Why Is My Home Still Humid With the AC Running? A Florida Homeowner's Guide

If you have ever set your thermostat to 74°F in a Florida home, walked into the living room an hour later, and thought "why does this still feel sticky," you have hit one of the most common HVAC problems in our market. Your AC is doing its first job (cooling) but failing its second one (removing moisture). Indoor humidity is the variable most Florida homeowners never measure but feel constantly.

This guide walks through what indoor humidity should actually be, the four reasons your AC stops dehumidifying well, and what fix actually solves it. Spoiler: setting the thermostat lower is the worst possible solution — it makes the problem worse and runs up your electric bill.

What Indoor Humidity Should Be in a Florida Home

EPA and ASHRAE both recommend 30 to 60 percent relative humidity (RH) indoors. For Florida specifically, the comfort and mold-safety sweet spot is 45 to 55 percent.

Pick up a $15 hygrometer at Home Depot or Amazon and place it in your living room. If it reads above 60 percent for more than a day or two, you have one of the four problems below. The number on the thermostat is meaningless without this measurement.

The "Two-Stroke" Job Your AC Has to Do in South Florida

Outdoor air in Broward County averages 75 to 85 percent relative humidity year-round. Every time you open a door, every time a leaky duct pulls attic air into the supply, every time a bath fan runs, that humid air gets dragged into your conditioned space. Your AC then has to do two things:

1. Cool the air by passing it across a cold evaporator coil (the "sensible" load).
2. Remove water from the air by condensing it onto that same coil and draining it out (the "latent" load).

The longer the AC runs, the more moisture it removes. A system that runs 35 minutes at a stretch will pull 4 to 8 times more water out of the air than one that runs three short 8-minute cycles. This is the key fact most homeowners do not know — and it explains why oversized AC systems leave Florida homes feeling sticky.

Cause 1: Your AC Is Oversized (The #1 Hidden Culprit)

Here is the counterintuitive truth that surprises every homeowner we explain it to: a 5-ton AC in a house that only needs 3 tons of cooling will leave you HOTTER and STICKIER than the right-sized 3-ton unit, even though it has way more cooling capacity.

Why: the oversized unit cools the air down to the thermostat setpoint fast (8 to 12 minutes), then shuts off. During that 8 to 12 minutes, the coil only has time to remove maybe 20 percent of the moisture it would have removed in a 25 to 30 minute cycle. The humid air left behind feels just as sticky as before. So you drop the thermostat from 74 to 72, the system cools faster and shuts off sooner, and you have made the humidity problem worse while doubling your bill.

This is the #1 reason builder-installed Florida AC systems fail to dehumidify properly. For decades, contractors sized AC by simple rule-of-thumb (1 ton per 500 square feet), which overshoots in any home with good insulation, new windows, or shade trees. Florida code now requires a Manual J load calculation for new installs, but the legacy systems in most pre-2015 homes were sized by the old method and almost certainly oversized.

How to confirm: Note your cycle times on a hot day. If your system runs less than 12 minutes per cycle and cycles multiple times per hour with high indoor humidity, it is almost certainly oversized.

Fix: The proper answer is right-sizing, either by replacing with a correctly sized system, by adding a two-stage or variable-speed system (which can effectively run at 50 to 67 percent capacity on mild days), or by adding a whole-home dehumidifier to handle the latent load that the oversized AC misses. We do the Manual J calculation as part of every replacement quote — see AC installation.

Cause 2: Short-Cycling (Sister Problem to Oversizing)

A short cycle is when the AC turns on, runs briefly (under 10 minutes), and shuts off, then repeats every 10 to 15 minutes. Short cycles look efficient on a thermostat ("it just turned off!") but they are terrible for humidity control. The coil never gets cold enough for long enough to pull water out of the air. By the time it would start removing moisture aggressively, the cycle ends.

Beyond oversizing, short cycles can be caused by:

• Thermostat in a bad location (direct sunlight, near a hot kitchen, near a supply vent that immediately cools it)
• Dirty filter or coil causing the high-pressure safety to trip
• Refrigerant level off, causing the low-pressure safety to trip
• Faulty thermostat or thermistor reading temperatures incorrectly
• Defective contactor or control board cycling power inconsistently

Fix: A tech needs to log run-time and pressure data to find the cause. Easy fixes (relocating a thermostat, replacing a filter) are under $200. Refrigerant or control issues run $300 to $1,500.

