What Temperature to Set Your AC When You're Away

When you’re away, set your air conditioner to about 85°F, which is roughly 7 to 10 degrees warmer than the temperature you keep when you’re home. Keep it closer to 80°F if pets stay behind or a warm spell is on the way; if the house is empty and the week looks foggy and mild, as it often does in Daly City, Colma and Brisbane, a high setpoint means the AC will barely run.

Away settings at a glance

These are starting points. Adjust a degree or two at a time.

Why a higher setting saves energy

A house gains heat in proportion to the difference between the indoor and outdoor temperature. When you let the inside warm up while you’re gone, that gap shrinks, heat leaks in more slowly, and the AC has less total heat to remove. The old idea that the system “works harder” to cool back down and wipes out the savings does not hold for a standard central AC: recovery takes longer, but it uses less energy than holding a cool temperature all day.

Two things to expect. First, recovery takes time. A closed house that sat at 85°F can need an hour or more to come down, because walls, floors and furniture soaked up heat too. Second, setting the thermostat to 65°F when you walk in does not cool the house faster. A typical single-stage AC delivers cooling at one fixed rate, so a lower number only makes it run longer and overshoot. Set your normal temperature and let a schedule start cooling before you get home.

Should you turn the AC off completely?

For a summer trip, turning cooling off is reasonable when nothing inside depends on the temperature and the forecast is mild. On this part of the coast that describes many summer weeks: the marine layer keeps afternoons cool, and a closed house often stays comfortable on its own.

Leave the AC on at a high setpoint instead when:

• A warm spell is possible. Locally, the hot days tend to come in late summer and early fall, when the fog pulls back, and they can arrive with little notice.
• Something inside needs steady conditions. Pets, aquariums, houseplants, wine and wooden instruments all do better without big swings.
• The house turns damp or musty when closed up. An AC removes moisture as it cools. With the system off, indoor humidity can creep up, and keeping it below about 60 percent relative humidity helps prevent mildew. Our fog belt humidity guide covers moisture in local homes in more detail.

Winter: what temperature to set the heat when away

If you are asking this in winter, the idea flips. Lower the heat while you’re gone: around 60 to 62°F for a workday and 55 to 58°F for a trip of several days. Avoid shutting the heat off for a long absence. A cold, closed house in a damp climate collects condensation on windows and cold walls, and mildew follows.

Heat pump owners need one extra step. When a heat pump has to raise the temperature several degrees at once, many thermostats call for the electric backup heat, often shown as “aux heat,” to help. Backup strips work, but they cost more to run than the heat pump itself. Keep winter setbacks to a few degrees, or use a thermostat with a heat pump recovery feature that warms the house gradually. In a mild coastal winter, a modest setback is usually all you need.

Fan: Auto, not On

Leave the fan on Auto while you’re away. On runs the blower around the clock, which adds to the electric bill, and in cooling mode it can blow moisture from the still-wet indoor coil back into the house between cycles. Auto runs the fan only while the system is heating or cooling.

Schedules, holds and smart thermostats

A programmable or smart thermostat does this for you. Set an away period at about 85°F in summer and have it return to your normal temperature an hour or so before you’re due home. A few settings trip people up:

• Hold overrides the schedule. A hold left on from a past trip keeps the house at that temperature until someone clears it.
• Heat-cool mode with a narrow gap. Too small a gap between the heating and cooling setpoints can make the system switch back and forth. Leave the gap at the default or wider.
• Away and geofencing modes. Smart thermostats may switch to their own away temperatures when phones leave the house. Check those numbers.
• Batteries. Many thermostats run on AA or AAA cells. Weak ones can blank the screen or drop the schedule, so replace them before a long trip.

If your thermostat cannot run a schedule or handle heat pump recovery, see thermostat installation. The wiring, the number of heating and cooling stages, and whether you have a heat pump all affect which thermostats fit and how much work the swap takes.

Mistakes that look like a broken system

• The house takes hours to cool after a trip. Normal after a long setback, especially following a sunny afternoon. If the vent air is cool and the temperature keeps dropping, the system is working.
• Short on and off runs. A setback does not cause them. If runs stay short once the house is back to normal, read about AC short cycling.
• A higher bill after time away. Usually a hold left on or backup heat during a winter recovery. If bills stay high with no setting to blame, see high energy bills.
• The AC runs nonstop and never reaches the setpoint. That one is a real problem, not a setting. Start with AC running but the house still hot.

Before you leave, and when to call

For a longer trip, put in a clean air filter, set the away temperature, clear any hold, leave the fan on Auto, and make sure leaves and debris are not piled against the outdoor unit.

Call if the system runs without reaching the setpoint, blows warm air, ices up, trips a breaker or starts making new noises. What a visit involves depends on what we find: a schedule or setting problem is a short fix, while a failing part, a refrigerant issue or a duct problem takes more time and parts. Daly City HVAC Masters answers the phone 24/7 at (650) 203-6702, and technician appointments run 8 AM to 8 PM, seven days a week. You get a written estimate before any work begins, and we explain what we found and why we recommend it, so you can decide.