Set the thermostat to 82-85°F before you leave for a week, not Off. That’s the range that balances energy savings against humidity control and mold risk. The exact number depends on your climate: drier climates can push closer to 85°F, humid coastal or Gulf Coast homes should stay at 82°F or below.
Do not turn the AC off. Here’s why.
What happens when you turn it off
With the AC off in summer, a closed house absorbs heat through the roof, walls, and windows. In humid climates, humidity follows: your home can reach 70-80% relative humidity within 24-48 hours. Above 60% RH, mold begins growing on organic surfaces, including wood framing, drywall paper, upholstery, and stored clothing.
Even in drier climates, an off AC means you’re asking the system to pull a 90°F, 70% humidity house back down to 75°F when you return. That’s a long recovery cycle that costs more than the steady 82°F maintenance would have.
The right way to set vacation mode
Setpoint: 82-85°F. The system runs occasionally to knock back humidity spikes even if the temperature is inside that range. Some thermostats have a “humidity setpoint” mode that runs the compressor specifically to control moisture at or below 60% RH. Use it if yours supports it.
Fan: Auto. Running the fan on “On” during a long absence re-evaporates coil moisture and works against humidity control. Auto-only.
Not a freeze-guard mode. Don’t confuse summer vacation mode with the low-limit “freeze guard” some thermostats offer (typically 45-55°F in winter). Summer vacation mode is a high-limit hold.
Geofencing: the smarter return
Instead of trying to time a pre-arrival pre-cool manually, use the geofencing feature on a smart thermostat:
- Nest (Home/Away Assist): uses your phone’s location and home activity sensors. Switches to Away mode when you leave, begins cooling before you return.
- ecobee (Smart Home/Away): similar phone-based detection with occupancy sensors.
- Honeywell Lyric / T9: geofencing mode with “radius” trigger you can set in the app.
Set the “Home” setpoint to your normal comfort (78°F) and the “Away” setpoint to 82-85°F. The thermostat handles the transition automatically. When you’re 30-60 minutes from home, it starts the cool-down so the house is ready when you arrive.
If you’ll be gone longer than two weeks
For extended absences:
- Walk through and check for humidity problems: a $10 hygrometer left in the house (or a WiFi version you can monitor remotely) shows you if humidity is creeping up.
- Set to 80°F rather than 85°F in very humid climates (Southeast US, Gulf Coast). The additional electricity cost is small compared to mold remediation.
- Make sure drain lines are clear. A clogged condensate line will trip the float switch and shut the system off, which is the last thing you want while you’re away for two weeks.
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If you’re coming back to a house that took 6+ hours to recover to a comfortable temperature after a week away, or one that has recurring humidity issues despite the AC running, the system may be undersized or losing efficiency with age. See Best Thermostat Settings for Summer for the full seasonal schedule, or Why Your AC Isn’t Cooling if recovery takes longer than expected.
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Related: Best Temperature for AC in Summer · AC vs Dehumidifier · Best Thermostat Settings for Summer · Thermostat Savings Calculator
