Dehumidifier FAQs
(Frequently Asked Questions)


Dehumidifier FAQs


Click on a question to see the answer.

1. What does a dehumidifier do?

2. What is relative humidity?

3. Where does the water come from?

4. What is the ideal humidity level for my home?

5. How does a dehumidifier work?

6. How long will it take to get the moisture under control?

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

8. How much water does a dehumidifier remove?

9. Can I use the water that collects in the bucket?

10. Do all dehumidifiers have a water container?

11. What are AHAM ratings and what do they mean?

12. How do I know how big a dehumidifier I need?

13. How does a dehumidifier prevent mold and mildew?

14. Do I need to run my dehumidifier all year round?

15. Could a dehumidifier dry the air too much?

16. How does a dehumidifier help to reduce allergic reactions?

17. Where is the best place to site my dehumidifier?

18. What is Continuous Drainage?

19. What is Porting?

20. What is the Condensate?

21. What is a Condensate Pump?



1. What does a dehumidifier do?


A dehumidifier removes water from the air until relative humidity is reduced to a pre-set level. It will then maintain that level indefinitely.

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2. What is relative humidity?


Relative humidity is a measure of how much water is in a body of air relative to the amount of water that air could hold if it was saturated. Relative humidity is expressed as a percentage. When relative humidity reaches 100% it is at the "dew point". In other words the air is "full" of water and that water will start to condense onto any surface.


Warm air holds more water than cool air so as air cools relative humidity rises even though the physical quantity remains constant. For every fall in temperature of 20°F/10°C the capacity of the air to hold water is halved. For example air at 80°F with a relative humidity of 50% will reach the dew point, 100% RH when it has cooled to 60°F. For a fuller explanation see our article about humidity.

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3. Where does the water come from?


Water is always in the air, both inside and outside the home. The amount inside the home is increased by many of the routine activities of daily life, such as cooking, washing, showering, making coffee and, of course, breathing. If we have uncovered water containers in the home, such as a fish tank, these will add to the amount of water in the air.

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4. What is the ideal humidity level for my home?


In normal conditions relative humidity of 50% is comfortable for you and your family, will protect your home and belongings, prevent mold growth and control the dust mite population. Unless you have unusually moisture sensitive items in a storage area or a particular, health related reason for needing a very dry atmosphere, there is no call to reduce humidity below the 45-50% mark. To do so will simply cost more in energy while serving no useful purpose.

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5. How does a dehumidifier work?


The vast majority work by drawing air from the room over a coil cooled to a very low temperature by a refrigeration system, just like the one that keeps your Coke cool in the refrigerator. The water vapor condenses on the cold surface and the liquid water drips into the water collection bucket underneath, or into a drainage pipe.


A smaller number of models, more commonly found in Europe than North America, pass the air over a rotor coated with a water absorbing material called a desiccant. The water is removed from the desiccant by a stream of warm air and then collected in a bucket.


Mini dehumidifiers may use the Peltier, heat transfer process to condense the water vapor or use a desiccant to absorb it. The Peltier type collects the water in a small bucket. The desiccant type is connected to the electricity supply every few weeks and the heat generated inside it dries the desiccant crystals. Units like these are only suitable for small enclosed spaces such as closets.


The heated rod type, typically used in closets and gun safes, does not remove water from the air but raises the temperature inside an enclosed space and thus lowers relative humidity. A much more detailed explanation can be found on our page about how these appliances work.

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6. How long will it take to get the moisture under control?


Provided that the unit is large enough for the area it is working in, and there is no water seepage into the room, a dehumidifier will normally reduce the relative humidity to the chosen level within a couple of days to a week. This will not apply if the room has been severely flooded, after which one or more units will be needed, supplemented by air movers and other specialist equipment. This work is usually undertaken by water damage restoration specialists.

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


An appliance described as a "70 pint" dehumidifier is capable of removing 70 pints of water from the air per day. This figure will have been obtained under specified conditions of temperature and relative humidity. In drier and/or cooler conditions the actual performance will be less. The figure does enable us to compare the performance of different models so we know, for example, that a 70 pint unit will always remove twice as much water as a 35 pint under the same conditions. In countries using the metric system the same applies but the unit of measurement is the Litre.


CFM stands for cubic feet (of air) per minute. The power of the appliance depends very much on the amount of air it can process in a given time. The higher the CFM rating the more powerful it is. This is why when you need your machine to work harder you select a higher fan speed. In areas using metric measures the air flow is measured in cubic metres.

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8. How much water does a dehumidifier remove?

This depends on three factors. Its power, the temperature in the room and the relative humidity of the air in that room. A 70 pint unit will rarely if ever remove 70 pints in a single 24 hour period. Once relative humidity has been reduced to your chosen level the appliance should only be operating intermittently to maintain that level so the amount of water collected in the bucket will be lower still.


