“I have AC running but the humidity goes up, not down. How is this possible? Why is my AC not removing humidity?”
An air conditioner not dehumidifying is quite a paradox. If you are experiencing relative humidity going up when air conditioning is on and cooling adequately, that is quite extraordinary. Normally, when you run an AC, the relative humidity should always decrease. AC cooling but not removing humidity is not normal behavior; something is probably wrong.
Example: Let’s say you have 85°F and 60% humidity. When you turn the AC on, you should see the temperature decrease to let’s say 75°F and the humidity drop below 40%. If you see AC cooling but humidity is not changing when AC is on (60%) or even increased to 65%, 70%, and so on, something must be wrong.
How is this increasing humidity explained? Is something wrong with your AC unit?
Now, there are 4 explanations (within reason) why we experience the relative humidity going up when AC is turned on. Let’s cover these. On your part, you can check if any of these culprits might be causing your AC to increase humidity while running:
4 Reasons Why Humidity Increases When AC Is Running
As we have said, a properly working air conditioner always decreases indoor relative humidity levels. If your AC is cooling but not removing humidity, something must be preventing the AC from performing its dehumidification function.
We are going to check culprits that cause AC cooling but not removing moisture levels one by one. You can check each one and evaluate if this or that particular culprit might be causing the humidity to increase when AC is on:
1. Air Conditioner Is Too Big (Oversized AC Leads To Short Cycling)
This is the most common culprit for humidity increasing while AC is running. If the AC is oversized, it will start to short cycle. You can read exactly what AC short cycling is here.
In short, that means that the refrigerant cycle doesn’t follow the whole circle because the AC is so big that it reaches the set temperature too quickly but cannot extract moisture. On top of that, the temperature decrease (cooling) is not homogenous; one room is much colder than other rooms, for example.
Other symptoms of AC cooling but not removing humidity due to oversized AC unit leading to short cycling include:
- You will have cold and humid rooms. This is usually the room where the indoor air handler is positioned (in the case of mini splits) or where a portable or window AC unit is located. AC will be able to lower temperature but unable to lower humidity levels.
- You will also have hot and humid rooms. When AC is too big, its too massive cooling output might reach the set temperature (let’s say 72°F) in 1 room but will leave other rooms hot. The result is the lack of cooling and dehumidifying.
- Air conditioner turning on and off every few minutes. This is a tell-tale sign of a too big AC unit short cycling.
If you think an oversized AC unit is causing cooling but not dehumidifying when turned on, the cleanest way is to install a smaller air conditioner. Of course, that’s an expense and not practical at all. You can also calculate how many BTU air conditioner you need here.
The more practical way to try to prevent AC short cycling is to run the unit at the lowest setting. A 3-ton unit might produce 3-ton of cooling output (36,000 BTU) when running on 100% output. If you set it to the lowest setting, you will end up with a 2-ton cooling output.
This reduced cooling output might prevent the AC from short cycling. As a result, you will no longer see AC cooling but not removing moisture; you will both get the cooling as well as dehumidification function back.
2. Thermostat Setting (To Prevent AC Cooling But Not Removing Humidity, Set It On ‘Auto’)
It seems quite reasonable to start your AC unit on the ‘On’ setting. That means that AC is running, right?
Well, the ‘On’ setting on an air conditioner tells the unit to run the indoor blower (fan) all the time. This is regardless if there is even a need for cooling or dehumidifying.
If the fan is constantly running, it might release the water condensed on the indoor coils back into your home. You will see this as AC cooling but not removing moisture. In fact, if the indoor coils are full of water, you will see humidity increasing while AC is on.
The solution here is quite simple:
Set the thermostat to ‘Auto’.
‘Auto’ thermostat setting tells the AC unit to run only when cooling is needed (temperature is above the set temperature). This will prevent the water accumulated on the indoor coils to be released back into your home. On top of that, adequately running AC unit will decrease the humidity (dehumidification) as well as decrease the temperature (cooling).
3. Indoor Evaporator Coils Might Be Dirty (Less Condensation)
Evaporator coils have two jobs:
- Primary function; cooling. The cold refrigerant cools the indoor coils, the fan generates airflow over these coils and we can enjoy a cooling breeze from the indoor air handler.
- Secondary function; dehumidifying. The cold refrigerant coils also absorb water by condensation. This is how AC can remove humidity.
Now, a properly functioning evaporator coil is a great heat exchanger and moisture absorber. If the fine copper metal these coils are usually made of is dirty, the heat exchanging and moisture-absorbing abilities will be impeded.
A dirty coil might still be cool and provide cooling. However, the dirt of the coil will prevent the water to condense on the coil effectively. The result of this subpar coil is us seeing AC cooling but not removing humidity adequately.
The fix here is simple as well:
Just clean the indoor coils.
You will have to open the unit, locate the evaporative coils and use the coil cleaning solution to clean them.
This involves quite a lot of brushing away the accumulated dirt. After cleaning the coil, turn the AC back on and you should see both cooling and dehumidifying functions go back to normal.
4. Rapid Temperature Drop Causing Increase In Humidity Levels
If the drop in temperature is very significant (20°F or more), you will experience the effects of the inverse relationship between temperature and humidity.
Namely, if you decrease the temperature, the relative humidity increases (if you don’t add or reduce moisture). We don’t usually detect this if the temperature changes for up to 10°F. Nonetheless, if you go beyond that, the humidity will go up just because the temperature drops.
With increasing humidity levels, the feeling of how cool your home is (just the feeling, the thermometer will show the same temperature) is obscured. This effect is quite nicely laid out by heat index:
In many cases when the temperature drop is so sudden, homeowners claim that AC is cooling but humidity increases (or is not reduced). This is just the effect of this temperature-humidity relationship.
Other Reasons Why Both Cooling And Dehumidification Function Are Imbedded
Thus far we have talked about situations when:
- Cooling is adequate.
- Moisture removal is inadequate.
If you see both cooling and moisture removal impeded, the culprits causing this range anywhere from freon leak (low refrigerant levels) and dirty AC filters to leaky ductwork. In general, you will have a lot more problems (and costs associated with fixing them) if both of these functions are inadequate.
All in all, if only humidity removal is impeded, check the AC size, thermostat setting (should be set at ‘Auto’), and the state of ACs indoor coils. The perception of humidity levels might also be impaired if the AC cools your home too quickly.