AC settings for an empty vacation rental or second home

Why an empty beach house gets damp

A lot of homes on this stretch of coast sit empty much of the time. Census figures (2020 to 2024) count 49% of homes in Destin as vacant, 53% in Santa Rosa Beach and along 30A, and 72% in Miramar Beach. Most of those are second homes and short-term rentals waiting for the next guest.

The AC is the only thing pulling water out of the air in a closed house. Turn it off and the humidity inside climbs toward what it is outside, and the beach rarely gets a break. The Destin-Fort Walton Beach Airport logs 106 nights a year with a low of 75°F or warmer, against 10 up in Crestview. The July average low on the coast is 78°F. Warm, wet nights mean the air outside never dries out, so neither does a house left to itself.

The EPA's guidance is to keep indoor relative humidity below 60 percent, ideally between 30 and 50 percent. That is the number to manage to. A closed condo on Okaloosa Island or a house off 30A with the AC off will be well above it by midsummer.

What to set between guests

A thermostat responds to temperature, not moisture. If you set it high to save power, there will be stretches, on a cloudy, wet day for example, when the house sits below the setpoint and stays damp, and the AC never runs. Over a season that is how a rental ends up with a musty closet and spots on the ceiling before anyone notices.

So the question to ask is how your system can run on humidity. The usual options:

• A smart thermostat with a dehumidify setting or a humidity sensor, so the AC runs when humidity goes up even if the temperature is fine. Many newer models have this; some systems need extra wiring to use it.
• A humidistat that controls the AC directly.
• A whole-home dehumidifier tied into the ductwork, which pulls moisture without cooling the house down. Ask how it and the AC should be set to work together.
• A daily run schedule, if none of the above is available. It is cruder, but it beats a long setback with no humidity control at all.

I'd ask the technician which of these your system supports before buying hardware. Then pick a setting that keeps the house under the EPA's 60 percent and check that it does with a sensor that reports to your phone. A cheap humidity and temperature sensor is the most useful thing a remote owner can buy. It tells you the house is getting damp long before a guest smells it.

Turnover checks for rental managers

A setting only helps if the system actually runs. Capacitors fail, breakers trip after summer storms, and drain lines clog in July whether a guest is there or not. At each turnover, or at least weekly in summer:

1. Look at the filter. A house that sleeps eight people runs harder than one that sleeps two, and a clogged filter cuts airflow and can freeze the coil. Keep spares of the right size in the unit.
2. Check the drain line and the float switch. A clogged condensate line is the common way an AC causes water damage in an empty unit. Look for water in the secondary pan under the air handler, and confirm water drains outside when the system runs. Our drain line guide covers it.
3. Reset the thermostat to your between-guest setting and confirm it took. If your thermostat can limit how low or high guests can set it, ask whether that makes sense for your property.
4. Close the sliders. A balcony door left open lets in humid air faster than the AC can pull it out.
5. Smell the closets and the air handler closet, and look at the ceiling near the supply vents for spots.
6. Check the breaker panel after a storm or a power outage.
7. Look at the outdoor unit. On the Gulf front, salt settles on the coil. A gentle rinse with a hose, power off first, helps. More in our salt air guide.

In a condo building the outdoor unit may be on the roof or a shared pad, and the association may have rules about access and who can work on it. Find out before the first service call, not during one.

When guests say it isn't cooling

Start with the easy checks: thermostat set to cool, filter, breaker, water in the drain pan. If the system runs all afternoon and the house still won't cool down, it may be undersized, low on refrigerant or dirty, and our page on an AC that can't keep up explains how to tell. If the house feels cold but clammy, that is often a system that cools too fast to pull out moisture, which our humidity page covers.

If you come back to a musty house

Don't start by booking a duct cleaning. Find out why the moisture got in first: AC off, a drain clog, a leak, or a system that short cycles. If there is visible mold on surfaces or in the air handler, the EPA's mold guidance applies.

The season here is long. In Crestview the first 90°F day has recently come around April 24 and the last around October 16. For a rental, "summer settings" covers most of the year. Area pages: Destin, Miramar Beach, Santa Rosa Beach and 30A. More numbers are on our heat and humidity data page.