“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.
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Thank you. Turned ac to auto. Its cooling so so just not as good as usual. I keep front filter clean
Wiped down outside but not moisture to be.seen in the unit from the outside.
Hello Linda, yes, turning AC to ‘Auto’ will primarily help with the humidity. Hope the cooling improves.
I live in a 2.5 year old senior living building. Around 160 apartments..each has their own heating/A-C unit. My A/C cools the 1 bdrm apt very well…but humidity in each room (I have 5 temp/humidity indicators… in bathroom, bedroom, kitchen and 2 in living room) has been between 48 & 55 %…sometimes as high as 62%.
Is it possible that this newer bldg has individual apt heating & cooling that doesn’t remove humidity in summer ?
I just found the AUTO setting on my thermostat…it’s 82 out. With 83% humidity. Tomorrow, 102 is expected high.
Temp is set at 72, Auto, Cool. I normally keep it at 74 to 75 degrees.
It’s 8:30 PM…I’ll recheck humidity levels in 12 hours.
No one ever explained the AUTO setting. I hope it helps…thanx for your time.
Hello Tim, the exceptionally high summer temperatures come hand in hand with high humidity levels. AC units are under quite a stress. Now, if you have problems with the AC unit reducing humidity levels, you can try to switch the unit to ‘Dry’ mode. This is a special AC mode that increases dehumidification rates. You should see that the AC unit will produce less airflow; moving air slower over cold coils increases the air moisture adsorption rates. Hope this helps.
I moved into a new construction apartment unit that is rather small. The humidity level (from thermostat) has been between 66-69% since I moved in. After a month and several calls to maintenance who kept telling me they don’t know why my apartment is so humid (they haven’t even checked my ac unit) I finally got them to give me a dehumidifier. Ran it for a week and it helped bring humidity down to the 30-40’s. Then I slowly increased the setting by 5 until I turned the dehumidifier off. Humidity is creeping back up into 60s again. What do I tell my landlord to do because they have no clue. I can’t run this thing forever, my electric bill is going to be tripled and it’s unfair that I moved into this situation and landlord is not helping fix the root cause. I’m concerned about mold growth, I already have book lice all over the apartment and my dogs are sick. Cost me several hundred dollars at the vet and my dogs are on medication now for allergies, which they’ve never had until we moved here. I would call a heating and cooling professional, but per my lease I’m not allowed to do so. I don’t know what to do.
Hi Hannah, this is quite an unfortunate situation. Humidity levels above 60% are a no-no and you already see the consequences. Running both AC and dehumidifier at the same time is a waste of electricity, as you have adequately pointed out.
Now, in most cases when AC is cooling alright but is not removing humidity (OK temp but humid rooms), you either have an oversized AC or inadequate air distribution (this usually happens in bigger homes).
Oversized AC might be the issue. There are two things that you can try:
1. Check if you have ‘Dry’ mode on the AC. This should increase AC dehumidification rate while holding the temperature constant.
2. Try running the AC on ‘Low’ mode. At low mode, you get less airflow, and the indoor evaporator coils inside the AC have more time to condense the humidity from the indoor air.
Hate so hear about all the dealings with landlords and especially about the dogs. Hope tell will be alright. Let us know if these two things work.
We replaced a 3 ton for a 4 ton pump with new air handler. They said our duct work was adequate for it, and when the sw florida summer and humidity kicked in it started pulling humid air from pocket doors and under cabinets causing mildew and mold issues. They came out 3 times, first I needed insulation, so we spent 3600 on spray foam in the attic, then they said a part was bad, it wasnt…then they said we need another air return. I’m getting nowhere the insulation didn’t change anything and I’m running out of $ by doing all the things they suggest when the salesperson came out, looked at everything and approved the unit to work with existing duct and air return. My old unit although it broke was a 3 ton and we never had humid, mold or mildew issues. Our ac doesn’t shut on and off, it runs all day with very few shutoff times because the thermostat always says higher then what it’s set at. If it’s pulling air from the walls is it that we indeed more air returns? A bigger one or additional? It gets around 100° with 90% humidity here in the summer but we didn’t experience anything like this with our old unit. Option of a smaller unit isn’t possible, we’re already $10,000 plus foam attic insulation into this. Need help, humid weather will be back soon. Thermostat set at 74-75 and in the summer the house approaches 80°
Hi Kate, this is quite a messy situation. Even professional HVAC guys will have problems finding adequate AC systems to fight these extreme summer temperatures and Florida is known for having extremely high humidity. Upgrading your insulation is a good move. However, it seems that your AC system is too small. The air conditioning system has to deal with both the extremely high temperature and extremely high humidity; if it’s running non-stop and cannot adequately lower the temperature (thermostat says 74-75 but the indoor temperature is close to 80 degrees) and if you are finding high humidity pockets (causing mildew, mold issues), it’s quite clear that it just can’t handle these conditions.
Upgrading to 5-ton system or adding a 12,000 BTU window or portable air conditioner would make sense in this case. Given how hot and humid summers are becoming, it will be very difficult to adequately solve this problem with your insulation upgrades. Hope this helps a bit.