As a rule of thumb you may need to empty the bucket at intervals ranging from once every two days to twice a day, depending on the conditions. Only in exceptional circumstances will a properly sized unit need emptying more frequently unless you have just put it to work, for the first time, in a very moist room.

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9. Can I use the water that collects in the bucket?


The water collected in the bucket, known as the condensate, is ideal for use in your steam iron and other uses, such as to water your plants. On no account should it be drunk by humans or animals as it may contain traces of substances that could make you, your pets or your livestock unwell.

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10. Do all dehumidifiers have a water container?


In general the answer is yes, at least so far as portables are concerned. Basement, crawl space and whole house models do not have a bucket and discharge the water by continuous drainage only. A few of you may use small, desiccant models designed for industrial use. These do not produce liquid water but expel the moisture as vapor, to the outside of the building, via ducting.

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11. What are AHAM ratings and what do they mean?


The water extraction rate of a dehumidifier varies dramatically as the air temperature and/or relative humidity change. The warmer and more humid the air the more water is collected. As the air cools and the humidity falls the amount of water collected falls too.


For example, at 90°F (32°C) and 80% Relative humidity the unit may collect, say 50 pints per day. If the temperature is only 80°F (27°C) and relative humidity is 60% that same unit will collect only 25 pints.


Is that a 50 pint or a 25 pint dehumidifier? The answer is that in Europe it is a 50 pint and in the USA it is a 25 pint unit.


AHAM (Association of Home Appliance Manufacturers) is a trade association which sets common standards for comparing some home appliances distributed in the USA. Among those appliances are dehumidifiers. All suppliers who subscribe to the AHAM standard have their appliances independently tested at the 80°F/60%RH standard. This enables consumers to make fair comparisons between the performance of two models from different manufacturers.


When purchasing one of these appliances in the USA it is always advisable to ask whether its stated water removing capacity was tested at AHAM standards. If it was not our advice is to halve the manufacturer's stated capacity in pints and only then to compare it with a unit that was tested at AHAM standards.

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12. How do I know how big a dehumidifier I need?


This can be a little complicated but it boils down to the size of the room and how wet the room is. The size of the room normally means the floor area, in square feet or square metres. This assumes that you have a ceiling in the room of typical room height, say around seven to eight feet above the floor. If your ceiling is much higher an allowance needs to be made for the additional volume of air in the room. So if your ceiling was 50% higher than the typical ceiling height it would be necessary to increase the actual floor area by 50% to arrive at a "notional" floor area against which to size your appliance.


The "wetness" of the room is a little more difficult to assess but you will find a useful guide by following this link to the Energy Star Site (opens in new window).


In general terms we would always recommend that you err on the side of caution and if in any doubt buy a dehumidifier a size larger than you may appear to need. Larger units remove more water per energy dollar spent and run for fewer hours in a given space than smaller ones. The energy saving will almost certainly be greater than the difference in purchase price.


For more information see our article on sizing.

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13. How does a dehumidifier prevent mold and mildew?


It prevents mold and mildew growth in two ways. First it dries out the room, drawing damp from the contents and fabric of the room on which mold and mildew will inevitably form and prevents that damp from recurring. Second, provided that the appliance is set to maintain relative humidity at 50% or slightly below, the air is too dry for mold and mildew spores to develop. Fungi need airborne water as well as surface water to grow. You will find more detailed information on our mold page.

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14. Do I need to run my dehumidifier all year round?


The answer depends on where you live and your particular circumstances. If you live in a warm, humid region you will normally feel the need for drier air during the summer months. In temperate or cold areas these appliances are usually needed most during the winter.


In both cases this is because the outdoor temperature is far from comfortable and your windows and doors remain closed. In warm areas, during the summer, this is necessary to make use of your air conditioning system, if you have one. In colder areas, during winter, you need to retain heat in the house. When effective ventilation is difficult or impossible a dehumidifier is often the only way to maintain a comfortable and safe level of relative humidity.


In some homes a particular area, such as a basement, is difficult to ventilate and may suffer from excessive humidity throughout the year. In this and similar circumstances it is essential to run the appliance at all times.


If you have a model with an adjustable humidistat, which is recommended, it will operate only when relative humidity is greater than the programmed level. This device ensures that relative humidity remains at a comfortable level and prevents mold and dust mites from multiplying. In one sense, therefore, you can let the appliance "decide" when it should be on and be assured that your home and family are protected at all times.

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15. Could a dehumidifier dry the air too much?


Yes, under certain circumstances, but it is easy to avoid.


For personal comfort relative humidity has to be within a certain range. Too high and we feel hot and sticky and breathing may seem like an effort; too low and we may suffer from dry skin, cracking lips and irritation in the breathing passages, particularly in the nose. This range is quite large and not all authorities agree on the precise upper and lower limits but a range of 30% to 70% provides a rough guide.


To control condensation, prevent mold growth and to limit the number of dust mites in our homes a relative humidity of around 45% to 50% is sufficient and falls comfortably within the broad 30-70 range. In very cold weather some sources recommend a level as low as 30%. A typical portable dehumidifier, using conventional refrigerant technology, will struggle to maintain such a low level.


