Florida humidity · AC on, house still sticky

House Humid With the AC Running in Florida

If the thermostat says 75°F but the house feels clammy, the AC is cooling but not drying enough. Measure the humidity first: the EPA says keep indoor relative humidity (RH) below 60%, ideally 30–50%. Then work in this order: thermostat fan on AUTO, let the system run full cycles, clear the airflow, cut indoor moisture, and look for return leaks pulling in attic or garage air. A dehumidifier comes last, not first. Every number on this page comes from the EPA, ENERGY STAR, FPL, the Florida Solar Energy Center (FSEC), UF/IFAS Extension, or the U.S. Department of Energy, linked at the bottom.

Stop DIY and call a licensed HVAC tech if: you see ice on the copper lines or indoor coil; water is pooling at the air handler or soaking a ceiling; you smell burning, hear buzzing at start, or the breaker trips; or anyone wants to “add Freon.” Refrigerant work needs an EPA Section 608–certified tech. Do not open the refrigerant circuit or dig in the outdoor disconnect.

Humidity reading checker

Enter what a humidity meter shows in the main living area after the AC has been running normally for a while, plus three things you can see from the thermostat and the house. You get the EPA / ENERGY STAR band and your next steps in order. Not a mold test or a diagnosis. Saves on this device only.

Indoor RH right now (%) Where did you measure? Thermostat fan switch How does the AC run on a hot afternoon? Where is the indoor unit (air handler)?

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Enter your RH and tap Check.

Why a cold house can still be humid

An air conditioner dries the air in one way only. Warm, wet room air crosses the cold indoor coil, water condenses on the fins, drips into the pan, and leaves through the drain line. The thermostat does not read humidity. It shuts the system off when the air near it reaches the set temperature. UF/IFAS Extension puts it plainly: with air conditioners that run on room temperature, humidity is not controlled directly, and any humidity control is a by-product of temperature control. So anything that lets the house hit the set temperature fast, or puts coil water back into the air, leaves the house cool and sticky.

The coil has to get wet before it dries anything

In lab tests on a 3-ton heat pump with the fan on AUTO, summarized in an FSEC report, moisture was actually added to the supply air right after start-up, and drying began only after 60 to 150 seconds. Full moisture-removal capacity took 3 to 15 minutes to settle. In FSEC’s own lab tests of cooling coils, it took 12 to 33 minutes of running before water first dripped off the coil. A coil still wet from the last cycle drains sooner, but short runs spend much of their time just wetting the coil.

A running fan puts the water back

When the compressor stops and the fan keeps blowing, room air picks up the water sitting on the coil and carries it back into the house. FSEC measured this in a Florida home: at a 77°F setpoint, about 50% RH with the fan on AUTO and 60–65% with the fan on ON. In FSEC simulations, fan ON raised the hours above 60% RH two to four times.

Short cycles waste the drying time

ENERGY STAR: air conditioners start at low efficiency and reach peak efficiency in about 10 minutes, and oversized units that short-cycle remove less humidity and can let some moisture evaporate back into the air. At one monitored field site FSEC cites, a single-speed system with the fan running constantly did virtually no net drying when the compressor ran less than 40% of the time.

Mild, damp days are the worst

FSEC notes that this part-load loss makes indoor humidity drift up, especially on days when the cooling load is modest, and that in efficient homes humidity can climb in weather that is humid but mild. In Florida that means cloudy, rainy stretches when it is not hot enough for the AC to run long.

The 8-step sticky-house fix, in order

Do these in order; the first ones are free. Tap a box as you go. It stays on this phone until you clear it. Print if you want paper.

