How Does a Dehumidifier Work?

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Last updated: 
February 21, 2018

When we’re asked how does a dehumidifier work, the questioner is either curious or is really asking why their dehumidifier isn’t working as it should. For both the inquisitive and frustrated, this post explains how a dehumidifier removes excess moisture from the air.

The Steps section below details how a dehumidifier works and includes tips on what to do if it isn’t doing its job.  After reading this post, “How does a dehumidifier work,” you’ll both understand the process and how to get maximum benefit from your appliance.

Portable dehumidifier next to couch

Dehumidifier Steps and Solving Common Problems

Here’s an overview of the process. A dehumidifier works by pulling in moist air and condensing moisture out of it to dry the air. The unit contains tubing filled with refrigerant. The refrigerant is circulated by the compressor first to an evaporator where it collects heat from the air and then to a condenser where heat is squeezed out of it. But heat isn’t the issue. Moisture is.

The purpose of using refrigerant to capture heat from the air around the evaporator is to make the evaporator cold enough for moisture to condense on it. The condensed water drains into a collection bucket or drain tube. Voila, the air is dehumidified! Now, let’s break down “how does a dehumidifier work” steps and solve common dehumidifier problems.

Step 1. The Dehumidifier Pulls in Moist Air

A fan in the dehumidifier pulls humid air into the front of the appliance. If this isn’t happening, the most common problem is that the humidistat, indicating how dry you want the air, is set below the humidity level in the room. Turn it up, like turning up a thermostat until the furnace comes on, to make the dehumidifier start. If you turn it up all the way, and nothing happens:

  • The collection tank is full, prohibiting the unit from turning on. Check and dump the tank, if needed.
  • The air is drier than the driest setting on the humidistat, so the compressor won’t start circulating refrigerant.
  • The fan or electrical components aren’t working and need repair.

Step 2. Refrigerant Captures Heat in the Evaporator

If warm air is being blown out the back of the dehumidifier, it is collecting heat as it should.

If the fan is running, but no heat is being removed from the air:

  • Your unit might have a continuously running fan, but the humidistat setting is too low to cause the compressor to come on and start circulating refrigerant. Adjust the humidistat to a drier setting.
  • The dehumidifier might be low on refrigerant, so it’s not collecting heat. It requires repair or replacing.

If the dehumidifier is working, but less effectively than it used to:

  • The refrigerant might be slowly leaking out, so repair or replacement is needed.
  • The air filter at the front of the dehumidifier might be dirty, reducing the intake of air. Take it out, and clean it.

Step 3. The Evaporator Gets Cold and Condenses Moisture

If the unit seems to be running normally, but little moisture is being condensed and collected or drained:

  • Turn up the humidistat until the compressor starts (you might hear it come on)
  • Check the evaporator, and clean it if it is covered with debris. Dirt prevents heat transfer.
  • Check to see if the float that descends into the collection tank is stuck in the “up” position that automatically shuts off the compressor to prevent overflow. If it is, gently pull it down, and remove dirt and debris from the float arm to free up its movement.

Step 4. Moisture Drains into the Tank or Drain Hose

If the unit shuts off when the air is still humid:

  • Empty the tank if it is full
  • Turn the humidistat to a drier setting

If the dehumidifier leaks:

  • Check to see if the float is stuck and not rising as the water rises. Turn off the dehumidifier, and gently push the float up. Clear debris from its arm that might be causing it to stick.
  • The drain hole, if it has one, might be plugged by debris, preventing water from entering the drain line. Search for and clear a clogged drain hole.

Step 5. Warm, Dry Air Comes Out the Back

We addressed a failed fan in Step 1.

If the fan is running, but the airflow seems diminished, clean the air filter.

In case the air coming out is cool, review Steps 2 and 3.

If the air that comes out smells bad:

  • Clean the filter if it is dirty.
  • Replace the filter or clean it with an oxygen bleach solution if it is moldy.
  • Empty the tank, and if it is moldy, clean it with an oxygen bleach solution

How does a dehumidifier work illustration

Tips for Maximum Dehumidifying

We’ve answered the basic question: “How does a dehumidifier work.” These tips will help you get the most from your unit.

  • Check the filter every few days, and clean it as needed.
  • If you have a drain hose attachment option, use it. This allows you to run the dehumidifier continuously. If you collect water in a tank, it will shut off when full, and you might not be able to empty it for hours or days. Simply make sure the hose runs to a floor drain.
  • Keep doors and windows shut. If humid air from outside continuously flows into space, it will never be dehumidified.
  • Run a vent hood fan when boiling liquid on the stove and the bathroom exhaust fan when taking a shower to remove humidity.
  • In warm weather, run a central AC or window unit to assist with dehumidification. Air conditioning uses the same technology, except that an AC moves warm air to the outside of your home while dehumidifying the indoor air.
  • If the space is below grade and humidity issues persist, contact a foundation specialist about damp-proofing your home’s basement or crawlspace.

Many homes have humidity issues that make them uncomfortable. When humidity leads to mold growth, the homes are unhealthy too. Many of your friends and social media followers probably wonder, “How does a dehumidifier work?” Passing along this post can help them, their families and pets live in a home that is comfortable and healthy.

Images sources: 1, 2

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