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How does a dehumidifier work, exactly?

Keep the mugginess down in your living area, your basement, and everywhere else.

How does a dehumidifier work, exactly? Credit: Getty / Marianna Lishchenco

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It’s finally summer, which means lazy days at the beach and… humidity in the house. Or you might have a basement that is always unpleasantly humid as water seeps in from outside. Either way, the culprit is humidity, another term for the amount of water in the air.

There's always some water vapor in the air, but when the amount increases, things get unpleasant because you can’t cool off by sweating. This excess humidity can also be unhealthy, encouraging mold and fungi to grow. Luckily, dehumidifiers make things more comfortable by removing some of this water vapor, making it easier for your body to cool itself and discouraging the growth of mold.

Ever wondered how dehumidifiers work? Here's what you need to know about these handy tools.

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What is 'relative humidity?'

What is humidity?
Credit: AcuRite

Humidity levels are dependent on the temperature.

Measuring humidity can be complicated because the amount of water vapor that the air can hold varies with temperature. The hotter it gets, the more water vapor the air can retain.

So, we use a measure called the relative humidity—that’s the number you see on weather reports. It’s a percentage that shows how much of its maximum capacity of water vapor the air has in it. The higher the relative humidity, the more saturated with water vapor the air will be. When it reaches 100%, the air can’t hold any more moisture, and you get fog or rain.

To remove this water vapor from the air, there are two types of dehumidifiers: desiccant and refrigerant. The following is a breakdown of how each type operates.

What are desiccant dehumidifiers?

Desiccant dehumidifiers in multiple sizes
Credit: Eva-Dry, Meaco

Desiccant dehumidifiers come in a few sizes and form factors. They use less energy, but the small and inexpensive ones need to be recharged or disposed of.

If you’ve seen those little packages of gel balls labeled “do not eat” in pill bottles, you’ve seen how desiccant dehumidifiers work.

This type of dehumidifier contains a special gel that absorbs water, also called a desiccant. In a pill bottle, this keeps the pills from sticking together by absorbing the moisture in the air. In a desiccant dehumidifier, a fan blows air over the desiccant, which absorbs the water vapor.

When the desiccant has absorbed all of the water vapor it can hold, it's heated up. This forces the material to release the water it's holding onto, which is then collected in a tray or pumped away. The humidifier then starts the process again, absorbing more water vapor from the air.

Larger units can be effective for drying the air in small rooms but they are usually not effective enough to keep a larger room dry.

Smaller desiccant dehumidifiers can be great for tight fits like lockers and closets, or where there are no electrical outlets, like a boat. Because of their size, however, they need to be replaced or electrically recharged after a few days or weeks.

The upside of desiccant dehumidifiers is that they don’t use much energy, but the downside is that the system can only remove a relatively small amount of moisture.

What are refrigeration dehumidifiers?

Refrigeration dehumidifier
Credit: Ivation

Refrigeration dehumidifiers remove more moisture from the air.

On the other side, there are refrigeration dehumidifiers. These are the more common variety. If you Google "humidifier," you'll likely be looking at a refrigerant dehumidifier. They can remove more water much more quickly than the desiccant-type dehumidifier. However, they do tend to be heavier and more expensive.

To understand how this style of dehumidifier works, remember that cold air holds less water vapor than hot air. So, if you quickly cool hot, humid air, the amount of water vapor that it can hold will decrease, and the water vapor in the air will condense into water. And, as an added bonus, the air will also be cooler, which feels great on a hot, sticky day.

How does this process happen? Refrigeration dehumidifiers are basically a fridge without a door. Inside them, you'll find a long, sealed tube (usually made of copper), a pump and a valve. The tube contains a refrigerant, a special liquid that turns from a gas into a liquid when compressed. Physicists call this a "phase transition." When the pump compresses the refrigerant into one part of the tube, it becomes a liquid. When the valve is released, the liquid refrigerant rushes through the valve into the empty part of the tube, expands and becomes a gas.

Because phase transitions require energy, the expanding gas absorbs heat from the tube, which becomes very cold. This tube is covered in fins, and a fan blows air over these fins, where the air quickly cools and the water condenses out. At the bottom of the dehumidifier, a tray captures this water and directs it into a container.

As the air cools, the refrigerant heats up and becomes liquid once again. The pump compresses it, and the cycle starts again. This cycle is why refrigerant dehumidifiers run intermittently—the system has to repeatedly stop cooling to compress the refrigerant. Once it's compressed and ready, it can open the valve, and you get that lovely blast of cool, dry air.

The major benefit of refrigeration dehumidifiers is that they remove a lot of water from the air and also cool it, so you get a double effect of dryer and cooler air. The downside is that compressing the refrigerant and pumping it around takes quite a lot of energy and can be noisy. As such, most refrigerant dehumidifiers will need a dedicated power circuit, and you have to put up with the noise of the fan and the pump whirring and clunking away while they work.

Now that you understand how dehumidifiers work, you can appreciate the effort they put into keeping your home cool and comfortable on muggy summer days.

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