Keeping indoor humidity under control in a Miami home

How an AC removes moisture

An air conditioner dries the air as a side effect of cooling it: when humid air passes over the cold coil, moisture condenses and drains away. That only works while the coil is cold and the system runs long enough for water to drip off. A system that runs for a few minutes and shuts off lowers the temperature without removing much moisture.

Common causes of a cool but damp house

Oversizing is the most common. A system too big for the house drops the temperature fast and stops before it has dried the air. The fan setting matters too: with the fan set to on rather than auto, air keeps blowing over the wet coil between cycles and puts moisture back into the house.

Leaky ducts are the third cause. A return duct with gaps in a hot attic pulls in humid attic air, adding moisture the system has to remove. Sealing the duct often makes a noticeable difference on its own.

Measuring humidity at home

An inexpensive digital hygrometer, placed away from bathrooms and the kitchen, tells you more than how the house feels. Readings that sit in the mid-40s to mid-50s percent are in the comfortable range. Readings that stay above 60 percent for days, especially with the AC running, point to one of the causes above. Musty closets, mildew on shoes and leather, and condensation on windows or supply grilles are the physical signs that the number is too high.

Why Miami houses fight moisture

The outdoor air in Miami carries a lot of water for most of the year, and every gap in the house lets some of it in: around doors and windows, through attic penetrations, and through leaky return duct. A tighter house, sealed duct and a correctly sized system that runs long, steady cycles are what keep that moisture out of the living space, and they work better together than any one of them alone.

Settings that help

Set the fan to auto, not on. Avoid very low setpoints that make the system short cycle on mild days. If your system is two-stage or variable-speed, make sure the thermostat supports it, because the low stage is where most of the moisture removal happens.

When a dehumidifier is the answer

If the AC is sized correctly, the duct is sealed and the settings are right, and the house still runs damp, a ducted whole-home dehumidifier is the next step. It runs on a humidistat independently of the AC and sends drier air through the existing duct. Many homeowners find they can raise the thermostat a couple of degrees and feel more comfortable. If the AC is oversized and near the end of its life anyway, a correctly sized variable-speed system may solve both problems.