What good indoor humidity looks like

For South Florida homes, target indoor relative humidity is 45–55% in summer. Above 60% you start feeling clammy at any temperature; above 65% you risk mold growth, dust mites, and warped wood floors. Below 35% (rare here) you get dry skin and static.

You can buy a $15 hygrometer from any hardware store to measure. Most smart thermostats display indoor humidity too.

Cause 1 — Oversized AC (the most common)

An AC that’s too big cools the air to setpoint very fast — but cooling is what removes humidity. A 4-ton system in a house that needs 3 tons might hit 75°F in 6 minutes, then shut off — never running long enough to actually dehumidify.

Symptom: Cool air at setpoint, but feels clammy. Short run cycles (under 12 minutes).
Fix: Variable-speed system replacement (Manual J sized). Or, in the short term, a whole-home dehumidifier ($1,800–$2,800 installed).

Cause 2 — Thermostat set to “FAN ON” instead of “AUTO”

When the fan runs continuously but the AC isn’t actively cooling, the moisture that condensed on the coil during the cooling cycle evaporates back into your air.

Fix: Set thermostat fan to AUTO. The blower will only run when cooling.

Cause 3 — Dirty evaporator coil

Dust, pet dander, and biofilm accumulate on the indoor coil over years. This insulates the coil and reduces the temperature differential — which means less moisture condenses out.

Symptom: Cooling capacity slowly degrading over years. Higher bills.
Fix: Professional coil cleaning, $250–$400. Best done as part of an annual maintenance plan.

Cause 4 — Duct leakage

If your supply ducts run through an unconditioned attic and are leaking, hot humid attic air gets sucked into the return side. Studies show 20–30% leakage is typical in older South Florida homes.

Symptom: Some rooms much more humid than others. AC runs constantly. High utility bills.
Fix: Aeroseal duct sealing ($1,400–$2,200) or manual mastic sealing ($800–$1,500).

Cause 5 — Refrigerant overcharge or undercharge

Either condition reduces dehumidification — overcharge raises evaporator temp (less condensing); undercharge causes coil ice (no condensation while frozen).

Fix: Tech with gauges, charge to spec. $200–$300.

Cause 6 — Drainage backup pulling water back

If the condensate drain has a partial clog, water that should leave the system stays in the drain pan and re-evaporates back into your supply air.

Fix: Clear the drain (wet/dry vac at outdoor termination, vinegar at indoor cleanout). Annual drain treatment included in our maintenance plan.

The South Florida humidity playbook

For homes that consistently can’t hit 50% indoor humidity in summer, the most reliable fix sequence (cheapest first):

1. Switch fan to AUTO. (Free.)
2. Replace filter, hose off outdoor coil. (Free / $0.)
3. Have indoor coil professionally cleaned. ($300.)
4. Have refrigerant charge verified. ($250.)
5. Aeroseal or manually seal ducts. ($1,400–$2,200.)
6. Add whole-home dehumidifier (Aprilaire E100/E130, Honeywell DR90). ($1,800–$2,800.)
7. Replace AC with right-sized variable-speed system. ($11,500+.)

Most homes solve the problem at step 4–5.

FAQ

Why does my house feel humid only in certain rooms?
Either localized duct leakage in those rooms’ supply runs, or those rooms have outsized latent loads (kitchen, bathrooms, rooms with lots of glass).

Will running AC at a lower temperature help?
A little. Lower setpoint = longer run time = more dehumidification. But the right answer is fixing why the AC isn’t dehumidifying at the proper setpoint.

How much does a whole-home dehumidifier cost to run?
About $25–$40/month in summer in Boca Raton. Pays for itself in comfort, mold prevention, and the ability to set your thermostat 3°F warmer (which saves more on cooling than the dehumidifier costs to run).

Need it diagnosed today?

AtlantFlow runs same-day service across Palm Beach, Broward, and Miami-Dade. Licensed FL CAC1824422.

Call (561) 510-9414