How Does a Dehumidifier Work?

So, how does a dehumidifier work? A dehumidifier removes water from the air. The result is lower relative humidity.

Warm air "holds" more air than cooler air. For simplicity's sake imagine the air in a room is a bucket, with a volume of, let's say, 4 pints. When the air is cooled it "shrinks", the bucket gets smaller. If the air in the room is cooled by 20°F (10°C) it can now hold only half as much water, 2 pints, as it could before.

Relative humidity tells us how full the bucket is as a percentage of its volume. So, if at 80°F it is half full the relative humidity is 50%. If the temperature goes down to 60°F the bucket has now halved in size. The relative humidity is doubled, to 100% even though the physical amount of water in the air has not changed.

When relative humidity is 100% we have reached the "dew point". The bucket is full and the water starts to "spill". This is the point when condensation starts to happen and the water vapor turns to liquid water on the surfaces in the room.

How does a dehumidifier work to prevent condensation? It removes water from the air, reducing the amount in the bucket. That lowers the relative humidity which, in turn, lowers the dew point. This can be critical in some parts of the home, particularly basements and crawl spaces which are often the coolest. The temperature in the main house may be above the dew point but down in the basement it may be below it. This is why basements are often dank and moist when the rest of the house is not affected.

Lowering the relative humidity also helps to dry out damp rooms. Instead of condensation onto surfaces we now get evaporation from surfaces. There is now enough room in the bucket for water to return to the air and for the fixtures and other items in the room to dry out, which answers the question how does a dehumidifier work so effectively to dry out a home after flooding?

Condensation is not the only problem caused by high humidity. Even when the temperature is above the dew point we may want to lower relative humidity.

How does a dehumidifier work, for example, to make us feel more comfortable in our homes? Relative humidity above 70% is uncomfortable, particularly in warm weather. This is because it makes our natural cooling system, sweating, work less efficiently. The higher relative humidity the slower sweat evaporates from the skin and the warmer we feel. Lowering relative humidity helps us cool more quickly and feel more comfortable.

What about mold and dust mites, how does a dehumidifier work to stop them? Condensation is not necessary for mold and dust mites to thrive. The key is to keep relative humidity no higher than 50% which should prevent mold growth and reduce dust mites to a tolerable level.

Not all dehumidifiers use the same process to do this. The following sections explain the most common types of dehumidifier, how they work and the strengths and weaknesses of each.

How Does a Dehumidifier Work?
Refrigerant Dehumidifiers

Also described as a "mechanical/refrigerative" or "heat-pump" unit, this is the most common type chosen for use in the home. How does a dehumidifier work using the refrigerant process?

If you are partial to ice cold drinks you will have noticed how condensation forms on the outside of your glass as the warm air makes contact with the cold glass. Over time a small pool of water will form at the base of the glass.

This type of machine uses the same principle. A fan draws in air from the room and passes it over a cold coil. Water in the air condenses on the coil and drips into a water bucket in the bottom of the appliance. The dry air is then passed over a warm coil before being returned to the room, both much drier and a little warmer than before.

The heart of the refrigerant type is the refrigeration system. This consists of a compressor, which places a liquid, the refrigerant, under pressure and an evaporator coil in which that pressure is suddenly released. As the refrigerant absorbs heat energy part of it evaporates. The heat transfer to the refrigerant is so fast it cools the evaporator coil. This makes the evaporator coil cold enough to condense water vapor from the air. The condensed water then drips into the water collection container.

The third part of the system is the condenser coil where the refrigerant returns to liquid form. This process causes the refrigerant to give up its heat energy which warms the evaporator coil. This accounts for the fact that the air leaving the appliance is a few degrees warmer than the ambient air.

The refrigerant circulates through the system in a continuous cycle. The pressures are very great which in part explains why a these machines usually have a shorter life span than some other home appliances. So if we are talking about the refrigerant type the simple answer to the question how does a dehumidifier work? is, "in the same way as a refrigerator".

All refrigerant models will come with the basic refrigeration mechanism described above, a fan, a bucket to collect the water and a switch to turn them on and off. Beyond this there are a number of "options" which may be helpful or even essential depending on the area you wish to keep dry.

How Does a Dehumidifier Work?
Capacity

Imagine your house was on the Moon. There is no atmosphere on the Moon so you would need an air tight house with an artificial atmosphere. Healthy life requires an oxygen content of around 20%. As low as 10% and we lose consciousness and die. If your atmosphere generator produced insufficient oxygen you would be in serious trouble. There is no middle way here, you either have enough oxygen or you have nothing.

