desiccant type of dehumidifier does not actually produce liquid water, but instead expels the moisture as vapour via ducting to the outside of the building. 


 How do I know how big a dehumidifier I need?

To work out how big your dehumidifier needs to be, you will need to establish:

  • the square footage of the area you wish to dehumidify
  • the room's conditions - which will help you to estimate the amount of moisture you need to remove per day, and whether that is average, or perhaps above average, for that sized room

For a slightly damp room, you will probably be able to manage with a dehumidifier with the rated capacity for your square footage. If the area is particularly damp, however, you will need to choose a dehumidifier with a little more capacity. If the room is extremely wet, and therefore the unit will be dealing with extremely tough conditions, you will need a large capacity to reduce relative humidity levels to a comfortable and healthy level.


 How does a dehumidifier prevent mould and mildew?

Fungi require airborne water as well as surface water in order to grow successfully. Dehumidifiers work to dry out the room. This removes damp from the fabric and contents of the room, which would otherwise provide a surface for mould and mildew to form on. In addition, once the air achieves relative humidity of 50% or slightly lower, it is too dry for mould and mildew spores to develop. 


 Could a dehumidifier dry the air too much?

The simple answer is yes, but it is easy enough to prevent this from happening simply by setting the desired humidity to be within the recommended range. 

If relative humidity is too high people feel hot and sticky, and it may feel like it takes more effort to breathe. If relative humidity is too low it can cause us to suffer from dry skin, respiratory irritation, and cracked lips. If you set your relative humidity to a lower level (e.g. under 40%) it can also over-dry certain materials in your home, such as wood and leather.

For optimum personal comfort most experts agree that the ideal relative humidity is within a range of 45-50%. This level will be ideal for personal comfort and will also be adequate for controlling condensation problems, preventing mould growth, and limiting the population of dust mites in the home.

Most models of dehumidifier come with an adjustable humidity level control called a humidistat. You can set this to your desired level of humidity and it will work to maintain this level, stopping and starting work as required.


 How does a dehumidifier help to reduce allergic reactions?

When there is excess moisture in an indoor environment it creates an ideal breeding ground for several allergens, including dust mites, mould, mildew and bacteria. All of these can cause allergic symptoms in those who suffer from allergies, causing unpleasant symptoms such as itchy eyes, skin rashes and respiratory problems. 

The use of a dehumidifier helps to extract excess moisture from a room and create a dryer, healthier environment with fewer of these particular allergens -- thus helping you to control many allergy symptoms.

Mould can grow and multiply at a relative humidity as low as 60%. Dust mites will thrive at any relative humidity level above 50%. So if you can maintain relative humidity of between 45-50% you should be able to prevent mould growth and reduce the dust mite population. 


 Where is the best place to site my dehumidifier?

To achieve top performance it's crucial to position your dehumidifier correctly. Most manufacturers recommend placing the unit at least six inches away from walls or any other structure that could prevent optimum air flow in and out of the dehumidifier. Always check the manufacturer's guidelines for product-specific recommendations.


 What causes condensation?

You can't prevent moisture from being in your indoor environment, and even normal, day-to-day activities will produce this water vapour in relatively large quantities. Just breathing, washing up, showering or bathing, cooking and washing or drying clothes will generate a lot of moisture in the air. Heating -- particularly paraffin and flue-less gas heaters - also create moisture in the air. And if you have large volumes of uncovered water lying about your home – such as those in a fish tank – this will also contribute to the moisture levels.


 What are the running costs?

When you are browsing and either buying or renting a dehumidifier it is important to have an idea of how much it will cost you to run. As a guide, it costs about 10 to 12 pence per hour for 1kW of electricity, so you will need to look at the wattage of the units you are interested in. For example, a 300 watt machine would cost about 3 to 4 pence an hour, depending on what you pay for your electricity from your specific supplier. The average dehumidifier would cost you about 3 to 4 pence per hour if you are running it continuously, although adsorption machines usually cost more to run (at about 5 pence per hour). It is worth bearing in mind, however, that some adsorption machines achieve their task more quickly at temperatures below 20 degrees C, and therefore in some conditions may reduce overall costs in comparison with a conventional refrigeration dehumidifier. Once the relative humidity levels in your home have been brought under control to a healthy level, maintaining this will cost you a lot less because your dehumidifier will need to operate less.


 What is “continuous drainage”?

Portable dehumidifiers are fitted with a water collection container, which is often called a "pan" or "bucket". This collects all the water that is removed from the air. Usually these containers are relatively small in order to keep the overall design of the dehumidifier compact and portable – so they need to be emptied at least once a day (and potentially more frequently if the relative humidity is very high).

This emptying task can be very inconvenient, however, if you are not at home during the day, or need to go away from home for a period of time.  So most models can also allow the water to be drained directly as it is being collected – a system known as “continuous drainage”. A small hole is cut into the casing to enable you to attach a small bore hose, through which the water can drain. When you purchase the unit this hole (know as a “port”) is usually filled with a rubber bung, or a small "knock-through" panel, which you can remove if you wish to adapt it for continuous drainage (also known as “porting”). 

The water simply drains away thanks to gravity, so to be effective you need to make sure your dehumidifier is raised up a foot or two above floor level, and that the hose runs to a floor level drain or sump. If you do not have a convenient outlet close by or at floor level, you will also need a condensate pump to push the water up to a higher level, or over the longer distance.

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