The simple answer is “Set it to what ever temperature that makes your family comfortable.”

But comfort can be a complicated (and entrenched) behavior. My father-in-law told me a story about his cousin who always set their air conditioner to 65°F in the summer, so she could wear a sweater. In the winter, she set her heat to 80°F and then go around in short sleeves.

Let’s shoot for an answer that keeps you comfortable and saves you money.

Play the (Humidity) Percentages

There’s lots of truth in the old saying, “It’s not the heat, but the humidity.” When it’s hot, it’s the relative humidity making people really feel uncomfortable, because your body’s sweat glands don’t work as efficiently as they can. Sweat works to help maintain body temperature by evaporating from your body and carrying away some of your body heat (called “latent heat“). In fact, in order to regulate body temperature, human skin sweats automatically at 98.6° Fahrenheit (37° C). To understand the effects of humidity better:

"When the air is dry, your sweat evaporates readily and you feel comfortable. An 80°F day feels like 80°F when the relative humidity is 40%. However, as humidity rises, more water vapor is in the air which makes its more and more difficult for your sweat to evaporate. So when it’s steamy hot and humid, it feels much hotter and you sweat in buckets. An 80°F day with 90% humidity has a “heat index” of 86°F."

So, the first thing you want to do is reduce the relative humidity in your house. Fortunately, air conditioners are extremely good at this, especially if you have air sealed your home and have vapor barriers in your basement or crawlspaces. EPA recommends the “ideal” humidity level of 60% during summer and 25 to 40% in the winter. Many programmable thermostats will display the relative humidity at the push of a button.

Meanwhile, personal fans and ceiling fans feel great, because they blow air across your body, and moving air is very good at evaporation. Thus, fans don’t lower the temperature of a room, but they make it feel cooler.

One common humidity problem is your air conditioner might be too big for the home. The system will run for a short time and cool the house, but will not run long enough to dehumidify. If this is the case in your home, you will need to discuss it with a contractor.

Numbers Game

How high can you set the thermostat on your central air and still be comfortable? Some people like it cool; others want it warm. Some people also get fixated on a setting, say 72°F. Yet, they find themselves being very comfortable in a room where it is 78°F without their knowing it. Like we said earlier – comfort is complicated. Also, our normal circadian rhythm fluctuates body temperatures during the day so the comfort-goal posts keep getting moved.

So, how do you find a sweet spot? Try this experiment on your family.

"When everyone is home, secretly set your thermostat to 81°F for few hours. See how your family reacts. Over the next few days for the same period of time, drop the temperature 2°F until you reach 71°F. Chances are that your family will be generally comfortable between 73°F and 79°F when they are active."

Not surprisingly, the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) finds “The preferred temperature range for occupants dressed in summer clothes (0.35 to 0.6 clo) is 73° to 79°F (22.5° to 26°C).” Choose the average of what you discover to be your family’s comfortable range and set your thermostat to that. For convenience, we’ll say it’s 78°F.

When everyone’s away for the day, there’s no real need to cool your home — but you still want to keep the humidity low so you will want to run your AC, just not as often. By raising the temperature by 10°F (86°F) when you are away, you can save money off your electric bill. Will your AC use more energy to bring the temperature back down? No. The higher interior temperature actually slows the flow of heat into your home. With a programmable thermostat, you can set your AC to begin cooling down your home 30 minutes before you arrive.

At night when you go to sleep, your core body temperature lowers and heat radiates from your extremities. A National Institute of Health study found the best sleep happens as the body reaches “thermoneutrality” when environmental temperatures are at 86°F (nude and uncovered) or 60 to 66°F (wearing pajamas and covered by one sheet).

This suggests the key to getting a good night’s sleep during the summer is to raise your thermostat setting to a somewhat warm setting, say 80°F for example. Then use the appropriate amount of bedding and pajamas for late June to feel comfortable. You can also help yourself drift off by using a ceiling fan to gently waft a breeze down onto you. To help you wake up, you can program your thermostat to return to the waking temperature a half hour before you get up.

Check Those Savings

How much can you save by making these adjustments to your thermostat? Each degree that you are able to raise the thermostat saves you 3 to 5% on your air conditioning costs. So if normally have your thermostat pegged at 74°F and you are paying $150, just raising it a mere 4 degrees to 78°F could reduce your bill by $25. By adjusting your thermostat to your family’s schedule, or using a programmable thermostat, you can save even more.

Isn’t that a comforting thought?