Cause 3: Dirty Coils or a Failing Blower

A dirty evaporator coil cannot absorb heat OR condense moisture as efficiently as a clean one. Over years of even moderate filter maintenance, a thin layer of dust, smoke residue, and biofilm builds up on the coil. The system runs longer to reach setpoint but produces less dehumidification per minute of run-time.

Same principle applies to the blower: if it is dirty, weak, or has a failing capacitor, airflow drops, run-time gets weird, and dehumidification suffers.

Signs the coil or blower is your problem:

• System used to handle humidity fine; gradually got worse over a year or two
• Musty smell when the system kicks on
• Air handler is the original equipment that came with the house and has never had a coil cleaning
• Visible dust buildup on the supply vents or louver fins

Fix: Professional evaporator coil cleaning runs $200 to $450 and is included in our Comfort Club tune-ups. Blower wheel cleaning is typically $150 to $250 if done at the same time. This single service often restores a 10-year-old system's dehumidification performance to near-new. If you also notice a musty smell, read our guide to musty AC odors.

Cause 4: Leaky Ductwork Pulling Humid Attic Air

Almost every Florida home routes ducts through the attic. Attics in South Florida hit 120 to 140°F in summer with relative humidity in the 60 to 80 percent range. If your return duct has a leak, a torn flex section, or a poor seal at the air handler connection, the system will pull that humid attic air directly into your supply, where it gets mixed with the conditioned air and pushed throughout your house.

This is the most insidious cause because it can sabotage even a brand-new, properly sized system. We have measured Florida homes where 25 to 40 percent of total return air was attic air, not house air.

Signs of leaky ductwork:

• One or two rooms always feel stickier than the rest of the house
• Excessive dust on furniture even when you change filters religiously
• Higher-than-expected electric bills
• Attic feels noticeably cool when the AC is running (your cold air is leaking into the attic, and humid attic air is being pulled into the return)
• Visible disconnected duct sections or torn insulation when you look in the attic

Fix: A duct pressure test ($200 to $400) measures leakage. Sealing typically runs $500 to $2,500 depending on how much of the system needs work. Read our duct cleaning vs duct sealing guide to know which one your home needs.

The Real Fix for High Humidity: A Whole-Home Dehumidifier

For homes where you have already right-sized the AC, cleaned the coil, and sealed the ducts, but humidity still creeps over 55 percent on rainy spring days, the proper answer is a dedicated whole-home dehumidifier integrated into the ductwork. These units (Aprilaire and Honeywell are the common Florida brands) remove 70 to 130 pints of water per day and run independently of the cooling cycle.

Why a whole-home unit beats a portable:

• Controls humidity for the entire house, not one room
• Drains automatically to a condensate line (no tank to empty)
• Runs only when needed, controlled by a separate humidistat
• Uses less energy per pint removed than a portable
• Hidden in attic or closet, no floor space lost
• Allows you to keep the AC at 76 to 78°F and still feel cool, because lower humidity feels cooler than dry 72°F air

Installed cost runs $2,000 to $3,500 in Broward County. Most Florida homes that have one save 10 to 15 percent on their electric bill because the AC can run at a higher setpoint while feeling more comfortable. See indoor air quality services for details.

When Musty Smell Is Your First Warning

If your home is humid AND smells musty, you likely already have mold or mildew growing somewhere — on the coil, in the drain pan, in the ductwork, or on porous surfaces in the house. Sustained humidity over 60 percent provides the conditions for biological growth within 48 to 72 hours.

This is more than a comfort issue. Indoor mold can trigger asthma, allergies, and chronic respiratory irritation, especially in children and seniors. If you smell musty air, do not just buy a dehumidifier and hope the smell fades — you need to find and remove the existing growth, then control humidity to keep it from coming back. Our musty smell troubleshooting guide walks through the specific sources to check.

What to Do This Week

1. Buy a $15 hygrometer and place it in your main living area. Measure for 3 to 5 days.
2. If humidity is above 60 percent, replace the air filter and time your AC cycles. Cycles under 12 minutes mean the system is oversized or short-cycling.
3. Check for musty smell when the AC kicks on after being off for an hour. If present, schedule a coil and drain pan inspection.
4. Have ductwork visually inspected for obvious disconnections or tears, especially at the return drop.
5. Get a humidity consult if the above does not fix it. We measure the actual latent load on your home and recommend the right combination of system right-sizing, duct sealing, or whole-home dehumidifier.