A dehumidifier is similar, although fortunately we are not talking about life and death for most people. If the appliance won't draw enough water from the air for the space you need to keep dry, it stays wet! You will have paid for the unit and the electricity to run it and you will have little of no benefit.

Dehumidifiers come with a range of capacities, expressed as pints or litres per day, ranging from those designed to keep a closet dry to others that can control moisture throughout an entire house.

If the appliance is designed to be used in the home the manufacturer will usually specify the area, in square feet or meters, or the volume, in cubic feet/meters, that the machine should keep dry. These figures provide a guide but should be treated with caution.

The maximum water extraction rate quoted has usually been measured at an ambient temperature as high as 90F/32C and a relative humidity of as much as 90%.

Refrigerant models remove water at a rate that depends on the conditions in which they operate. They will remove more water at a higher temperature and/or relative humidity than at lower values of one or both. Under conditions typically found in our homes extraction performance will be lower than the maximum quoted. This is less of a problem in our living spaces but can be an issue in cooler areas, such as a basement, crawl space or garage.

Because of their variable performance in different conditions it is always a good idea to consider going one capacity higher than you need, particularly if, according to its specification, a given machine is only just enough for the space you wish to dry.

How Does a Dehumidifier Work?
Defrost Feature

A refrigerant model is sensitive to the ambient temperature in a second, important way. Because it uses a refrigeration coil to condense water vapor the condensed water will freeze if the temperature of the space being dried is too low.

Such freezing can damage the appliance and will usually, therefore, cause the machine to shut down until the ice has melted.

Once again, this is less of problem in the usual living areas of a home, although if heating is switched off at night, and the nights are cold, the temperature may be low enough to cause freezing.

In the cold areas, basements, crawl spaces, garages etc., this is more critical. The basement is always the coldest part of any house because warm air rises. Garages are typically neither heated nor insulated so are generally colder than the house.

Most portable dehumidifiers have an "auto-defrost" feature. This detects the formation of ice and suspends the refrigeration process; that is it turns off the compressor. The fan continues to run and the flow of air from the room melts the ice.

Ice formation can happen at any temperature from about 65°F (18°C) down. It will be obvious that the cooler the air the more quickly ice will form and that cooler air will melt the ice more slowly than warmer air, which means longer and more frequent periods of down-time. For this reason a refrigerant model with auto-defrost becomes significantly less effective as the ambient temperature falls.

Such a model will operate at temperatures as low as 40-42°F (about 5°C) with auto-defrost but will not be genuinely effective. Claims that a unit of this type will operate in low temperature conditions refer only to the fact that it can operate without being damaged.

How does a dehumidifier work in low temperatures? Some come with an active defrost feature, often called hot gas defrost. Hot gas defrost systems work in different ways but essentially they reverse the flow of air warmed by the condenser coil to speed up thawing on the evaporator coil. This results in much shorter periods of down-time and makes this type capable of effective operation at low temperature.

If your need is for an appliance that can operate in temperatures below around 60°F (15°C) for extended periods please ensure that the one you choose has an active defrost system, not merely auto-defrost.

How Does a Dehumidifier Work?
Humidistat

A humidistat is a device with a function similar to that of a thermostat. Whereas a thermostat measures the heat of air or water, turning your space heating or water heating on or off according to the temperature you have pre-set, a humidistat will turn your unit on or off at a pre-set level of relative humidity.

Most portable models have a humidistat. Some come with an adjustable humidistat with which you can choose the level of relative humidity you prefer, usually in increments of 5%. Some, however, come with a limited number of preset levels, between two and four. An adjustable humidistat gives more control and is, in our view, preferable. This is important for two reasons:

First, economy; a relative humidity of 50% or less will usually be sufficient to prevent damage, discomfort or health problems arising from damp. If this is your main objective running the appliance once the desired level of relative humidity has been reached wastes expensive electricity.

Obviously, if you have a short term need to dry out a wet or very damp space or, perhaps, to speed up the drying of laundry inside, you will choose a lower setting.

Second; protecting your home and possessions; Relative humidity of 40% or less will cause wood and leather, for example, to dry out and even crack, as well as making the air too dry for personal comfort.

Our recommendation is that in almost all circumstances an adjustable humidistat is an essential feature.

It is important to note that the humidistat on a typical portable will have a margin of error which may be as much as +/-5%. It is a wise precaution to purchase a good quality hygrometer, a humidity measuring device, and check the performance of your appliance against it. If your unit is under or over measuring relative humidity you can adjust the humidistat to compensate for the error.

