A homeowner pictured in front of her AC's living room evaporator wiping sweat off her brow

As you know, your air conditioner does more to control a room’s temperature than simply produce cool air. It also dehumidifies as a byproduct of its cooling process.

But what can you do if your AC stopped dehumidifying?

This post will explain why your air conditioner may be removing humidity and what you can do to correct the issue. It’ll also explain some basic maintenance procedures you can perform to better prevent these problems from developing in the future. 

Let’s begin!

Why Is Your AC Not Dehumidifying? 

Your air conditioner may have stopped dehumidifying because the unit is too large, your thermostat settings hinder dehumidification, part of your AC is damaged, or a component is dirty. Thankfully, these problems are easy to fix. 

Now let’s look at ways to remedy each of these issues.

1) Your Thermostat Is Set to “On,” Not “Auto”

This issue is by far the most common problem faced by AC owners. Your system’s thermostat controls the unit’s internal fan. Specifically, when the fan is active and blowing cool air into the room.

A smart thermostat on the wall in our second net-zero home

Users who start using their air conditioner without reading the user manual might think leaving the fan setting at “on” is the best choice. However, if the fan is always on, it risks blowing condensation from the evaporator coil back into the room. From there, it returns as humidity.

How To Fix

Your system’s thermostat has a setting that will maintain a cool temperature while reducing humidity: auto. Switching to the auto setting will only turn the fan on when it detects temperatures have risen to a predetermined point. Auto gives condensation time to evaporate when the room is adequately cooled.

The auto setting has additional benefits, too. First, since the unit will run only when needed and not constantly, you’ll extend its life. Second, switching from On will save you money on the energy costs required to run your machine. 

2) Your AC Is Too Big for Your Room

AC units have specific size requirements for the rooms they are installed in and the size of your home. These dimensions are usually listed somewhere on the box and in the manual. 

Because an air conditioner detects the room’s temperature, the size of the unit will determine how long it runs when in “Auto” mode. 

When a unit is too large for the space it’s installed in, it will only run for a brief period. However, it will ultimately run more frequently in a process called short cycling. When this occurs, the water collected on the evaporator coil will get blown back into your home instead of draining away.

How To Fix

Replacing your current AC with a smaller unit is the best solution to short cycling. However, replacing an air conditioner can be a significant expense. Additionally, you could be misdiagnosing the problem, in which case a replacement isn’t necessary. 

To ensure your unit is the appropriate size for your home, call an HVAC specialist to make an assessment. Not only can they confirm the issue, but they can also make appropriate size recommendations for a new air conditioner. If the size isn’t the problem, they can make the correct diagnosis with their expertise. 

3) Dirty Evaporator Coils

Your air conditioner’s evaporator coils are responsible for creating cold air and removing moisture from the air. However, condensation builds up on the coils to absorb and release heat. Eventually, this water runs off and down a drain line.

However, these coils become dirty due to constant contact with absorbed water vapors. In severe cases, it can even rust. In addition, a thick buildup prevents the rings from absorbing heat and forces the system to work overtime.

How To Fix

Evaporator coils can be cleaned, so you should have them looked at once a year to address dirt and corrosion. While there are guides online on how to clean the coils yourself, they’re a sensitive component of your air conditioner, so it’s best to call on a professional to handle it. Improper cleaning can also damage surrounding parts of the unit. 

The evaporator coil of an air conditioner in the grass outside a home
A central air evaporator coil. Courtesy of Rescue Air

Severe cases of corrosion could result in the coils getting damaged. In such cases, your best option is to replace them entirely. Unfortunately, you cannot repair most physical damage to these parts.

4) A Clog in the Drain Line

When everything in your unit runs smoothly, condensation will go down the drain. But the drain line, like all drains, can clog over time, especially if debris gets in the air conditioner. However, moisture can carry dirt from the coils and deposit it in the drain. 

A clogged drain is difficult to diagnose visually. However, the telltale signs are water pooling around the drain’s opening inside the unit and large amounts of condensation developing on the coils. The inability to rid the air conditioner of moisture prevents it from working efficiently.

How To Fix

Unclogging an AC drain requires partial uninstallation to access the problem. Leave clog removal to an experienced HVAC specialist to ensure no further harm is done to the unit. Allow them to clean the drain to remove any buildup that hasn’t yet developed into a clog.

5) A Leak in Your Air Ducts

Ductwork connects your HVAC components (including your air conditioner) to the vents in your home. Air travels both ways through the system when it’s active. When undamaged, the ductwork keeps circulating air inside the system.