1. Measure RH. Don’t go by “feels sticky.” The EPA says keep indoor humidity below 60% (ideally 30–50%) and that an inexpensive humidity meter from a hardware store can measure it. Put it in the main living area, away from a supply vent and out of a bathroom right after a shower. Check it at a few times of day, including a mild, cloudy one.
2. Set the thermostat fan to AUTO FPL: AUTO runs the fan only while the AC is cooling; ON runs it nonstop, uses more energy, and if your AC normally cycles off about 30% of the time, ON adds more than 200 extra fan hours a month. FSEC: on ON, coil water goes back into the house (the 50% vs 60–65% Florida home above). FSEC also found that air handlers normally ship with a short fan run-on after the compressor stops (“fan overrun”); in one simulation a 90-second overrun raised the hours above 60% RH by 80%. That one is an air-handler setting. Ask a licensed HVAC tech whether yours can be disabled; don’t open the unit yourself.
3. Don’t chase a lower number UF/IFAS: when a system isn’t drying the house, the usual reaction is to turn the thermostat down, and every degree below 78°F can add up to 8% to cooling costs. The U.S. DOE says a colder-than-normal setting will not cool the house any faster. FPL’s guidance is 78°F, or the warmest you are comfortable at, when home. See what temperature to set your AC in Florida.
4. Fix the airflow you can reach Replace a dirty filter with the right size (FPL lists clean filters among its efficiency tips; size help: air filter size finder). FPL: keep vents open; closing them makes the system work harder. Open interior doors that cut rooms off from the return (closed doors & return air). Clear growth away from the outdoor unit (outdoor coil clearance). Ice on the lines: stop, and see AC freezing.
5. Time one full cooling cycle On a warm afternoon, use a phone timer from when the outdoor unit starts to when it stops, then how long it stays off. A few minutes on and then off, over and over, is short cycling: use the short cycling checklist, and check the thermostat isn’t sitting in a supply-air blast (thermostat near a supply vent). Running nearly nonstop and still sticky is a different problem: see AC runs constantly.
6. Cut the moisture coming in UF/IFAS: outdoor air coming in through poorly sealed windows and doors, open fireplace dampers, and bath and kitchen vents causes most of the moisture load in a home, and even opening and closing exterior doors adds humidity; bathing, cooking, cleaning, and plants add more. EPA: vent clothes dryers outdoors, run the bath fan or open a window while showering, and use exhaust fans when cooking. The DOE advice UF/IFAS passes on is to use bath and kitchen fans sparingly while the AC runs, so run them for the shower or the stove, then switch them off. Keep the fireplace damper shut in summer.
7. Look, don’t touch: return leaks pulling attic air See the attic-air section below. Check that the filter cover or filter door is in place and closed, and look for gaps where return grilles meet the wall or ceiling. Note what you see and ask a licensed HVAC tech for a duct leakage check. Don’t crawl the attic or open the air handler to hunt for leaks.
8. Only then: decide on a dehumidifier ENERGY STAR: whole-home dehumidifiers can bring significant benefits to relatively well-sealed homes with central air that see readings above 55% consistently throughout the house. FSEC recommends trying the no-cost and low-cost AC fixes before paying for separate dehumidification. Capacity education: dehumidifier pint planner.

Attic air: the Florida humidity leak you can’t see

If your ducts or air handler sit in the attic or garage and the return side leaks, the blower pulls attic or garage air straight into the system. FSEC’s summary of humidity fixes says that when ductwork or the air handler is outside the conditioned space, duct leakage can pull in humid outside air and significantly increase the moisture load the AC has to remove. FSEC measured how common this was in 160 central Florida homes (report FSEC-CR-397-91, 1991):

The blower pulled outside air in. Air leakage into the house averaged 0.91 air changes per hour with the air handler running and 0.28 with it off: about three times higher, which FSEC read as large leaks in the duct system.
Return leaks were common. On average, 10.7% of the air coming back to the air handler came from outside the house. 67% of homes had return leaks of 5% or more, and 36% had more than 10%. FSEC notes that much of that return leakage comes from the attic or garage, which run hotter than outdoors.
An indoor closet is not automatically tight. Average return leakage was 9.8% with the air handler in the attic, 9.0% in a closet, 10.1% in the garage, and 16.3% for outdoor package units.
Where the leaks were, biggest first: the return plenum (the platform box the air handler sits on, framed into closet or garage walls; leaks there commonly ran 5–25% of total air-handler flow); unsealed return grille connections that pull air through wall cavities; air-handler cabinet panels and filter doors that don’t seal; and, in a few homes, return ducts that had come loose.
Fixing it paid. After duct repairs in 50 homes, air-conditioner energy use dropped an average of 17.2% in the 46 homes with data.

That study is from the early 1990s, and newer homes with tested ducts may be tighter. FSEC’s 2008 paper notes the Florida Energy Code at that time gave an energy credit for tight, tested ductwork. Use the numbers to see how common return leaks were, not as a reading of your house. Only a duct leakage test tells you what yours does.

