What Temperature Should You Set Your AC in Summer?

The standard advice is 78°F when you're home and a few degrees higher when you're out. In a North Texas summer that's sound, with one caveat worth knowing: on a 100°F afternoon, most residential systems physically cannot reach much below 78 anyway.

The 20-degree rule

A correctly working residential AC can pull indoor temperature roughly 20 degrees below outdoor. On a 100°F day that's about 80°F indoors, at full tilt.

So if it's 102 outside and your house is sitting at 80, your system probably isn't broken — it's at its physical limit. That's worth knowing before you pay for a diagnostic.

If the gap is much smaller than 20 degrees, that is worth investigating. It usually points to low refrigerant, a dirty coil, or duct losses.

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Setting it lower does not cool faster

An AC doesn't have a throttle. Setting the thermostat to 65 instead of 75 doesn't make it work harder — it just makes it run longer, past the point you wanted, using more electricity to overshoot.

What actually helps

• Close blinds on the sun side. Free, and surprisingly effective in a Texas afternoon.
• Ceiling fans in occupied rooms. They cool people, not rooms — so turn them off when you leave.
• Raise it 4–7 degrees when out, rather than switching off entirely. A system restarting a 15-degree climb in August works harder than one holding a smaller gap.
• Keep interior doors open so the house equalises.

If you can't hold temperature at all

If the house won't hold 78 on a normal day, or one room never gets there, that's a fault worth diagnosing — most often low refrigerant, a failing capacitor, or duct problems.