If there’s a seal anywhere in the ductwork, it can have disastrous effects on your air conditioner. The biggest issue is warm air being pulled through the ducts and returning prematurely to your home, releasing humidity rather than trapping it. 

Likewise, duct leaks can cause the unit to run over time, as the leaking air will prevent the air temperature from changing significantly. 

How To Fix

Seeking out and sealing leaks in your ductwork is the best solution. Sealing them is easy—you can use duct tape in most cases, but finding them can be challenging. If they aren’t visible, you might be able to hear the air escaping if you listen closely. 

A technician applying metal-backed tape to an cylindrical duct to prevent air duct leakage

An HVAC professional can utilize special tools to detect leaks throughout your ductwork and even your home. They’re also better equipped to seal large holes or cracks, should they exist. However, if you haven’t stayed on top of maintaining your ductwork, parts may need to be replaced. 

6) Dirty Air Filters

When we talk about air filters, we mean two things: the filters in your AC (if it has its own filters) and those in your vents. 

Both play an essential role in catching dirt and other debris traveling through your ductwork and your unit. However, since they don’t play a direct role in the air conditioner’s ability to cool, we tend not to think of them as part of the unit.

But what happens when the filters get dirty? Airflow gets obstructed, which forces your system to work harder and longer to change your home’s temperature. Ultimately, this struggle will cause other parts of the unit to strain and possibly fail.

How To Fix

Keeping filters clean ensures that airflow will remain unhindered. You can clean some brands of filters, but most need replacing once they are thoroughly used up. You can keep airflow on track by replacing these filters once a month.

A homeowner replaces an air filter to improve HVAC efficiency

Regarding the vents inside your home, you can maintain good airflow by keeping them uncovered and giving them as much space as possible. That means you should move furniture placed over or near vents. Otherwise, circulation could be affected.

7) Low Refrigerant Levels

While the coils are responsible for evaporating moisture, the refrigerant attracts heat in the first place. Refrigerant runs through your system as a liquid and a gas, getting heated and cooled as it runs through a line set. When cold, the AC’s fan blows across the lines, cooling the air. 

The amount of refrigerant in your system is crucial because it determines how much heat gets absorbed. The amount won’t drop as a result of everyday use. However, if there’s a leak in the refrigerant lines, your air conditioner will struggle to dehumidify and cool your home, worsening as more refrigerant leaks. 

How To Fix

Identifying refrigerant leaks is difficult for laypeople because they’re mostly out of sight. However, leaks are audible while they occur. While the system is running, try listening for a hissing sound where the lines are installed and run. 

Another method is to observe how the air in your home feels while the air conditioner runs. It could be due to insufficient coolant if it remains warm and humid. However, while this indicates a problem, it could be related to something other than refrigerant levels. 

Repairing refrigerant lines is a two-part job for HVAC professionals. First, they will need to locate and fix the leak, which may require uninstalling parts of the AC. Then, they will need to replace the lost refrigerant so your unit will run properly again.

8) Frozen Evaporator Coils

Condensation gathers on the evaporator coils as they release heat. Ideally, that moisture will eventually drop off and run down a designated drain line. 

A ductless mini-split condensate drain at the bottom of line near the compressor
Condensed water vapor should run down the condensate drain line.

However, if the internal temperature of your unit gets too cold, that water will instead freeze on the coil, preventing it from trapping and releasing more moisture.

Dirt on the coils, low refrigerant levels, airflow problems, and a low outdoor temperature can all contribute to frozen coils. While the latter situation will fix itself as the temperature changes, the others require your attention to resolve. 

Otherwise, prolonged exposure to ice can damage the coils and other internal components. 

How To Fix

Pinpointing why your evaporator coils have frozen can be tricky because there are many possible causes. 

However, you can eliminate some guesswork by addressing other issues that can lead to frozen coils. Specifically, keep your filters clean, ensure there are no air or refrigerant leaks, and have your coils regularly cleaned. 

If you see that your coils are frozen, you should shut off your AC in addition to following these steps. Keeping the unit off will allow the coils to thaw out. Once they are, you can better check the system for problems. 


The best way to ensure that your air conditioner can adequately dehumidify your home is to create the ideal settings for it to operate. Then, use the correct settings, maintain good airflow, keep your unit and its components clean, and repair leaks in the connected ductwork. 

Keeping on top of these things will help keep your unit running for years to come.


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