What's the Best Temperature for the Air Conditioner When No One Is Home?

POC woman's hand reaches for thermostat in central air conditioning home

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Q: We’ve been turning off the air conditioning each morning when my husband and I leave for work, and turning it back on again when we get home at 5:30 p.m. The problem is that when we get home, it's sometimes unbearably hot in the house — topping 85 degrees on the worst days. Not to mention that I feel like my air conditioning is working in overdrive and possibly costing me more money to operate. I’m wondering, in these lazy, hazy days of late summer, is it better to turn off my air conditioning when I leave and turn it back on when I get home, forcing the AC to work harder to cool my house from, say, 82 degrees to 75 degrees, or is better to leave it at a constant temperature all day so that it doesn’t have to work as hard?

A: Good question.

Contrary to popular belief, air conditioners do not “work harder” when they're cooling your house down after a hot day. They are actually working more efficiently at full blast than at a lesser power running constantly throughout the day.

In some scenarios, it's a good idea to turn off your central air conditioning when you leave your house, or if it’s going to get really hot on a particular day, set the thermostat several degrees higher than you would if you were at home. You will save energy by heating (or cooling) your house only as much as is necessary during the time your family is at home, not on a constant basis. As far as single-room air conditioners go (such as window units), the same principle applies. But unlike your central air conditioning, you usually can’t set those to go on before you get home, and will have to wait a bit once you turn it back on to actually feel cooler.

Did you know that for every degree above 72 that you raise your thermostat, you can save 3 to 5 percent on your energy bill? In our house, we usually have the thermostat set to 77 or 78 at night. My husband likes to call our bedroom the desert, but I prefer to think of it as a tropical island getaway. Either way, we all (kids included) sleep in tank tops and shorts, some with a light blanket and some without, and we all manage to sleep pretty well.

My sister, on the other hand, keeps her house at a positively frosty 71 degrees, forcing our family to huddle under our blankets when we go for a summertime visit to her house. I’ve compared our energy bills, though, and keeping your house warmer definitely chisels off a few bucks.

There’s been a lot of buzz recently about whole house fans. A whole house fan is usually installed in the attic and pulls fresh air into the house from open windows and exhausts it out the attic. Often, in many summer climates, a whole house fan together with ceiling fans and open windows is enough to keep you comfortable. These fans should be installed by a professional though — and you always need to make sure you have enough ventilation in the attic once you have it installed. Otherwise, the fan could create a backdraft in your furnace, water heater or gas-fired dryer, pulling dangerous things like carbon monoxide into your home.

And don't forget: There are plenty of ways to save on your energy bill this summer and year-round.