Leaving Florida for the Summer? AC Settings for an Empty House

The mistake that costs the most

Switching the air conditioning off entirely. It feels like the obvious way to avoid paying to cool an empty house, and in Ohio it would be. In South Florida it is how people come back to a mold problem, warped cabinet doors, curling floors and a smell that does not leave.

The reason is humidity. Outdoor relative humidity here sits high for months, and a closed house without air conditioning does not stay dry — it equalises with outside. Sustained indoor humidity above roughly 60% is enough for mold to establish on soft furnishings, in wardrobes, behind pictures, and inside the air handler itself. It takes weeks, not months, and it is far more expensive to remediate than the electricity would have been.

What to set

• Thermostat: 77 to 80°F. This is the working range. High enough to save meaningfully, low enough that the system still runs and dehumidifies. We would suggest 78 as the default.
• If your thermostat has a humidity setpoint, set it around 55%, and let it override the temperature to run for dehumidification. Many modern and smart thermostats do this — if yours can, use it, because humidity rather than temperature is what you are actually managing.
• Fan: AUTO, never ON. Continuous fan in an empty humid house re-evaporates moisture off the coil back into the air, which is precisely wrong here.
• Do not set it above 80. Above that the system may barely run, and a system that barely runs does not dehumidify.

Why not just use a dehumidifier?

A portable dehumidifier can supplement, but on its own it is a poor substitute for a whole house: it only treats the room it is in, its tank fills and stops unless it is plumbed to a drain, and its own failure mode is a puddle. If you have a whole-house dehumidifier integrated with the system, that is a different and genuinely good answer — set it around 50–55%.

The pre-departure checklist

• Have the condensate drain flushed and the float switch tested. This is the single highest-value item on the list. A drain that clogs while you are away either shuts the system off — and the house then sits hot and humid for months — or overflows into the ceiling for weeks with nobody watching.
• Change the filter and fit a clean one, not the one that is nearly due.
• Confirm the secondary pan and its float switch if the air handler is in an attic or above finished space.
• Get the system serviced before you leave, not after you return. A marginal capacitor becomes a dead system in July with nobody there.
• Fit a smart thermostat if you can. Remote monitoring is the point — a thermostat that emails you when the indoor temperature or humidity climbs turns a disaster into a phone call. Set alerts for high temperature and high humidity.
• Turn off the main water supply, or at least the supply to the water heater and any appliance hoses. Most catastrophic empty-house damage is water, and much of it is plumbing rather than AC.
• Leave interior doors open and closet doors ajar so air circulates everywhere, including the places mold prefers.
• Close blinds and curtains to cut solar gain and reduce the load.
• Do not close supply vents in unused rooms. Those are the rooms most likely to develop a problem.
• Arrange for someone to check the house every couple of weeks — a neighbour, a manager, a service. Ask them specifically to feel whether the air is cool and dry, and to look at the drain line.
• Consider a surge protector on the system. Storm season is exactly when you are away.

If you leave during hurricane season

Clear the area around the outdoor unit, and make sure nothing stored nearby can become a projectile. Confirm the unit’s hurricane tie-downs are in place and sound. Photograph the equipment, including the data plate, and keep it with your insurance documents — if you later need to prove condition and age, you will be very glad you did. The claims side is covered in hurricane damage and insurance, and the seasonal checklist is in hurricane season AC prep.

If a storm causes an extended power outage while you are away, the house may sit hot and humid for days. That is survivable. What is not survivable is the same conditions for two months because a $3 float switch did its job and nobody knew.

What it actually costs to leave it running

A system holding 78 degrees in an empty, shaded, closed-up house runs far less than one keeping an occupied house at 74 with doors opening, cooking, laundry and people in it. The load is genuinely low. Work your own number using the method in what it costs to run your AC, and then compare it with a mold remediation quote. It is not a close call.

Coming home

Set the thermostat back to your normal setting a day before you arrive if you can do it remotely, and give the system time. Do not set it to 68 on arrival to catch up quickly — it does not work, and on a system that has been running lightly for months it is a good way to freeze the coil. If the house smells musty on return, that is information: see the musty smell article and get the coil and pan looked at before the season starts properly.