Is the AC too big for the house?

ENERGY STAR’s right-sizing fact sheet says oversized units short-cycle, and short run times don’t let the system remove humidity well. UF/IFAS says an oversized air conditioner cools the home too quickly to remove moisture, which leaves it “cool but clammy.” If the house short-cycles even with a clean filter, open returns, and a well-placed thermostat, and still reads above 60% RH, size is a fair question.

Size comes from a load calculation, not square feet. UF/IFAS: the load must be calculated, not just estimated from square footage, and ACCA Manual J (or a method based on it) is the standard. A Manual J gives both the moisture (latent) and temperature (sensible) load. ACCA calls Manual J the national ANSI-approved standard.
High SEER is not the same as good drying. UF/IFAS notes many high-SEER units remove humidity less well, so the contractor should confirm the unit chosen can remove the calculated latent load. FSEC suggests a sensible heat ratio on the low end (for example, under 0.73 at rating conditions) for humid-climate homes.
Oversizing also costs energy. In an FSEC-cited field study of more than 300 Florida homes in the early 1990s, systems sized 20% above Manual J used 3.7% more cooling energy, and 50% above used 9.3% more. FSEC notes energy codes often limit oversizing, for example to 115% of the Manual J size.
Ballpark yours: the AC tonnage calculator (use “Refine for your house”) and 3-ton vs 3.5-ton in Florida humidity. Neither replaces a Manual J.

Dehumidifier: when it helps, and when it’s a band-aid

The sources disagree a little here, and the order matters. ENERGY STAR says whole-home dehumidifiers bring real benefits to relatively well-sealed homes with central air that read above 55% consistently throughout the house. UF/IFAS cautions against running a dehumidifier at the same time the AC is running, because it adds to the cooling load and makes the AC work harder. FSEC’s advice is to try the low-cost and no-cost AC fixes first: fan on AUTO, no fan overrun, lower airflow if the tech says it’s safe, right size, and airtight ducts. Only then add the cost of separate dehumidification.

The upshot: if the house is leaky, or the fan is on ON, a dehumidifier treats the symptom. If RH still sits above 55% after steps 1–7, it is a reasonable next conversation. Use the dehumidifier pint planner for the capacity questions.

What to ask a licensed HVAC tech

These are adjustments behind the panels, so they’re not DIY. Asking the right questions gets them looked at:

“Is fan overrun (the fan off-delay) on, and can it be turned off?” FSEC recommends disabling it in hot, humid climates; the installation guide usually shows how.
“What is my airflow per ton, and can it come down without icing the coil?” FSEC: less airflow increases drying. About 350 cfm per ton is suggested when the air handler or ducts are outside the living space; too little can ice the coil or make ducts sweat.
“What did the Manual J say my latent load is, and can this equipment remove it?” (UF/IFAS, ACCA)
“Can you test the ducts for leakage and seal the return plenum and air-handler joints?” FSEC says the duct-to-air-handler connection, where pressure differences are greatest, needs to be airtight.
“Is the indoor coil clean and the refrigerant charge right?” UF/IFAS calls a dirty indoor coil the single most common cause of poor efficiency, and says checking charge takes a service contractor.

Humidity bands and settings (sourced)

Item	What the source says	Source
30–50% RH	Ideal range; “optimum” for a building	EPA; ENERGY STAR
Below 60% RH	Keep-below target	EPA
Above 55% RH, consistently	Where whole-home dehumidifiers bring benefits in well-sealed homes with central air	ENERGY STAR
Thermostat fan	AUTO, not ON; no fan overrun in hot, humid climates	FPL; FSEC
Thermostat when home	78°F or the warmest you are comfortable at	FPL
Each degree below 78°F	Up to 8% more cooling cost	UF/IFAS

Signs it’s time to stop and call a licensed HVAC tech

Ice on the copper lines or the indoor coil, water pooling at the air handler, or a wet ceiling.
A burning smell, buzzing at start-up, or a breaker that trips.
Air from the vents isn’t cold, or the AC runs all afternoon and never reaches the set temperature.
RH stays above 60% after steps 1–7, or the house short-cycles with a clean filter and open returns.
Anyone suggests adding refrigerant. That is licensed, EPA Section 608–certified work.