How Does a Dehumidifier Work?
Energy Efficiency

While a humidistat will prevent a dehumidifier from running unnecessarily it will not prevent it from running uneconomically when in use. Selecting an appliance with a higher Energy Factor will clearly save more money on power.

In North America the Energy Star® standard defines the energy factor, EF, as the number of litres of water removed from the air for each kilowatt-hour of electricity used, under the test conditions — air temperature of 27°C (80°F) and relative humidity of 60%. For reasons explained above the actual performance, and therefore the actual cost, will be different at other temperatures and relative humidity levels.

The key factor is the relative difference in cost between Energy Star® qualified and non-qualified models. The difference claimed by Energy Star® is a saving on qualified models of 15% over non-qualified equivalents. This, they say, amounts to an energy cost saving of $20 a year for the typical user.

There is of course a trade-off here. Energy efficient units are more carefully designed and constructed than less efficient models, which comes at a price. Of course that generally means better quality, fewer problems and, usually, longer warranty.

We cannot recommend what you should buy because the trade-off between running costs versus initial outlay, bearing in mind quality and reliability issues, will depend on your individual circumstances, judgements and priorities. We do suggest however that you take account of this factor while researching the market and making your choice.

There are two points to consider. The life-time cost of a dehumidifier with a typical pattern of use will be more than 100% greater than the initial purchase price. Larger models have a higher energy factor than smaller ones, they remove more water per energy dollar expended. A larger unit is also likely to work fewer hours per day in a room of a given size.

How Does a Dehumidifier Work?
Continuous Drainage

These appliances remove water from the air and that water has to go somewhere. All have a bucket, fitted inside, where the water collects but these buckets have a limited capacity and must be emptied. They can be easily removed by hand and the water tipped into a sink or drain and this will need to be done at least once, more often twice in 24 hours; even more in extreme circumstances.

When the bucket is full the machine switches off, so no flooding, but the machine is then not operating until the bucket is emptied. This may not be a problem for you.

If, however, you have a memory that is less than entirely reliable, or, more importantly, the unit is located somewhere that you don't or can't get to at least once daily, it may be a problem.

Many machines come with a continuous drainage facility. This simply means you can attach a hose (sometimes but not always supplied) and direct the water to a sink or drain so it doesn't have to be emptied daily.

This refinement may be attractive to anyone for the convenience it offers but is essential for anyone with a disability that prevents them from emptying the bucket by hand, or who needs to control moisture in an inaccessible location continuously.

The drainage from portable models is almost always by means of a gravity feed. Put simply, the water must travel "down-hill" to drain. Raising the unit a couple of feet from the floor will make this work more effectively. If you need to drain over any distance, or to a drain on the floor above, a separate condensate pump will be needed. A very few brands, Winix models for example, have an internal condensate pump.

How Does a Dehumidifier Work?
Auto-Restart

An interruption is our electricity supply is always possible. If we are away from home for more than a day or two, or use our dehumidifier somewhere we can't readily get to, a power outage means loss of humidity control until we can restart the machine by hand.

Automatic restart means the machine will restart itself when power is restored. It is preferable that any settings we have selected are also restored. You will be able to judge from your own circumstances whether this is a benefit you need.

How Does a Dehumidifier Work?
Air Filter

While all refrigerant models come with an air filter some are better than others.

The first purpose of the air filter is to protect the appliance. Any device with a fan to draw in air will also draw in dust and other fine debris. This dust can damage the machine and/or degrade its performance. The filter is usually easily removable and should be taken out and cleaned in line with the manufacturers instructions. Failure to do this regularly can reduce the efficiency of your machine.

Some models come with a more effective filter that traps smaller particles. This can be a benefit to anyone sensitive to air quality, such as asthmatics. Please note that dehumidifiers are not equivalent to air purifiers, even with a high quality filter.

How Does a Dehumidifier Work?
Noise Level

As with any device with major moving parts these devices make some noise and with some makes and models this can be intrusive and annoying. Those built to a higher standard, and sold at a higher price, will tend to make less noise although this is not always the case.

The causes of noise are these. The fan makes the most noise and in many models it runs continuously to sample the air for the humidistat. The compressor is noisy but less so than the fan and runs intermittently. It stops when relative humidity is at the pre-set level and it also stops when the unit is defrosting. The third cause of noise is secondary vibration. This tends to afflict cheaper, less well built units. It will also manifest itself as an appliance ages and wears.

If yours is placed in a living or sleeping area the noise level may be more important to you than if it's in, say, a laundry room. We rely very much on feedback from users for information about the noise levels of particular makes and models.

