How Do Dehumidifiers Work?

Dehumidifiers demystified

"It's not the heat, it's the humidity." These frequently spoken words have become the stuff of stand up comedians.  Although we have all experienced the sensation associated with standing in a hot, muggy room, the physical principles at work in mechanically controlling humidity take place on a molecular scale.

What is humidity, really?

Measuring up

Humidity is the amount of water vapor suspended in the air at a given time.  As air changes temperature, it's ability to hold water changes.  Warm air can hold more water vapor than cold air.

Humidity can be measured many ways, but the two most common measurements are absolute humidity and relative humidity.  Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature.  The temperature must be specified, since the amount of vapor contained in that volume of air would change if it heated up or cooled down.

Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which again depends on the temperature). A reading of 100% relative humidity means that the air holds the maximum amount of water vapor it could possibly hold and could only take on more by changing temperature.  In short, the overall amount of water vapor the air is currently holding, means less to us than knowing how saturated the air is.

Why should I use a dehumidifier?

Benefits of dehumidifiers

Most residential air conditioners accomplish the goal of dehumidifying the air in a room.  However, they consume a great deal of energy and can be quite noisy.  A dehumidifier has several benefits:
 

For your home

By creating dry air, a dehumidifier helps prevent damage to home and property.  Moisture wears down painted surfaces, photographs, and anything else that isn't waterproof; drier air slows down the gradual erosion and degradation of these materials inside the home.  Museums use commercial dehumidifiers to protect priceless works of art in this way.  The same principle applies to residences and all the things contained inside.  Especially useful for basements, where many heirlooms and important documents are stored in porous cardboard boxes, a dehumidifier can help preserve these important keepsakes and records.

Reducing moisture with a dehumidifier cuts down on mold spores and house mites dramatically. House mites live in bedding and leave waste that can aggravate asthma.  They thrive in relative humidity over 50%.  Mold needs very wet air to grow.  Keeping the relative humidity below 50% creates a hostile environment for these pests and promotes respiratory health.  In addition to the health benefits for sufferers of asthma, allergies, and emphysema, most people find drier air more pleasant.

Moist air demands more energy to heat than dry air does, due to its greater mass.  A relative humidity of less than 50% cans save energy by making the air easier to heat.  The same amount of energy can be used to achieve greater impact on the air once the unwanted moisture had been removed.

For your health

Human skin responds to changes in humidity. When we perspire, our body is attempting to rid itself of excess heat.  Normally, sweat evaporates off the surface of the skin into the air.  If, however, the air is at 100% relative humidity, the sweat can't escape because the air is already holding as much water as it can. The upshot is that humid air really does feel hotter, even at the same temperature. People tend to feel most comfortable at a relative humidity of around 45%. 

Home air dehumidifiers remove moisture from the air.  Dehumidifiers take advantage of the variable capacity of air to carry water vapor by drawing moisture out of the air as condensation on a cold surface.  Dehumidifiers convert electrical current into a region of cold that drains moisture out of the air, leaving behind liquid water.  A vent takes air in and a blower distributes the conditioned air back out.  Most home dehumidifiers function using a mechanism similar to an air conditioner that has both its hot and cold coils in the same unit:  a fan draws the room's air over the cold coil of the air conditioner to condense the moisture (which normally drips into a designated receptacle). The dry air then passes through the hot coil to return it to its original temperature before exiting the loop.  The amount of moisture in the resulting conditioned air can be monitored and controlled by the unit.

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