This is a homeowner humidity checklist, not a mold inspection, energy audit, or repair quote. The study numbers come from specific FSEC test homes and lab coils, and the bands and settings are published guidance from the EPA, ENERGY STAR, FPL, UF/IFAS, and the DOE. Your home, ducts, and equipment change the result.

Common questions

Why is my house humid with the AC running in Florida?

The AC only dries air while the indoor coil is cold and wet long enough for water to drip off and drain. Short cycles, a thermostat fan left on ON, leaky returns pulling in attic or garage air, and an oversized system all leave a house cool but clammy. UF/IFAS notes that humidity control is only a by-product of temperature control. Measure RH, set the fan to AUTO, time a cycle, and check airflow before you turn the thermostat down.

What indoor humidity should I aim for in Florida?

The EPA says keep indoor relative humidity below 60%, ideally 30–50%, and that an inexpensive humidity meter can measure it. ENERGY STAR calls 30–50% the optimum range for a building and says levels above that may promote mold growth.

Should the thermostat fan be on AUTO or ON for humidity?

AUTO. FPL says ON runs the fan nonstop and can add more than 200 extra fan hours a month if the AC normally cycles off about 30% of the time. The Florida Solar Energy Center measured a Florida home at a 77°F setpoint at about 50% RH with the fan on AUTO and 60–65% with the fan on ON, because the running fan blows water off the wet coil back into the house.

Will turning the thermostat lower fix a sticky house?

Usually not. UF/IFAS says turning the thermostat down is the typical reaction when a system isn’t drying the house, and each degree below 78°F can add up to 8% to cooling costs. The U.S. Department of Energy says a colder setting will not cool the home any faster. Fix fan mode, cycle length, airflow, and leaks first.

Can duct leaks make my house humid?

Yes. FSEC says leaky ducts or an air handler outside the living space can pull humid outside air into the system and add to the moisture load. In FSEC’s study of 160 central Florida homes, return leaks averaged 10.7% of air-handler flow, and air leakage into the house was about three times higher with the air handler running than with it off. A licensed HVAC tech can test and seal ducts.

Could my Florida AC be too big?

It can be. ENERGY STAR says oversized units short-cycle and do not remove humidity well, and UF/IFAS says an oversized system cools the home too quickly to remove moisture, leaving it cool but clammy. The size should come from an ACCA Manual J load calculation, not square footage alone.

When do I need a whole-home dehumidifier in Florida?

ENERGY STAR says whole-home dehumidifiers can bring significant benefits to relatively well-sealed homes with central air that see humidity above 55% consistently throughout the house. FSEC recommends trying the no-cost AC fixes first, and UF/IFAS cautions that running a dehumidifier at the same time as the AC adds to the cooling load.

Also here

AC tonnage calculator · What temperature to set your AC · Short cycling · AC runs constantly · Dehumidifier pint planner · 3-ton vs 3.5-ton · AC freezing · Closed doors & return air · Thermostat near a supply vent · Air filter size finder · Outdoor coil clearance · Florida AC maintenance checklist · All tools

