Last Friday the National Weather Service had the Riverside and San Bernardino valleys under a heat advisory, “mid 90s to 102,” and my 1966 ranch house did what it does: soaked it up all afternoon and handed it back to me at dinner. I know the reflex, because I did it for twenty years. You walk into an 88-degree living room, you want 76, so you set the thermostat to 65. Get there faster, then ease it back up. As of mid-September 2026, with October’s heat still ahead of us, here is why the people who measure this wish you wouldn’t.
What the Energy Department says
The U.S. Department of Energy’s thermostat guidance is one sentence long on this point: “Avoid setting your thermostat at a colder setting than normal when you turn on your air conditioner. It will not cool your home any faster and could result in excessive cooling and, therefore, unnecessary expense.”
The same page takes on the belief that runs the other way, that a system has to “work harder” to bring a house back after you let it drift up while you were out. The department calls that “a common misconception.” The unit runs at the same rate either way; the only thing that changes is how long it runs.
Why the number on the dial isn’t a throttle
A Houston contractor put it better than any manual I’ve read: think of the thermostat as a target, not a throttle. It tells the system what temperature to aim for. It does not tell it to blow colder or work harder. Whether you set 72, 68 or 65, an 88-degree house cools at the same speed, because the air conditioner can only pull so much heat out of the air per hour. The only thing a lower number changes is when the system decides to stop.
In an older house like mine, with a single-speed compressor, that’s literal. The compressor is either on or it’s off. Set it to 65 and it will run straight past 76, the temperature you actually wanted, and keep going until you notice you’re cold or the bill arrives. Set it to 76 and it shuts off at 76. Same time to get there, less time running.
What actually moves the bill
The department’s numbers are about where you leave the thermostat, not how far you swing it. Turning it back 7 to 10 degrees for 8 hours a day, the hours you’re at work or asleep, saves as much as 10% a year on heating and cooling. Running at 78 instead of 72 saves 6 to 18 percent on the cooling bill, and the smaller the gap between inside and outside, the smaller the bill. One detail I hadn’t thought about: a lamp or TV next to the thermostat makes the unit run longer, because the thermostat is reading the appliance, not the room…