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Dehumidifiers: The How and Why

By Jason Earle Google+

Source: Habitat Quarterly, Summer 2010

Dehumidifiers: a rather dry topic, but depending upon where you live and the time of year, they can be an essential household appliance. Controlling humidity is important for comfort, helping to keep you feeling cool, but it's also a critical element in maintaining a healthy home free of mold growth and dust mite proliferation.

Although your air conditioner does behave like a large dehumidifier, often it can't keep up with the infiltration of humid air from outside, especially in very humid climates. In other cases, dehumidifiers are needed in spaces which aren't affected by your heating or cooling systems, such as crawlspaces. Plus, you still need to control humidity during the nine months when your AC is not running, especially in basements and other scenarios prone to dampness, such as houses built on wooded lots.

When deciding which dehumidifier is best for your specific needs, there are a few key things which you, as an informed consumer, should know.


Get a grip on your humidity issue. How bad is it? You shouldn't modify what you don't quantify. Until you begin measuring the temperature AND relative humidity (RH) in your house on a regular basis, there's no point investing in a real dehumidifier. (We'll talk about what a "real" dehumidifier is in a bit. They are not all created equal.)

We suggest everyone have a digital temperature/humidity gauge. We really like the ones made by Oregon Scientific because they have remote sensors which you can strategically place in areas you rarely visit (like crawlspaces, attics, etc.). The unit I have (RMR500A Multi-Room Climate Monitor) in my house serves me well and you can pick them up directly from the manufacturer. Here's a link: http://www.oregonscientificstore.com.

According to the people who make the "rules" and spend lots of time thinking about things like this, the target range for human comfort is 40% to 60% RH. Without getting into a complicated discussion about psychrometrics (the study of air and water vapor), which would surely put you to sleep, just know that the term "relative" means that the humidity reading is affected by temperature.

When the temperature drops, condensation - which occurs when there is 100% relative humidity, the maximum - is more likely. That's why cold surfaces get wet on humid days and why dew develops on leaves. That's how a dehumidifier works, which you'll read more about below, but let's stay focused. We recommended you target 45% RH. That number will help you stay in the safe zone. The safe zone is where mold growth and the creepy crawlies who are attracted to high humidity don't like to visit. It's a good place where you'll be comfortable but they won't.


The next thing to do is learn how a dehumidifier works. In essence, there are refrigeration coils inside these units which get cold enough that condensation develops (100% relative humidity) and drips into a bucket. A fan forces presumably warm, humid air across these coils with the intention of stripping as much water from the air as possible.

The problem occurs when you place a dehumidifier into a damp space which is NOT warm, like crawlspaces and basements in most climates north of the Mason-Dixon line during the cooler months. In this case, sometimes the coils will get too cold and the condensation will freeze. No dripping means no dehumidifying. So, the key is getting a unit which will operate at lower temperatures. The good news is that you can find them very easily because they prominently advertise this nifty feature. They are called "Low Temp." The other ones are toys, but will cost just as much to purchase and operate.

As an aside, you should also know that dehumidifiers are similar to small refrigerators in terms of electrical usage. In some cases, it can add $20 to $30 a month to your utility bill. Before you balk at this cost, understand how much it costs to clean up a mold problem. This can be VERY expensive. Often the difference between a clean, healthy livable space and a $5,000 to $10,000 mold remediation bill is the proper dehumidifier and a minor additional charge on your monthly electric bill. Don't be cheap here. Trust me on this one.


As with most products, the way things are advertised can confuse us. There is an organization called the Association of Home Appliance Manufacturers (AHAM) and they publish a nice little piece about selecting a dehumidifier. Here's the link. http://www.aham.org/ht/a/GetDocumentAction/i/863.

Why is this important? Well, simply put, the numbers don't always add up. They all publish how many pints of water per day they will remove, but that hinges upon the temperature, as we described above. You can easily buy a unit which is too small and then you'll wonder why you're having a hard time hitting your target 45% RH.

Also, you know from walking through the hardware store that many dehumidifiers are around $100, but what you may not know is that the best ones, and perhaps the one you might need, can be over $1,000. You want to know as much as possible before you make that kind of a commitment, so do your homework.


The other consideration is where you're going to put this daily collection of drippage. You've got three choices:
  • You can go and manually dump it every day, which most people don't do, or...
  • You can have it drain into a pit with a sump pump, which will discharge outside, or...
  • You can buy an additional auxiliary pump which will send the condensation on its merry way to whichever destination you intend.
Now, if you don't empty it, it will fill up and stop working. That's the best case scenario, but still not ideal. The worst case - and this happens all too often - is when the shutoff switch doesn't work and the unit conveniently proceeds to flood the place you were working hard to keep dry. If you are planning to go empty everyday, just remember the gym membership you purchased: Out of sight means out of mind. Eliminate the manual dumping of your condensation bucket on your dehumidifier and go to the gym instead.

My suggestion is to either get an auxiliary pump which is approved by, or made by, the manufacturer of the dehumidifier, or use a sump pump if you have a sump. Two words of advice on the sump pump: First, keep the sump covered. You don't want the water to evaporate right back out, do you? Second, get a battery backup unit. It's worth it. You'll need that pump most when the power goes out. Think hurricanes and thunderstorms.


While there are far more things to say about selecting and using dehumidifiers than any reasonable person would ever expect, I will refrain from deploying the rest of my hard-earned wisdom here and leave you with a final useful tidbit. A dehumidifier works best when the difference between the ambient temperature of the air and the temperature of the refrigerant coils in the unit is greatest.

So, I know this is a summer issue of Habitat Quarterly, but for best results in a cool basement in the winter, see if the furnace or other appliance is throwing off a lot of heat. If so, place the dehumidifier near that heat source. Otherwise, you want the unit as close to the middle of the room as possible.


Regardless of the choice you make, it's up to you to protect your home and your family. Controlling moisture is a fundamental step in the right direction. When it comes to preventing mold and moisture problems, the old cliché that an ounce of prevention is worth a pound of cure holds quite true here.

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Dehumidifiers: The How and Why
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