Sources

U.S. EPA, A Brief Guide to Mold, Moisture and Your Home (retrieved Oct 16, 2026): keep indoor humidity below 60 percent (ideally 30–50 percent); measure with an inexpensive humidity meter; use air conditioners and/or dehumidifiers when needed; vent moisture-producing appliances outdoors; bath fan or window when showering; exhaust fans when cooking. epa.gov/mold/brief-guide-mold-moisture-and-your-home
ENERGY STAR, Dehumidifiers (retrieved Oct 16, 2026): optimum RH for a building generally 30–50%, above that may promote mold growth; whole-home units carry significant benefits for relatively well-sealed homes with central air that see readings above 55% consistently throughout the house. energystar.gov/products/dehumidifiers
ENERGY STAR, Right-Sized Air Conditioners fact sheet (retrieved Oct 16, 2026): efficiency is low at start and reaches peak in about 10 minutes; oversized units short-cycle, remove humidity poorly, and can let moisture evaporate back into the air. energystar.gov/…/RightSized_AirCondFS_2005.pdf
FPL, Ask the Expert: ON or AUTO (retrieved Oct 16, 2026): AUTO runs the fan only while cooling; ON adds more than 200 extra fan hours a month if the AC cycles off about 30% of the time; keep vents open; clean filters. fpl.com/blog/ask-the-expert/ac-settings.html
FPL, Energy Saving Tips (retrieved Oct 16, 2026): 78 degrees or the highest comfortable temperature while home. fpl.com/save/lower-bill/tips.html
Florida Solar Energy Center, FSEC-PF-443-08, Shirey, AC System Equipment Specification, Installation and Operational Options for Improved Indoor Humidity Control (2008; retrieved Oct 16, 2026): Florida home at 77°F about 50% RH on AUTO vs 60–65% on constant fan; constant fan 2–4× the hours above 60% RH in simulation; air handlers ship with fan overrun enabled, 90-second overrun raised hours above 60% RH by 80% in simulation; virtually no net dehumidification below 0.4 runtime fraction at a monitored single-speed site; lower SHR (e.g., <0.73); about 350 cfm/ton when the air handler or ducts are outside conditioned space, too low risks coil icing; 300+ Florida homes, 20% / 50% oversizing used 3.7% / 9.3% more cooling energy; codes often limit oversizing (e.g., 115% of Manual J); duct leakage outside conditioned space adds latent load; try these before separate dehumidification. fsec.ucf.edu/…/FSEC-PF-443-08.pdf
Florida Solar Energy Center, FSEC-GP-151-06, Shirey and Henderson, Dehumidification at Part Load (ASHRAE Journal, April 2004; retrieved Oct 16, 2026): 12 to 33 minutes for condensate to first fall from lab test coils; a partly wet coil drains sooner; no latent removal below 40% runtime at a field site with continuous fan; humidity drifts up on days with modest cooling loads. fsec.ucf.edu/…/FSEC-GP-151-06.pdf
Florida Solar Energy Center, FSEC-CR-1537-05, Shirey, Henderson and Raustad, Understanding the Dehumidification Performance of Air-Conditioning Equipment at Part-Load Conditions (final report; retrieved Oct 16, 2026), literature review of 3-ton heat pump lab tests with AUTO fan: moisture added right after start-up, dehumidification began after 60–150 seconds, latent capacity took 3–15 minutes to reach steady state. fsec.ucf.edu/…/FSEC-CR-1537-05.pdf
Florida Solar Energy Center, FSEC-CR-397-91, Cummings, Tooley and Moyer, Investigation of Air Distribution System Leakage and Its Impacts in Central Florida Homes (1991; retrieved Oct 16, 2026): 160 homes; infiltration 0.91 ach with air handler on vs 0.28 off; return leaks averaged 10.7% (67% of homes ≥5%, 36% >10%); by location attic 9.8%, closet 9.0%, garage 10.1%, package 16.3%; return leaks largely drawn from attic or garage; leak sites in order (return plenum 5–25%, register connections, cabinet and filter door, disconnected ducts); repairs cut AC energy use 17.2% on average in 46 homes. fsec.ucf.edu/…/FSEC-CR-397-91.pdf
UF/IFAS Extension, FCS3262/FY1026, Energy Efficient Homes: Air Conditioning (revised January 2025; retrieved Oct 16, 2026): humidity control is a by-product of temperature control; oversized systems leave homes cool but clammy; up to 8% more cooling cost per degree below 78°F; load must be calculated (ACCA Manual J), not estimated from square feet; many high-SEER units remove humidity less well; outdoor air through windows, doors, fireplace dampers, and bath/kitchen vents causes most of the moisture load; use bath and kitchen fans sparingly while the AC runs; avoid running a dehumidifier at the same time as the AC; dirty indoor coil the most common cause of poor efficiency; refrigerant charge needs a service contractor. ask.ifas.ufl.edu/publication/FY1026
U.S. Department of Energy, Energy Saver: Programmable Thermostats: a colder-than-normal setting will not cool the home any faster. The live energy.gov page returned 404 on Oct 16, 2026; text verified from the archived copy.
ACCA, Technical Manuals (retrieved Oct 16, 2026): Manual J Residential Load Calculation, the national ANSI-approved standard. acca.org/standards/technical-manuals
U.S. EPA, Section 608 Technician Certification Requirements (retrieved Oct 16, 2026): technicians who maintain, service, repair, or dispose of equipment that could release refrigerants into the atmosphere must be certified. epa.gov/section608/section-608-technician-certification-requirements

Electrical trouble? Shut the AC off at the breaker and call a licensed HVAC tech. Do not open panels, the outdoor disconnect, or unit covers.