A properly sized, well-maintained air conditioning system reduces indoor humidity. It chills indoor air across a cold evaporator coil; water vapor condenses on the coil and drains away, so supply air is both cooler and drier. Aim for a relative humidity (RH) of 30–50% for comfort and health.
Key takeaways
- A right sized, well maintained air conditioning system lowers indoor humidity; aim for 30–50% RH and keep it under 60%.
- Cool but clammy? Common culprits are oversizing, the fan left on “On” (not Auto), humid air leaks, or deferred maintenance. Switch to Auto, seal leaks, clean the coil/drain, and keep a fresh filter in place.
- Use a hygrometer and replace the HVAC return filter about every 90 days to keep airflow and drying strong.
- Filterbuy provides snug-fit MERV 11–13 filters.
- Filterbuy HVAC Solutions handles maintenance and repair to stabilize RH and improve comfort.
How air conditioning lowers humidity
When warm, humid air passes over the evaporator coil of an air conditioning system, the coil—being colder than the air’s dew point—causes water vapor to condense. The moisture collects in a drain pan and exits through the condensate line. For window units, running the fan at a lower speed on humid days can improve moisture removal by increasing contact time at the coil.
Why humidity stays high
Even with an air conditioning system running, some homes feel damp. Here are common causes:
- Oversized equipment and short cycles: A system that’s too large cools the space quickly but shuts off before removing much moisture. Right-sizing prevents this.
- Fan set to “On”: When the fan keeps running after the compressor stops, it can blow evaporated moisture back into the air. Set it to “Auto” so it shuts off with the compressor.
- Humid outdoor air leaks in: Open windows or a leaky building shell can introduce excess humidity. In humid regions, tighten the envelope and use energy recovery ventilators (ERVs) instead of HRVs—they help manage moisture along with heat.
- Maintenance issues: A dirty filter, clogged return grille, or biofilm on the coil impairs both heat exchange and dehumidification. A blocked condensate line can also cause backups or leaks.
Best Settings for Humidity Control
Set your thermostat’s fan to “Auto”. That way, it only runs with the compressor, helping the air conditioning system remove moisture efficiently. If you want extra airflow, use ceiling fans or portable fans in individual rooms.
How Ductwork Affects Humidity
Leaky return ducts in attics or crawlspaces can suck hot, humid air into the system. Leaky supply ducts can depressurize rooms, pulling in moist air from outside. Sealing and insulating ducts helps your air conditioning system dehumidify properly.
Seasonal and Climate Considerations
- In humid regions, RH can rise on mild days when the air conditioning system doesn’t run often. A dedicated dehumidifier can help.
- In dry climates, strong cooling can lower RH too much, causing dry skin or nasal discomfort.
- Always use a hygrometer and target 30–50% RH.
Measure and Maintain
- Place inexpensive digital hygrometers in key living spaces and bedrooms.
- Replace your air conditioning system’s return filter on schedule—typically every 90 days, sooner during wildfire season or high dust.
- Keep vents unblocked, coils clean, and the condensate drain flowing freely.
- These steps ensure both cooling and drying performance.
Window, Portable, and Mini-Split Expectations
- Window air conditioners remove moisture best when sized correctly and set to lower fan speeds in humid weather.
- Portable units (especially single-hose models) often struggle with humidity—dual-hose models perform better.
- Mini-splits with inverter compressors run at low speed for longer cycles, which helps maintain consistent moisture control.
Basements and In-Betweeen Seasons
Basements have cool surfaces that can sit at or below the air’s dew point, so moisture condenses even when upstairs is comfortable. In spring and fall, outdoor air may be mild but humid, and the air conditioning system may not run enough to dry the house. A stand-alone dehumidifier is the right tool here to keep RH under 60%.
Five-Minute Humidity Check
- Put a hygrometer in the main room and the primary bedroom.
- Set the thermostat fan to Auto and run a normal cooling cycle for 30–60 minutes.
- Watch RH—it should trend down, not up.
- Look for steady condensate draining at the indoor unit.
- If RH stays high, check the filter, coil cleanliness, and condensate line, then look for open windows or shell leaks.
Support Better Cooling and Humidity Control with Pleated MERV Filters
Snug, correctly sized MERV 11–13 pleated return filter (standard or custom) preserves airflow so the coil removes heat and moisture efficiently. The Odor Eliminator option adds activated carbon for smells. Fast, free shipping and Auto Delivery keep changes on schedule.
FAQs
Does AC increase or decrease humidity?
It decreases humidity when sized and maintained correctly.
What indoor humidity should I aim for?
Target 30–50% relative humidity. Keep it below 60%.
Why is my home humid even when the AC runs?
Common causes are an oversized unit, the fan set to On, air leaks, dirty filters or coils, a clogged drain, or low refrigerant.
Should the thermostat fan be On or Auto?
Use Auto. This prevents re-evaporating water from a wet coil.
Can a dirty filter cause high humidity?
Yes. It restricts airflow and reduces moisture removal at the coil.
Do mini-splits dehumidify well?
Yes. Inverter mini-splits run long, low-speed cycles that remove moisture effectively. Many include a Dry mode.
Is Dry mode better than Cool for humidity?
Use Dry on mild but humid days. It prioritizes moisture removal with lower airflow.
Do portable ACs dehumidify?
Single-hose units can draw in humid air from outdoors. Dual-hose units perform better.
Do I still need a dehumidifier?
Often yes in basements and during spring or fall when the AC does not run enough to dry the air.
How do I measure humidity at home?
Use a low-cost digital hygrometer in the main room and the primary bedroom.
How often should I replace my AC filter?
About every 90 days for most homes. Sooner with pets, smoke, or heavy dust.
What MERV rating should I use?
Use the highest MERV your system supports, typically MERV 11–13, in the exact size your return slot requires.
Can duct leaks raise indoor humidity?
Yes. Leaky returns can pull humid attic or crawlspace air into the system.
When should I call a pro?
If RH stays above 60%, the unit short-cycles, the coil ices, you see water around the air handler, or comfort requires running the fan on On.
