Lower the Thermostat or Add a Dehumidifier?

What each one actually does

Your air conditioner removes moisture only as a side effect of cooling, and only while it is running. Lowering the thermostat makes it run longer, which does remove more moisture — that is why it half-works. But you are now paying to over-cool the whole house in order to dry it.

A dehumidifier removes moisture directly, independent of temperature. That means it can dry the house without dropping it to 70°F, which is both more comfortable and usually cheaper to run for that purpose.

Measure before you buy anything

A hygrometer costs very little and turns this from an argument into a number. Target for a South Florida home is roughly 45–55% relative humidity. Above about 60% you get the familiar symptoms: clammy air, condensation on vents or windows, musty smells, doors sticking, and conditions that favour mould and dust mites.

Also time a cooling cycle. Reaching setpoint in under ten minutes and shutting off, repeatedly, tells you the system is oversized and short cycling — which is the actual cause in most of these houses.

Try these before buying equipment

• Fan to AUTO, not ON. Continuous fan re-evaporates water off the coil straight back into the house. This one is free and it is the single most common self-inflicted humidity problem.
• Raise the setpoint slightly and see what happens. Counter-intuitive, but longer run cycles at a higher setpoint often dehumidify better than short blasts at a low one.
• Seal the obvious leaks. Return-side duct leakage in a hot attic pulls humid air directly into the system. Door sweeps, penetrations, a badly sealed attic hatch all add load.
• Service the system. A dirty coil dehumidifies badly.

When a dehumidifier is genuinely the right answer

• A well-sealed house that still reads above 60% with the system working correctly.
• A home that sits empty for weeks — seasonal residents especially. A dehumidifier protects the house when the AC has little reason to run.
• A house where the cooling load is genuinely small relative to the moisture load, so the AC rarely runs long enough to dry anything.
• Somewhere with a specific moisture source — a pool room, a poorly ventilated laundry.

Whole-home units are ducted into the system and controlled by their own humidistat. Portables handle one room, add heat to that room, and someone has to empty them — useful as a stopgap, not a solution.

The alternative worth knowing about

If you are near replacement anyway, a variable-speed (inverter) system changes this problem substantially. It runs long, slow cycles instead of short blasts, and long low-speed run time is exactly what removes moisture. In this climate that is the strongest argument for inverter equipment — stronger than the efficiency numbers.

The short version

Cranking the thermostat down is the expensive way to treat a symptom. Measure first, fix the free things, and buy a dehumidifier when the numbers still say you need one — not before.