Your feedback on this is very valuable. This is particularly the case as many people have no experience of these appliances when they buy their first one and have no bench mark for making comparison between machines. An unusually noisy machine may be faulty but it's difficult to recognise this if it is the first you have owned.

How Does a Dehumidifier Work?
Warranty

This page is entitled "How does a Dehumidifier Work"...but what if it stops working?

Every reputable manufacturer offers a warranty on their product but the length of that warranty, and what it covers and, more importantly, what it does not cover varies widely.

Models with longer, more comprehensive warranties tend to be more expensive than those with inferior warranties. If a manufacturer offers a long warranty, covering most of the likely eventualities, he probably has confidence in his product.

Although this cannot be relied upon in all cases it does offer the reassurance that faults are unlikely and that, if they do occur, they will be professionally corrected.

Once again, it's a trade-off between initial purchase price and longer term operating costs. Bear in mind that a cheap unit that fails outside its warranty period may well be uneconomic for you to repair and may have to be replaced.

Whatever you decide on this point, ensure that you read the terms of the warranty so you don't get any unpleasant surprises if something does go wrong. You may, for example, discover that your five year warranty on the sealed refrigeration system does not cover the costs of shipping the unit to and from the repair depot.

How Does a Dehumidifier Work?
Service

This item relates to the previous one. Availability of service for your machine within a reasonable distance of your home is clearly an advantage. The standard of service customers experience varies from one manufacturer and/or supplier to another.

Other pages on this site report the experiences of customers and any you may have, both good and bad, will be welcomed for the benefit of other visitors.

As you can see this is quite a complex machine, a fan, two coils, a compressor, buttons and dials to operate a control system, often a humidistat, hot gas defrost on some models, a water bucket and drain system, an air filter and so on. Some might be tempted to ask "how does a dehumidifier work...at all?"

Unsurprisingly these appliances are not cheap and the more capacity and sophistication you need and the better the overall manufacturing quality you prefer, the more expensive the dehumidifier. The refrigerant type also requires a constant source of electricity and will use a significant amount of power when in use.

The good news is that these devices do work, justifying the initial expense both through the improvements in comfort and convenience they bring about and the costly water damage they can prevent. Air conditioners too will reduce the moisture content of the air but some modern units are air coolers first and dehumidifiers second so may be less effective in this respect. When all is said and done the refrigerant type remains the preferred choice of the majority of those buying for use in the home.

How Does a Dehumidifier Work?
Desiccant Dehumidifier

How does a dehumidifier work using a desiccant? It could equally well be asked, how does a dehumidifier work without a compressor?

A desiccant model uses a water absorbing material, a desiccant, to capture molecules of water from the air. The desiccant is mounted on a revolving rotor over which the air is passed. It is the surface of the desiccant that absorbs the water so the rotor is cleverly designed to present the largest possible surface area to the air and to draw that air through a large number of carefully designed pathways.

The desiccant type has a fan to draw in and expel the air which passes across the rotor. The rotor rotates slowly and during every cycle a segment of it passes in front of a stream of warm air which dries the rotor. This air is heated by the appliance and driven by a second fan.

This process has traditionally been employed in large scale applications, industrial and commercial premises for example. Industrial models using desiccant technology vent the warm, moist air outside the building using ducting. Desiccant dehumidifiers are less energy efficient than refrigerant models due to the cost of heating the air that dries the desiccant. They also tend to be more expensive to buy than refrigerant units.

The higher purchase and energy costs, combined with the need for ducting, have restricted the use of desiccant dehumidifiers in the home. Recent technological advances have now made desiccant models for the home affordable. Portable desiccant units are now available in Europe which collect water in a bucket, in a way similar to the refrigerant type, so ducting is not required. Energy efficiency has been improved, although it cannot yet match that of refrigerant units, and they are comparably priced.

The advantages of a desiccant model are its ability to operate effectively at low temperatures as there is no risk of ice formation, its lighter weight and quieter operation. Light weight and quiet operation are benefits of the desiccant process because there is no refrigeration process involved and, therefore, no compressor.

A portable desiccant model is about half the weight of a refrigerant unit of equivalent power. The reduction in noise is much less dramatic. The main source of noise is the fan. A desiccant unit has two fans. The compressor is a significant but lesser source of noise and the noise reduction achieved by desiccant home dehumidifiers has been over stated and led to disappointment among consumers.