In addition to personal comfort an excessively low relative humidity may be damaging to some materials used in our homes and their contents. Wood and leather, for example, can be over-dried if relative humidity is maintained below 40% for long periods.


Most models are equipped with an adjustable humidity level control, a humidistat. With this control you may program the level of humidity you want the appliance to maintain. When the level is at or below the programmed percentage the machine stops removing water and only restarts when the level rises again.


If your humidistat is set at 45% or 50% there is no risk of over-drying, unless your humidistat is not working correctly. It is wise precaution to check the relative humidity with a good quality hygrometer from time to time. This is even more important if you have reason to choose a relative humidity setting which is below 45%.

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16. How does a dehumidifier help to reduce allergic reactions?


Allergic reactions are triggered by a variety of substances, such as pollen, pet dander and food-stuffs. A dehumidifier cannot help with many of these allergens but it does reduce two of the most common, mold spores and debris left by dust mites.


It does not remove these particles from the air directly, like an air purifier but, by drying the air, it makes your home a hostile environment for the sources of the particles, mold and dust mites.


The EPA advises that mold can grow and multiply at a relative humidity as low as 60% while dust mites will continue to thrive at any level above 50%. By maintaining relative humidity in the home at between 45% and 50% mold growth should be prevented and the dust mite population greatly reduced.


The basements and crawl spaces of our homes are the most likely to suffer from mold growth and dust mite infestation. They are the coolest areas of any house and suffer, therefore, from higher relative humidity even when the absolute humidity may be no higher than elsewhere in the home.


Because warm air rises the allergens generated in the lower reaches of the house will be drawn up through the upper floor(s) and affect the occupants, even if those upper floors are relatively dry. This is one of the reasons why the most popular use of large, portable units is in the basement.

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17. Where is the best place to site my dehumidifier?


In an ideal world a dehumidifier would be silent and invisible. The next best thing is to be out of sight and earshot. Unfortunately the first is impossible and the second, while achievable in some cases, will be more expensive in cost and effort.


If you need to lower humidity throughout your house a whole-house model is the best option. It is usually located next to your heating boiler, typically in the basement. It is integrated into your existing HVAC ducting and is the cheapest solution for whole-house humidity control.


If you are using a portable in a small to medium sized room, a laundry room or a bedroom for example, it is not particularly important where you position it and most people prefer it to be close to one wall.


In a basement of any size the problem is a little more complicated. If you are using a large (50 to 70 pint) portable model you have little choice but to place the unit at or near to the center of the basement for greatest effect. If you have to site the unit in a corner or against one wall of a larger basement you may find that you also need a couple of fans to increase air circulation.


If you decide on a basement model but want to operate it as a free-standing unit, the same applies. All basement models, however, can be ducted. This means that you may locate it where you choose and by ducting the incoming and outgoing air flows an even level of relative humidity can be maintained across the whole area of a large basement. This allows you to locate the appliance where it is not in sight and, in some cases, less audible.

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18. What is Continuous Drainage?


All portable dehumidifiers have a water collection container, often called a "pan" or "bucket" to collect the water extracted form the air. These buckets are quite small, to keep the overall size of the appliance down, and need to be emptied at least once a day.


Because it may not be convenient, or possible, to empty the bucket daily almost all portable models are designed to allow the water to be drained directly as it is collected. This is known as continuous drainage.


In the casing a small hole has been cut to allow a small bore hose to be attached through which the water can be drained. This hole is usually filled with a rubber bung or a small "knock-through" panel. The hole is called a "port" and the process of continuous drainage is sometimes referred to as porting.


The water is drained by gravity alone so for effective continuous drainage the appliance needs to be raised a foot or two above the floor and the hose run to a floor level drain or sump. If a convenient outlet is not available at floor level, or is at a higher level, a condensate pump (see question 21) will be needed to force the water to a higher level or over a longer distance.

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19. What is Porting?


Porting is another word used to describe continuous drainage (see question 18).

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20. What is the Condensate?


The condensate is the water collected by a dehumidifier. A refrigerant unit condenses water vapor on cold coils so the condensate is the condensed water. Strictly speaking this word only applies to refrigerant models since all other types use a process of water absorption to remove water from the air and so condensation does play a part.

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21. What is a Condensate Pump?


A condensate pump is used to pump the condensate, the water collected by the appliance, to a drain or other outlet. It is used to propel the water over a greater horizontal or vertical distance when gravity fed continuous drainage is not possible.


Condensate pumps differ in power and if a pump is needed to raise the water to a higher level a pump with a "vertical lift" capacity of at least 12, preferably 15, feet should be used.


A few portables are equipped with a built-in condensate pump, such as the Winix and some DeLonghi models. For other dehumidifiers separate condensate pumps may be purchased at prices ranging from $40 to $80.

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