How Dehumidifiers Work
Many people understand what dehumidifiers do, but not many people understand how dehumidifiers work, and more importantly what factors will help or hinder the dehumidifier you choose.
The Basics of Dehumidifier Operation
We've all witnessed water forming on the glass of a cold drink. The same principle is used by dehumidifiers to extract moisture from the air. When warm air is exposed to a cold surface, water is pulled from the air and collects on the colder surface. This is called condensation. A dehumidifier uses a fan to pull air across a metal coil that is cooled by a compressor (just like in your air conditioner). Once the water is extracted from the air and collected on the cold coils, the air is re-heated and exhausted. The water that was collected falls into a resevoir where it can be drained.
Air conditioners do almost the exact same thing, but they do not re-heat the air before exhausting it (warm air in, moisture extracted, colder air out).
How Air Movement Affects Dehumidifiers
Now that you understand the basics of dehumidifier operation, you need to also understand what characteristics a dehumidifier must have to work well. For starters, a dehumidifier must have a fan of some sort that actively draws air into the dehumidifier, across the cold coils, and back out again once the moisture has been extracted. The amount of air a dehumidifier can move is measured in CFM, or cubic feet of air per minute. The more CFM a dehumidifier can move, the better job it can do removing moisture from larger spaces. Without an effective air mover, the dehumidifier would only remove air from the space immediately surrounding the unit. Smaller portable dehumidifiers move less air and are typically best suited for single-room dehumidification. Large whole house units are equipped with larger fans and can extract moisture from much more air, and therefore much larger spaces (like an entire basement or crawlspace).
How Ambient Air Temperature Affects Dehumidifiers
Condensation occurs when moist air comes in contact with colder surfaces. If the air temperature is reasonably cold, the dehumidifier must be able to cool the coils even lower to extract moisture. If your dehumidifier is to be installed in an area with low ambient air temperatures (like a crawlspace or unfinished basement) you must make sure that the dehumidifier model you choose is designed to operate effectively in those conditions. Smaller or under-powered dehumidifiers often can't cool the coils to a low enough temperature to extract moisture.
How Relative Humidity Affects Dehumidifiers
Dehumidifiers have an easier time extracting moisture when it is readily available in the air. Dehumidifiers can extract much more moisture when they are running in a moist environment. It is much more difficult for a dehumidifier to remove moisture as you lower the relative humidity. So, if you are trying to bring a moist area (relative humidity of 70% or more) down to a normal level of 50%, many whole house dehumidifiers can accomplish that task. If, however, you are trying to get your relative humidity down to 20% you'll need a much more powerful dehumidifier for that task.