Humidity and Dehumidifiers


An appliance that reduces the humidity in a room by condensing moisture in the air on to a cold surface. A dehumidifier works in a similar way to an air conditioner, the main difference being that it has both its hot and cold coils in the same box. Air is drawn in by a fan and moisture from the air condenses on one set of coils much like an air conditioner’s evaporator coils. The water drips into a removable bucket or through a hose to a drain. The other coil warms the air, which is blown back into the room.

Although a dehumidifier doesn’t cool the air in a room (in fact it heats it slightly), it makes it more comfortable because it decreases the humidity. Most dehumidifiers don’t work well under about 65°F because at these temperatures the cooling coil would have to operate below freezing point to condense any moisture.


Dehumidifier capacity is usually measured in pints per 24 hours and is determined by two factors: the size of the space that needs to be dehumidified and the conditions that exist in the space before dehumidification. Use the chart below to estimate the capacity you are looking for.

Condition without dehumidification
Area (sq. feet) 500 1,000 1,500 2,000 2,500
Moderately damp (space feels damp and has musty odor only in humid weather) 10 14 18 22 26
Very damp (space always feels damp and has musty odor. Damp spots show on walls and floor.) 12 17 22 27 32
Wet (space feels and smells wet. Walls or floor sweat, or seepage is present.) 14 20 26 32 38
Extremely wet (laundry drying, wet floor, high load conditions.) 16 23 30 37 44

Source: Association of Home Appliance Manufacturers (AHAM)

Water removal options

Most dehumidifiers use a removable plastic bucket and warning lights to indicate when the bucket is full and needs to be emptied. There is typically an automatic shutoff when the bucket is full, so you won’t have to worry about the bucket overflowing. Most dehumidifiers also come with a fitting that allows you to hook a hose directly to the bucket, which can then be directed into a floor drain or sump pump, so there is no need to empty it.


Most dehumidifiers have top-mounted air discharge and can be placed against walls, but if you don’t have top-mounted discharge, make sure the dehumidifier is located away from walls and furniture, so that air can circulate freely around the unit. This will result in better operation of the product.

Doors and windows to the space being dehumidified should be closed while the unit is running. This will ensure that the space is dehumidified as efficiently as possible.

Locate away from sources of dust and dirt (like woodworking equipment), which can clog coils and grills.

Observe all manufacturer warnings regarding electrical safety. In particular, never set up water drainage or disposal near electrical circuits or devices. Make sure the dehumidifier is connected to a properly grounded outlet. Keep drain hoses away from electrical cords and connections.

If you are planning to use a hose to drain the dehumidifier’s water bucket, make sure the unit is located near enough to the floor drain or sump to avoid the need for a long and unwieldy hose. Don’t create a tripping hazard!

Don’t be alarmed if the air temperature directly around the unit is warmer than in surrounding areas. As a dehumidifier removes moisture from the air, it slightly warms the space around it.

Operating temperature

If the space being dehumidified has temperatures that typically fall below 65°F, you may want to consider buying a product that is specified for use at lower temperatures.

Frost can form on the condensing coils if the air temperature drops below 65°F, and may negatively affect the performance of the product by causing the compressor to cycle on and off repeatedly without removing moisture from the air. If this happens, the unit should be switched off and allowed to defrost before it is turned back on. Some dehumidifiers come with an antifrost sensor, which will automatically turn off the unit if the air temperature drops below a certain point.

Qualified ENERGY STAR models are available that are rated for use at temperatures as low as 42°F.

Relative humidity (RH) and humidistats

Humidity, the amount of moisture or water vapor in the air, is most often talked about in terms of relative humidity (RH). RH is the amount of water vapor actually present in the air compared to the greatest amount of water vapor the air can hold at that temperature. The optimum RH level for a building is generally considered to be in the range 30–50%. Anything above this range may promote bacteria growth. (In colder climates, during the heating season, humidity levels should be in the range of 30–40% RH to prevent window condensation.)

Many dehumidifiers include a built-in humidistat, a device that allows you to set the desired RH level that you would like for the room. Once the room reaches the desired RH level, the dehumidifier will cycle on and off automatically to maintain the level.

If your dehumidifier does not have a built-in humidistat, or has a humidistat that does not show RH levels purchase a hygrometer (a gauge that measures relative humidity) separately and hang it near the dehumidifier. The hygrometer will measure RH levels, and enable you to monitor when to turn the dehumidifier on and off.

Energy factor

The energy efficiency of dehumidifiers is measured by its energy factor, in liters of water removed per kilowatt-hour (kWh) of energy consumed or L/kWh. In general, a higher energy factor means a more efficient dehumidifier.

Other available features

Various performance and convenience features are available on dehumidifiers, which may add to their purchase price.

  • A timer will enable you to control the cycling of the dehumidifier, and set it to turn on at night. This may save you money on electricity bills, by taking advantage of off-peak electric rates.
  • A washable air filter will allow you to easily remove and clean the filter, so that the dehumidifier does not become clogged with dust and dirt and operate less efficiently. Additionally, the unit’s coils should also be regularly vacuumed or dusted, so that dirt does not build up there, either.
  • A pause button enables you to stop the dehumidifier for a pre-set short period of time if the sound from the dehumidifier interferes with a nearby activity.
  • A built-in condensate pump, which takes water from the dehumidifier’s water storage bucket and pumps it directly out of a window or into a sink for draining can be a big time-saver. This feature means you would never have to worry about the dehumidifier turning off automatically due to a full bucket, nor would you have to manually empty the bucket. (The advantage of a condensate pump over a hose for drainage is that the hose would be reliant on gravity, so it would require careful placement to ensure proper drainage into a floor drain or sump.)
  • Multiple fan speeds will mean that you have the choice of running the dehumidifier at a higher speed, which is generally more efficient or a lower speed, which is quieter.
  • If you plan to move your dehumidifier around, look for one with an easy-to-use carrying handle and/or wheels. This will make transportation of the unit a lot easier!

Any of the following will help reduce the levels of humidity in your home:

  • Install a dehumidifier compatible with the size and conditions of the room in which it will be used. A basement dehumidifier can reduce humidity around the entire house.
  • Improving the drainage around the foundation of your home may result in decreased humidity in your basement. Some ways to improve drainage are:
    • Extend downspouts from your gutters away from the foundation of your home
    • Keep gutters and downspouts clear and open
    • Ensure that the soil slopes away from your foundation, to avoid pooling of water around your home
    • Avoid over-watering of foundation plantings
  • Ensure that clothes dryers are properly vented to the outdoors. If you don’t use a clothes dryer, dry your clothes outdoors as opposed to hanging them inside for drying.
  • Use vent fans in bathrooms and kitchens to remove humidity at the source.
  • Repair leaking outdoor faucets. Not only will this help reduce humidity, but it will also save you money on your water bills!
  • If your home uses central air conditioning, consider installing an A/C vent in the humid space in your home and take advantage of the dehumidifying capabilities of your A/C system. This will also help with air circulation, improving airflow between humid parts of your home and drier parts.

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