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

Air conditioners are not dehumidifiers but they do quite a bit of dehumidifying as part of the cooling process. When air conditioners remove heat from the air, they also remove moisture.

They expel this excess moisture through a drain line that either goes into your basement drain or a condensate pump.

If you take a look at the drain coming out of your AC’s coil on a warm day, you’ll see quite a bit of water running out. This is water that was previously in the air.

If your AC is running and you are still feeling humidity in the air, there could be something wrong.

Why Is Your AC Not Dehumidifying? 

1) Your Thermostat Fan 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.

This is the blower on the furnace that circulates air throughout your home.

The fan can either be set to “Auto” or “On.” Auto is the default setting and runs only when the AC runs. When you set the fan to “On,” it runs continuously.

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 to humidity.

How To Fix

This is an easy fix. Simply go to the thermostat and check your fan settings. If the switch is in the “On” position, turn it to “Auto” and see if that solves your problem.

2) Your AC Is Too Big for Your Home

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

The air conditioner’s sensors are connected to the thermostat. If the unit is too large, it cools the area too quickly. While this might not seem like a problem, the unit needs to cool the area evenly to provide a comfortable environment.

When the AC is too large, the thermostat believes it has been satisfied before the room is completely cooled, causing the unit to turn on and off constantly as the indoor air temperature fluctuates.

This is called “short cycling” and is very bad for your AC equipment.

How To Fix

The only way to fix this is to remove your current AC unit and replace it with a properly sized one. This is why it is so important to have a licensed HVAC company do your installation.

3) Dirty Evaporator Coils

Dirty coils cause air restrictions and reduce your air conditioner’s ability to function. Which ultimately means it can’t cool your home effectively.

There are two coils in your AC system. The evaporator coil that is located inside your home, above your furnace (unless you have a downflow furnace, then the coil will be beneath it), and the condenser coil, which is the part that surrounds your outdoor AC unit.

How To Fix

If the evaporator coil is dirty, you will need to call an HVAC professional to clean it. They will need to disassemble the plenum to access the coil and carefully clean it without damaging the delicate fins.

If the outdoor condenser is dirty and plugged with debris (grass clippings, leaves, cottonwood, etc.), you can clean it yourself. Simply take a garden hose with a sprayer and rinse out the debris until the coils look clear.

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

4) A Clog in the Drain Line

A clogged condensate drain will overflow the evaporator coil’s drain pan and cause it to stop working effectively. If the drain is completely blocked, you’ll see water on the floor beneath the furnace.

This is a major problem that needs to be addressed right away or the furnace could be damaged by the water leaking down from the coil.

How To Fix

Luckily, this is an easy fix you can do yourself. Simply disconnect the drain hose and unscrew the drain fitting. You might get quite a bit of water coming out of the drain pan, so be prepared with a small bucket.

All you need to do is remove the debris clogging the drain fitting and reinstall it. It’s a good idea to turn the furnace fan to “On” at the thermostat for a few hours to dry things out.

5) A Leak in Your Air Ducts

Leaking ductwork won’t cause high humidity in most cases, but if the leaks and gaps are bad enough, you will see decreased performance from your AC, which could result in difficulty removing humidity.

How To Fix

If the gaps in your ductwork aren’t major, you could likely repair them yourself with DC-181 foil tape or mastic air duct sealant.

The tape is a little easier to use but mastic sealant can offer a better seal in hard-to-reach places.

If your ductwork is in bad shape, you will likely need to have an HVAC professional repair it.

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

6) Dirty Air Filters

As a homeowner, the best thing you can do for your HVAC unit is to keep a clean filter. If your AC can’t breathe, it can’t do its job.

Filters are often forgotten about during the summer months because they are part of the furnace. What many people don’t realize is that your furnace fan is what circulates the cool air throughout your house. Both units work together to provide you with cool, comfortable air.

Dirty air filters can cause enough restriction to make the unit stop cooling. And worse yet, it could cause the furnace fan or the outdoor AC compressor to overheat and fail.

How To Fix

If you have a 1’’ filter, change it once a month. For these types of filters, you don’t want to buy the expensive pleated kind. They were made to use the simple ones that allow decent air flow.

If you have a 4’’ or 5’’ filter, you can change these every six months to a year, depending on the traffic in your home.

A homeowner replaces an air filter to improve HVAC efficiency

Speaking of air restriction, it’s a good idea to inspect your vents (especially your return air vents) and clean them regularly. Make sure furniture isn’t blocking the vents so air can move freely through them.

7) Low Refrigerant Levels

Low refrigerant levels mean there is a leak somewhere in the system. You might notice the AC system blowing warm air or just struggling to maintain the temperature.

If you go outside and take a look at the unit, you might see ice buildup on the liens. You could also see ice on the lines of the indoor evaporator coil. If you see that, it is likely that the entire coil is a block of ice.

If you see any signs of ice, turn the system off and allow it to melt while you call an HVAC company.

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

This is a job for an HVAC company. They will first need to find the leak, repair it, and then recharge the system to the proper refrigerant levels.

This is not something you’ll be able to do yourself. Even if you somehow found the leaks, you likely don’t have the knowledge or the tools to repair them.

And when you go to add refrigerant, adding too much or too little can damage the system, sometimes beyond repair.


Higher than normal humidity could be a sign that your AC is malfunctioning. Some problems are easy to remedy yourself, while others require professional service.

If you notice your AC isn’t cooling well, first change the filter and check the drain for debris. If those are both clean, go outside and take a look at the condenser.

If it’s dirty, hose it off. Those are the three most common problems that homeowners can easily solve themselves. If you live in an extremely humid environment, your AC might not be able to handle dehumidification on its own.

First, have your AC serviced by a professional. If it gets a clean bill of health, you will likely need to invest in a separate dehumidification unit.

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