AC Is Cooling But Not Removing Humidity? Still High +50% RH?

Air conditioners have two jobs: 1) Decrease temperature, 2) Decrease humidity. We have seen a lot of cases when the AC is cooling but not removing humidity (think 70% humidity levels). We will explain why you see this high humidity in the house with AC running; it’s quite paradoxical, and there is a good reason why this happens in the first place.

Now, there are only these 2 options why the humidity stays very high despite running the AC unit:

  1. General humidity is very high. This happens in very limited cases when the house is flooded, for example.
  2. Air conditioner is oversized. In more than 90% of cases, humidity is high because the AC short cycles. And AC short cycles if it’s too big for the room/s you want to cool and dehumidify.
oversized ac is cooling but not removing humidity enough
This is a 3-ton Gree AC outdoor unit installed in a 1,047 sq ft home. It’s oversized; which leaves you with cool temperature but high humidity levels.

First of all, we are going to explain how oversizing an AC unit leads to high humidity. After that, we will spell out 3 tricks of how you can effectively reduce humidity even with an oversized AC unit (no need to throw it out, lose $1,000s worth of AC, and then need to spend additional $1,000s to get smaller units).

How Oversized AC Causes High Humidity With AC Running

Let’s say you have a 1,000 sq ft home. Adequately sized AC unit for such a home should produce 20,000 BTU of cooling output. Usually, we would just install a 2-ton unit (24,000 BTU); you can use this AC tonnage calculator to adequately size an AC unit. However, if you install a 3-ton unit, such an air conditioning system will be too big for the 1,000 sq ft home.

Now, running a 3-ton AC unit will produce the following results:

  1. Temperature will quickly decrease to the thermostat set temperature (72°F, for example).
  2. Humidity will not decrease enough. You might go from 70% relative humidity to 65%, but that’s still way below the 30% to 50% relative humidity, recommended by the EPA (they warn about mold growth at high humidity levels).

 

This is a classic example of how an oversized AC is cooling but not removing humidity. You are left with a cool but humid house.

Here’s why the oversized AC unit creates this humid environment:

The main goal of every AC is to achieve the thermostat set temperature (72°F). It reduces humidity as well, but the AC will turn off when the temperature is achieved, no matter what the humidity is.

mini split ac removing humidity levels via condensation
Mini Split AC removed humidity via condensation in the indoor air handler (moisture captured by the cold coils).

Now, the AC removes humidity by creating an airflow over cold indoor coils. The water in the humid indoor air condenses on these cold coils, turning water vapor into water droplets and removing the accumulated water.

The key insight here is that the AC removes humidity slowly. That’s why it needs a bit of time in order to reduce the humidity levels from, say 70% to 50%.

When the AC unit is too big (3-ton AC for 1,000 sq ft home), the AC will reduce the temperature too quickly, and turn off. The unit will simply not run long enough for the indoor air humidity to be adequately reduced by the condensation process. The result is usually that the AC will quickly turn on and off; something we refer to as short cycling.

Alright, if you are now sitting in a cool but humid room, thinking “Oh no, my AC unit is too big, will I have to buy a new smaller one to get rid of this crazy humidity?”, you are on the right track.

In the perfect world, you would just swap 3-ton AC unit from our example to 2-ton AC unit, and the unit would be both cooling and removing humidity adequately.

The big caveat here is, of course, the $1,000s cost for a new unit and the bother.

That’s why when we see that the AC is cooling but not removing humidity, we first try to do some trick with the existing oversized AC unit that can remove humidity more substantially.

Here are the 3 things you can try with an existing oversized AC unit:

3 Tricks To Lower Humidity With Oversized AC Unit

Here is the 1st trick:

You should try to run the AC on a “Low” fan speed setting. 

The 3-ton unit will produce 3-ton of cooling output when it runs at 100% output. However, if you cut that output by choosing a “Low” setting on the AC remote control, a 3-ton unit will start working as a 1.5-ton or 2-ton unit.

if ac is not removing humidity, you can run it on dry mode or low fan speed setting
Example of “Low” setting on portable air conditioner. The 2nd trick – as we will see further on – is to run the AC on “Dry” mode.

The result is that the temperature decreases slowly. This gives the dehumidification process (condensation on cold indoor coils) time to remove more water than before. You might have to wait a bit to get the cool temperature but you will also get low (below 50%) relative humidity levels.

If that doesn’t work, you can go all-in with this 2nd trick:

Run the AC on “Dry” mode.

Use the AC remote control, choose “Dry” setting, and see how the AC will start reducing humidity levels.

“Dry” or “Dehumidification” mode is designed to extract the maximum amount of moisture out of indoor air. It works by creating a slow airflow over very cold indoor coils, thereby allowing these coils time to absorb the water, and remove it from indoor air. You can read more about how “Dry” mode in AC works here.

If this still doesn’t work, use the 3rd trick:

Get a dehumidifier.

A dehumidifier is specifically designed to reduce humidity levels. You can go with simple portable 20-pint, 35-pint, or 50-pint dehumidifier; these cost less than $500. It’s a financially more viable solution than getting a brand new smaller AC unit.

These are the go-to 3 trick you can use when you see that AC is not pulling out humidity as it should.

If this doesn’t work, you might have to start thinking about downsizing the AC unit (which is, sadly, costly and bothersome).

We hope this helps a bit with your humidity issues.

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