# Leaving Town? Don't Turn Your AC Off By Fairfax Mechanical (@fairfaxmechanical) · Published 2026-08-25 Canonical: https://voce.com/@fairfaxmechanical/leaving-town-don-turn-off-dap2mq --- # Leaving Town? Don't Turn Your AC Off It feels thrifty to shut everything down before a trip — but in a closed-up Northern Virginia house, turning the AC completely off can cost you far more than leaving it running. Here's exactly what to set it to before you go. **Quick answer** - **Don't turn the AC fully off in summer.** Raise it to about 82–85°F so it still controls humidity. - **Don't turn the heat fully off in winter.** Set it to 55–60°F to keep pipes from freezing. - The real enemy while you're away isn't temperature — it's **humidity**. Keep indoor RH under ~60% to prevent mold and mildew. - Have a **heat pump?** Don't drop the winter setting drastically, or it may lean on expensive backup heat when you return. - Before you leave: fans **off**, blinds **closed**, thermostat set, pets' comfort protected. - A smart thermostat lets you monitor and adjust the house from your phone, anywhere. ## Why "off" is the expensive setting The instinct is understandable: you're leaving for a week, nobody's home, so why pay to cool an empty house? But a shut-down house in a Northern Virginia summer doesn't just get warm — it gets _damp_. With no air conditioning running, indoor humidity climbs steadily, and our region has plenty of it to give. Give moisture a warm, still, closed-up house for several days and you create ideal conditions for mold and mildew on walls, in closets, and behind furniture. The math is lopsided. The extra runtime to cool the house back down when you return is minor. A mold remediation job, warped hardwood, or musty furnishings are not. Leaving the system running at a smart "away" setting is cheap insurance. **Should I turn off my air conditioner when I go on vacation?** No — you should raise the setting, not shut it off. In summer, set the thermostat to about 82–85°F so the air conditioner runs occasionally and keeps indoor humidity below roughly 60%, which prevents mold and mildew in your closed-up home. In winter, set the heat to 55–60°F to protect your pipes from freezing. Turning the system completely off saves very little and exposes your home to humidity, mold, or frozen-pipe damage that costs far more than the runtime you'd save. ## The two numbers to remember **Summer away setting → 82–85°F.** High enough to cut runtime, low enough that the AC still cycles on and pulls moisture out of the air. That humidity control is the whole point. **Winter away setting → 55–60°F.** Never below 55°F. This keeps interior and exterior-wall pipes safely above freezing while you're away — a burst pipe is the costliest "savings" there is. These aren't daily-setback numbers. Nudging the thermostat back a few degrees while you're at work — the kind of thing we covered in our post on [thermostat setbacks](https://fairfaxmechanical.co/) — is about runtime savings. A multi-day absence is different: the governing constraint is humidity, so you set it high but keep it running. ## The heat pump exception (winter) If you heat with a heat pump, don't slam the winter setting way down. Heat pumps warm a home gradually with supply air around 90–95°F. If you set the house to 55°F for a week and then ask it to jump back to 68°F when you return, the system often kicks on its electric backup (auxiliary) heat to catch up fast — which is the most expensive way a heat pump can run. For a heat pump, a gentler "away" setback (a few degrees, not a plunge) is smarter. Straight gas furnaces and dedicated AC systems don't have this quirk. ## Let your phone watch the house A smart thermostat turns vacation HVAC from "set it and hope" into "set it and see." Most let you set a dedicated vacation hold, check the indoor temperature and humidity from anywhere, and get alerts if something drifts out of range — so a failing AC or a spiking humidity reading pings your phone at the beach instead of greeting you at the door. Some use geofencing to ease the house back to comfort as you drive home. It's the single best upgrade for anyone who travels; see our guide to [smart thermostats](https://fairfaxmechanical.co/) for the details. ## The pre-trip checklist - **Set the thermostat** to your season's away number (82–85°F summer / 55–60°F winter). - **Turn ceiling fans off.** Fans cool people, not rooms — running them in an empty house does nothing but add cost, as we explained in our [ceiling fans piece](https://fairfaxmechanical.co/). - **Close blinds and curtains.** Blocking direct sun cuts solar heat gain, so the AC works less to hold your setting — the attic already hits 130–150°F up there. - **Change or check the air filter** so airflow stays clean while you're gone. - **Protect pets left home.** If animals stay behind, don't go above ~80°F, and make sure they have water and airflow. - **Long trip?** Ask someone to check in, or add a smart temperature/leak sensor for peace of mind. > "The homeowners who come back to a problem are almost always the ones who turned everything off to save money. In a Northern Virginia summer, an empty house with no AC becomes a humidity trap in a matter of days. Set it high, keep it running, and let it protect the house while you're gone." — Fairfax Mechanical Technical Team, Fairfax, VA ## Vacation HVAC FAQ **What temperature should I set my thermostat to when on vacation in summer?** Set it to about 82–85°F. That's warm enough to reduce runtime and your cooling costs, but cool enough that the air conditioner still cycles on periodically and removes humidity from the air. Keeping indoor relative humidity below roughly 60% is what prevents mold and mildew in a closed-up Northern Virginia home. Avoid turning the system fully off, which lets humidity climb unchecked. **Is it bad to turn off the AC completely when away?** In a humid climate like Northern Virginia's, yes. With the AC off, indoor humidity rises over several days and creates ideal conditions for mold, mildew, and moisture damage to wood, furniture, and finishes. The small amount you save on electricity rarely covers the cost of remediation or repairs. Raising the thermostat to 82–85°F is the better move — it cuts costs while still protecting the home. **What should I set the heat to when traveling in winter?** Set it to 55–60°F, and never below 55°F. That range keeps your pipes safely above freezing while still saving energy. If you heat with a heat pump, avoid a drastic setback — dropping it too far means the system may rely on expensive electric backup heat to recover when you return, so a gentler few-degree reduction is smarter for heat pumps. **Can I control my HVAC while I'm away?** Yes, with a smart thermostat. It lets you monitor indoor temperature and humidity from your phone, adjust the setting remotely, set a vacation hold, and receive alerts if conditions drift out of range. That means a cooling failure or a humidity spike reaches you while you're traveling, rather than surprising you when you walk back in the door. **Should I have my system checked before a long trip?** It's a smart precaution, especially before summer travel. A pre-trip inspection catches weak parts like a failing capacitor, low refrigerant, or a clogged drain before they cause a shutdown in an empty house. Fairfax Mechanical is a licensed Virginia contractor serving Fairfax, Vienna, Manassas, Gainesville, Woodbridge, and all of Northern Virginia — verify any contractor's license at [DPOR](https://www.dpor.virginia.gov/LicenseLookup), and ask about the federal [25C tax credit](https://www.irs.gov/credits-deductions/energy-efficient-home-improvement-credit) (up to $2,000 on a qualifying heat pump) and Dominion rebates on upgrades. * * * **Leave town with a house that protects itself.** Set it right, keep it running, and let your system guard against heat, humidity, and frozen pipes while you're away. Fairfax Mechanical serves all of Northern Virginia with same-day service and licensed technicians. [Book your service](https://fairfaxmechanical.co/contact) or visit the [FAQ](https://fairfaxmechanical.co/faq). _Fairfax Mechanical — Licensed Virginia HVAC Contractor · EPA Section 608 Certified · Serving Fairfax, Vienna, Manassas, Gainesville, Woodbridge & all of Northern Virginia._