A Homeowner's Guide to Dehumidifiers

As its name implies, a dehumidifier removes excessive humidity, or moisture, from the air. While a dehumidifier isn't an everyday necessity in many homes, it can be a must-have device wherever excessive moisture is an issue, such as homes in damp climates, or in a basement during humid, rainy times of the year. A dehumidifier can mean the difference between a comfortable environment and a swampy, musty home harboring potential health issues.

Why Humidity Matters

Damp sandstone wall stained with mildew
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Humid conditions can foster the growth of a number of allergens, including molds and dust mites. Even for those without allergies or asthma, these indoor pollutants can be a problem. If an area is too damp, molds can grow on the walls, ceiling or floor. Over time, some molds can cause wood to rot, causing structural damage to your home. Some molds are even toxic—black mold (Stachybotrys spp.) is one example. Reducing the potential for mold proactively by keeping humidity levels low is much easier and less expensive than dealing with large-scale mold removal and cleanup, which can be quite complicated.

Ideal Humidity Range

Electronic hygrometer
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An indoor relative-humidity level in the range of 30 to 50 percent is ideal for both your home and your health. In the summer, aim for a range near 50 percent when running a dehumidifier, as attempting to keep humidity levels lower can lead to higher energy bills. The humid summer months are when dehumidifiers are most likely to be needed.

In winter, when the air is naturally dry in most regions, a relative humidity level of around 30 percent is acceptable. It's unlikely a dehumidifier will be necessary in winter, except in very moist tropical environments. In most temperate zones, winter air is more likely to require a humidifier to keep moisture levels up in the 30 percent range. Air that's too dry can cause nasal passages to dry out, leading to nosebleeds and making you susceptible to cold viruses.

Relative humidity levels can be measured with a hygrometer, a device available at a home improvement centers and hardware stores, as well as from online retailers. Position the hygrometer away from windows and drafts and set it up following the directions included with the device. Hygrometers are best used in rooms other than the kitchen or bathroom, where humidity levels fluctuate frequently.

How a Dehumidifier Works

A fan inside the dehumidifier draws humid air in, blowing it over cold evaporator coils. These coils are cooled by a circulating refrigerant, much the way an air conditioner works. Moisture from the room's air turns into water droplets on the coils in the same way that droplets of water condense on the outside of a glass of ice on a hot day. The water on the coils falls into the collection bucket, while the dried air continues flowing through the unit over a set of warm coils and back out into the room. Unlike an air conditioner, a dehumidifier does not cool the air in the room; it tends to warm it up a bit. A built-in humidistat, which monitors humidity levels in the room, turns the dehumidifier on or off depending upon your chosen humidity settings.

Portable Dehumidifiers

Dehumidifier and Electric Meter
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The simplest dehumidifiers are portable, collecting water in a bucket that slides out for manual emptying. Many also have a drainage hose option, allowing you to set up the appliance near a drain for continual draining. Some models also have a pump that allows the unit to be drained upwards through a hose; these are ideal for setting up in a basement laundry room, next to a utility sink.

Even the most basic portable models offer some sort of option for selecting humidity level in the room. The simplest ones may offer "more dry" or "less dry" as options, while deluxe portable units and whole-home systems offer a thermostat-style digital readout to optimize the ideal humidity level.

Pints Per Day Capacity

Dehumidifier capacity is measured in pints of water collected every 24 hours. Damper and larger spaces require a humidifier with larger capacity. For instance, a 10-pint-capacity dehumidifier is enough to tackle a 500-square-foot space that seems musty and damp during humid weather, while a 16-pint (or greater) dehumidifier is better if that same room feels wet all the time. As a comparison, whole-house dehumidifiers can remove 70 to 130 pints per day. If you rarely need a dehumidifier, a low-capacity option may do the trick, while a whole-house dehumidifier is ideal for a structure that feels too humid most of the time.

Whole-House Dehumidifiers

Aprilaire whole-home dehumidifier.
credit: Aprilaire
Aprilaire whole-home dehumidifier.

For ongoing needs for larger spaces, a whole-house humidifier offers a better option. A whole-house dehumidifier can be a free-standing device, not attached to ductwork at all, or it can be a model that connects to the HVAC system, working through the same ducts as the heating and cooling system. The freestanding versions may be installed in a basement, crawl space or even attic. A model that connects to your cooling system will likely be installed alongside the air conditioner for easier access to the ductwork. As with entire-home cooling or heating systems, a whole-home dehumidifier is much more expensive than a portable unit designed to remove moisture from a specific area of the home.

Concerns and Maintenance

Full Frame Shot Of Filter
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Any dehumidifier will have maintenance needs as well as usage issues. Portable dehumidifiers, in particular, can be quite noisy, especially if positioned on an uneven surface such as a basement floor. The bucket-filling option means you'll have to pay attention to the dehumidifier, emptying it every day or so when it is running at peak capacity. The dehumidifier will shut off when the bucket is full, but this also means no more moisture will be removed until you empty the bucket. The inside of the bucket can also develop scum or mildew if you don't empty it frequently or if you don't clean it regularly.

Both portable and whole-home dehumidifiers have filters that require cleaning from time to time. Read the manual to determine where the filters are on your model and how often they require cleaning. For instance, if you live in a home with pets that shed, or if the device is placed in a home woodshop, you may have to clean the filter more often.

Running the dehumidifier in temperatures below 65°F. may also cause the device's coils to freeze, rendering the unit ineffective. If this happens, unplug the dehumidifier and allow the coils to thaw. Select another location, if possible, where conditions are a bit warmer, then start the device again. Also make sure the unit is placed far enough from walls or structures so its ventilation ports aren't blocked.