Why is my AC not lowering indoor humidity?
An air conditioner that cools the house too quickly or is oversized will shut off before it has run long enough to pull moisture out of the air. This leaves indoor humidity stuck above 60 percent, which creates the conditions for mold to grow around the house within a few months.
What the numbers show
Counted from the threads quoted below. How we count
- The most recent one is from October 2026.
- This comes up 20.1× more often in r/hvacadvice than across the site as a whole.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1running dehumidifiersDid this help you?
- 2dropping the system setpoint to force longer cooling cyclesDid this help you?
- 3Retaping the plenum boxDid this help you?
- 4Using a humidifier (or dehumidifier) downstairsDid this help you?
- 5Bought multiple hygrometers to monitor humidity levelsDid this help you?
- 6Set up a portable Midea dehumidifierDid this help you?
- 7Had an HVAC company inspect the unitsDid this help you?
- 8Consulted ChatGPT for advice on air quality specialistsDid this help you?
- 9Opening the apartment window for 30 minutes a dayDid this help you?
- 10Running an air conditioner or bathroom ventDid this help you?
- 11Using a countertop dehumidifier with a continuous drain into a sink or bathtubDid this help you?
- 12Opening windows on a dry afternoon to temporarily drop humidityDid this help you?
- 13Tracking dewpoint instead of relative humidityDid this help you?
- 14Compiling a list of specific measurements for the technician to checkDid this help you?
- 15Opening windows and running fans for a cross breeze at night and running dehumidifiers during the dayDid this help you?
- 16Cracking just one or two windows in the room you are in with a fan blowing outwardsDid this help you?
- 17Using a portable AC unit that vents out a windowDid this help you?
- 18Turning off bathroom exhaust fans to prevent pulling in humid outside airDid this help you?
- 19Running the air conditionerDid this help you?
- 20Waiting for heating season to naturally drop humidityDid this help you?
- 21Using dehumidifiers with continuous drainsDid this help you?
- 22Suggesting a mini split added to the second story to dehumidify and cool the area.Did this help you?
- 23buying portable dehumidifiersDid this help you?
- 24running heating and air conditioning at the same timeDid this help you?
- 25Using a separate dehumidifierDid this help you?
- 26Leaving the window open to ventilateDid this help you?
- 27Experimenting with window positioning and fansDid this help you?
- 28Lowering the fan speedDid this help you?
- 29Using a smart thermostat to set minimum run times and increase cycle timeDid this help you?
- 30Running a large dehumidifierDid this help you?
- 31Turning off the humidifier control and water supply lineDid this help you?
- 32Contacting an HVAC professional or mold abatement companyDid this help you?
- 33Using a portable dehumidifierDid this help you?
- 34Upgrading to a thermostat with dehumidification modeDid this help you?
- 35Relying on HVAC cooling to lower humidity when activeDid this help you?
- 36Adjusting the crawlspace dehumidifier to be more aggressiveDid this help you?
- 37Using an infrared camera to check for cool spots representing moistureDid this help you?
- 38Configuring the thermostat to run the AC fan during dehumidifier callsDid this help you?
- 39Accepting higher humidity levels than the desired targetDid this help you?
- 40Switching the thermostat fan setting from ON to AUTODid this help you?
- 41Cleaning AC coilsDid this help you?
- 42Checking and clearing clogs in the condensate drain lineDid this help you?
- 43Adjusting blower speed or checking refrigerant chargeDid this help you?
- 44buying and installing a standalone dehumidifierDid this help you?
