Humidity and Mold Control While You're Away

The short answer

Leave the air conditioner on. In a closed-up South Florida house, the AC does more than cool. It's the main thing pulling moisture out of the air. Set it to run less than it does when you're home, but not so little that humidity climbs. Aim to keep indoor humidity below 60 percent, which is the EPA's guidance for preventing mold, and check that the drain is clear before you go.

Why empty houses get musty

An air conditioner removes moisture only while it runs: humid air crosses the cold coil, water condenses and drains away. Raise the thermostat a long way, or switch the system off, and it barely runs. Outdoor humidity keeps leaking in through gaps, walls and the slab all the same. The temperature might look acceptable on the thermostat while the moisture quietly builds.

Moisture is what mold needs. The EPA's summary is blunt: "The key to mold control is moisture control." University of Florida extension guidance notes that relative humidity above 70 percent for long periods "can eventually lead to mold problems, especially in stagnant air spaces such as closets." Those are exactly the spaces that get no air when a house sits closed for months.

Set a humidity target, not just a temperature

Sources give slightly different numbers, but they agree on the direction:

• EPA: keep indoor humidity below 60 percent, ideally between 30 and 50 percent.
• FSEC (University of Central Florida): a target between 45 and 55 percent, and humidity above 60 percent for several hours in an air-conditioned home is a sign something could be improved.

The practical goal while you're away: keep it under 60 percent. To know whether you're managing that, you need a measurement. Many smart thermostats show indoor humidity and can be checked from your phone; an inexpensive digital hygrometer works too, if someone can read it for you.

Thermostat strategy

There's no single right temperature for every home, because houses differ in how much moisture they let in and how well their system removes it. A reliable way to find your setting:

1. A week or two before you leave, raise the thermostat to the setting you're considering and watch the humidity.
2. If humidity stays under 60 percent, that setting works for your house in that season.
3. If it creeps up, lower the setting until it doesn't, or have the system itself looked at (see below).

Two settings matter regardless of temperature:

• Fan on AUTO, not ON. FSEC's guidance for hot, humid climates is to always leave the fan on AUTO. With the fan running continuously, water left on the coil after the compressor shuts off gets blown back into the house.
• Don't switch to OFF "to save energy." It saves some electricity, but it gives up the moisture removal that keeps a closed house dry.

If your system can't keep humidity down at any reasonable setting, that's information too. Common reasons are an oversized system that runs in short bursts (see AC short cycling), a dirty coil, or simply a house that lets in more moisture than the AC removes when it isn't working hard. In those houses a whole-home dehumidifier may be the better answer, and ENERGY STAR makes the same point for humid climates with central air. See humidity control.

Before you leave: a checklist

• Replace the air filter. ENERGY STAR's guidance is to check monthly and change it at least every three months. A fresh filter keeps airflow up while nobody is checking.
• Make sure the drain is flowing. A clogged condensate drain is one of the most common reasons a system shuts itself off or leaks, and those are the last things you want in an empty house. Manufacturer guidance for homeowners: turn the AC off, check the drain line's access cap, clear debris, and flush the line with diluted vinegar or a cleaner made for PVC drain lines. See AC drain line clogs.
• Open interior doors, including closets. Air that reaches every room keeps moisture from settling in the stagnant corners mold likes.
• Leave the thermostat on COOL, fan on AUTO, at the setting you tested.
• Ask someone to check in. A neighbor or house-watcher who can look for water around the air handler, a musty smell, or a thermostat that's blank. Make sure they know where the breaker panel is.
• Plan for hurricane season. The Atlantic season runs June 1 to November 30, the same months many seasonal residents are away. If water starts to enter the house, the system should be shut off at the breaker; after any flooding it shouldn't be turned back on until it's been inspected. See hurricane season AC prep.

A maintenance visit before you leave covers the parts you can't check yourself: the coil, the refrigerant charge, the electrical components and the drain from end to end.

When you come back

Walk through the house before you settle in:

• Smell. A musty odor near the air handler, the vents or the closets means moisture got ahead of the system somewhere. See AC smells.
• Look. Spots on ceilings around supply registers, on closet walls or on shoes and leather goods are signs of high humidity.
• Check the air handler. Water in the pan under the unit, or a system that's off because its float switch tripped, points to a drain problem.
• Read the humidity and compare it with your target.

If you find mold, the EPA says a homeowner can usually handle an area smaller than about 10 square feet (roughly 3 feet by 3 feet). Larger areas, or mold after significant water damage, call for a professional. And if you know or suspect the AC system itself is contaminated, the EPA's advice is not to run it until it's dealt with, because it could spread mold through the house. Cleaning without fixing the moisture source doesn't last. As the EPA puts it, if the conditions that caused the growth aren't corrected, it will come back.