The most highly valued advantage of the desiccant type is its ability to operate at temperatures close to freezing point. Desiccant models are popular in Europe for use in garages, boats and other unheated locations in cold weather. In the USA there are plenty of potential users in the cooler zones but the market has not responded and the home dehumidifier is still largely regarded as an appliance for use in hot, humid summers.

One reason why the question, how does a dehumidifier work? is important with respect to desiccant models is that some of you may be wondering where the heat that dries the desiccant rotor goes to. It goes straight back into the room, at around 20°F/10°C higher than the ambient temperature. This is clearly another reason why this type is better suited for low temperature applications.

In the USA the main application of desiccant technology to home dehumidifiers has been restricted to whole house models, notably from Novelaire.

We do not encourage the use of portable desiccant units for ordinary home use as they are more expensive to run than refrigerant equivalents. We do, however, recommend these units for use in cold locations such as garages and boats.

In most other respects answers to the question how does a dehumidifier work? are the same for desiccant as for refrigerant models and most of the points already made apply. These include those relating to capacity, humidistats, power consumption, continuous drainage, automatic restart, air filters, warranty and service. The issue of a defrost feature does not apply as freezing of components does not occur with this type. With regard to noise they should be quieter than the refrigerant type but please do not be surprised to find that the difference is less than the advertising suggests.

How Does a Dehumidifier Work?
Electronic Dehumidifier

Both the above types have moving parts, so how does a dehumidifier work without moving parts?

An electronic dehumidifier operates using a third process, without moving parts. It employs a peltier heat pump which uses the principle that applying electricity to a panel consisting of two different materials, one on top of the other, like two slices of bread, creates a "heat flux" at the junction between the two materials. Ones side get hotter while the other gets cooler. Water from the air condenses on the cool side and thus is removed from the air.

For a fuller explanation of the peltier heat pump please go to "Thermoelectric cooling" at Wikipedia

This technique is used for very small appliances which will only be effective in very confined spaces, such as a closet, and needs a permanent supply of electricity. A typical model, such as the "Eco-Egg Mini Electronic Dehumidifier" will extract 100ml of water per day and retails at around $60/£40.

Since there are cheaper, equally compact and efficient models that have comparable performance and do not require a permanent electricity supply we do not recommend products of this type for closets and similar spaces and would suggest that their water removal rate makes them only marginally effective in any but the smallest room.

One point to note is that some models are described as "electronic" because they have electronic rather than mechanical controls. They should not be confused with those that operate by means of a peltier heat pump.

How Does a Dehumidifier Work?
Rechargeable Dehumidifier

How does a dehumidifier work without either moving parts or a constant supply of electricity?

Often referred to as desiccant or mini dehumidifiers this type has similarities to the portable desiccant models described earlier but also important differences.

Like the desiccant type the rechargeable dehumidifier uses a water absorbing material which literally soaks up the water from the air. In principle it is no different to the small bags of desiccant you find when you unpack items such as cameras after purchase.

The main difference is that this type has no moving parts and requires no energy to operate. Over time the desiccant becomes saturated which is when the unit needs "recharging" by connection to mains electricity for about ten hours. The "recharging" is actually conversion of the electricity into modest heat to dry out the desiccant for re-use. Typically this type will need recharging every four to eight weeks.

This is a low capacity device, designed for very small spaces such as closets, storage boxes etc. It is silent in operation and can be easily placed where it is needed because it is independent of a permanent power supply. This type of dehumidifier is not suitable for rooms of any size.

If used for the purpose for which they are designed these units are effective.

How Does a Dehumidifier Work?
Heated Rod Dehumidifier

How does a dehumidifier work without removing any moisture from the air?

The final type dehumidifier is the heated rod variety. Technically this is not a dehumidifier as it does not remove water from the air but raises the air temperature which has the effect of reducing relative humidity and thereby lowering the risk of condensation.

It consists simply of a metal rod through which an electric charge passes constantly and has the same effect that a light bulb would have if the light was left switched on. The purpose designed rods are, however much safer as the maximum temperature they reach is only just too warm for a person to hold the rod in an unprotected hand, well below the level at which, if used according to the manufacturer's directions, it presents a risk of fire.

This type also is restricted to small, enclosed spaces, gun safes for example. Inside the gun safe it keeps the temperature above the level at which condensation will form and, if fitted to the floor of the safe, creates a circulation of air through convection.

Once again, if used for its designed purpose a heated rod is effective. Its popularity with gun owners is such that many gun safe manufacturers pre-drill a hole in a wall of the safe to allow a power cable to be passed through to the rod.

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How Does a Dehumidifier Work?