- 45improving exterior drainage with gutters and gradingDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“Am running two dehumidifiers with the bathroom exhaust fan on, all windows closed, but the in home humidity refuses to go below 60%.”source ↗
“New HVAC install, house humidity went from 55% to 65% and won't come down. What do I make them check?”source ↗
“House used to sit at 55% RH or lower. Now it's stuck at 65%, spikes to 73%.”source ↗
“Nothing in my house is pulling moisture out.”source ↗
“I ran large dehumidifier seemed to help but I started noticing my houses humidity is just generally always high like 60%.”source ↗
“Now after this nor'easter I took the trash out and noticed 2 lines of black mold behind trash cans in garage running all the way up the wall.”source ↗
“I noticed some mold inside the humidifier in the basement, this should be off along with the water line but I'm wondering if the needle valve isn't working.”source ↗
“Now that we have been cooling for 4 months we started noticing mold around the house.”source ↗
“It’s been 4 days and humidity levels are still averaging in the mid 60%.”source ↗
“The unit cools the space and cycles off before any dehumidifying happens.”source ↗
“During spring and fall (shoulder seasons), our house gets very humid with average reading of 60-68. Outdoor temps are mild, so neither heating nor cooling runs often.”source ↗
“immediately after it was done the humidity in my entire house started to rise. My house was pretty much 50% or below, but now it's 60% and up.”source ↗
“My fear is that my house is "too tight" now and that the only way to fix this is a whole home dehumidifier, the opposite effect I was hoping for.”source ↗
“I want to get my home humidity levels down, my unit can’t keep it below 60%.”source ↗
“I am back. And it feels awful in my home right now.”source ↗
“Today I woke up and noticed the musty smell was back and it was humid/damp feeling both up and downstairs.”source ↗
“The whole house is between 71 and 80 % humidity depending on the thermostat vs the actual humidifier.”source ↗
“This thing is only a year old so I feel at my wits end”source ↗
“I also want to add that the doors have swelled so much they are not closing. (For atleast 2 rooms and the front door)”source ↗
“Recently we found several clothing items in our master bedroom closet with mold/mildew on them.”source ↗
“Reading up to 85% at time in the house (we live in the southeast U.S. and heat and humidity has been extremely high for weeks)”source ↗
“Got a midea de-humidifier set up and it’s keeping it around 65% RH.”source ↗
“Whenever I come back from work, I can feel the air has moist smell.”source ↗
“What would be best way to fix the issue without breaking the bank and without installing portable room dehumidifier?”source ↗
“Since it's had tenants in, they've had significant increases in humidity (including moisture detectable on walls); an issue we never had.”source ↗
“While they say they're running the unit, they're also complaining about the power bill”source ↗
“Is it ok to run my heat so I can run my ac to dehumidify my house that’s at 67 percent humidity”source ↗
“Tenants complaining that AC, which is set on 72, feels hot today (cause it’s in the lower 60s outside) but it normally feels cool (when it’s in the 80s outside). 🤷♂️”source ↗
“Fighting the same issue. Two people with a ΔT of 19 and it’s 66 in their unit”source ↗
“Humidity is lower in the house when the ac runs all day. Now with lower outdoor temps it doesn’t run nearly as much and humidity in the house rises.”source ↗
“Humidity levels have increase in our house. I had a temp/humidity meter in our house to check temp for baby and it was always around 40% and never had any issues with humidity with the older system.”source ↗
“Now the levels are around 48-52% constantly even on dry days. When we wake up the temp isn't necessarily bad but we can feel the humidity in our house.”source ↗
“Also the new AC system turns on/off alot more but stays on shorter. So maybe it's working too well or overpowered?”source ↗
“House is cooling but feels pretty humid.”source ↗
“unsure where / if I have an air handler or if its an all in one.”source ↗
“Havent had an issue in two years so curious whats changed and how to fix it.”source ↗
“After indoor humidity reached 80–90% and caused mold, we hired an HVAC company to install a dehumidifier.”source ↗
“The dehumidifier has now run nonstop for four days with a 50% target. Humidity stays around 65–70%, with indoor temperatures of ~69°F.”source ↗
“Does this install look right, and should I expect this unit to get me to our target of 50% (or lower)?”source ↗
“Apparently the humidity even when I don't cook is 66%. Is it worth buying one to remove these pests ?”source ↗
“there is just one big window in the whole apartment, the bathroom has a vent. I open that window 30 minutes a day. Maybe because I live at the 16th floor? no idea.”source ↗
“Went in the crawl space and it’s very humid in there. No standing water, but a lot of moisture. The vapor barrier is in horrid condition, so we’ve had a couple contractors out, but they both have refused to quote for a new vapor barrier and insist on upselling us to a full encapsulation system.”source ↗
“I had mine encapsulated in July and the humidity is still at 72% ish. I don’t have any standing water or really even evidence of moisture. Not sure what to do about it. The whole basement is 70-80%.”source ↗
Where this came up
People with this problem also raised
- 7How to stop mold and high humidity ruining a basement
- 4Do I need crawlspace encapsulation for high humidity?
- 5Why is there condensation and damp spots on walls only on laundry days?
- 3How do I know if high indoor humidity is actually a problem?
- 6How to fix bad air quality in a rental apartment
- 3Why does my house get dusty so quickly?