What Temperature Should We Set Our A/C and Heat To?

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I just read an interesting article “You Can Stop Arguing Over the Thermostat: This Is the Optimal Temperature for Your Home in Summer and Winter” in Yahoo News that basically says we should set our A/C on 70^o and heat between 68^o and 70^o in the daytime. I find it very interesting that the author asserts this as an absolute solution to the “battle of the thermostat” discussions that probably occur in nearly every home, as I know full well it’s definitely not that simple. Here’s why:

Different Strokes for Different Folks

First off, we’re all wired differently when it comes to temperature. I actually happen to resonate with 70^o for A/C, and that’s exactly what I set my central A/C to when I was living alone. However, today, that temperature would never work, as it would likely drive the rest of my family out of the house and add another $200 to my summer electric bills. Hence, we usually settle somewhere around 75^o. l just sit under a fan when I get warm. For heating, I would tend to go lower than the author’s guidance, as I’m good with 65-67^o during the day and 60-62^o at night. But, again, that wouldn’t go over well at all with my family. So, we usually settle on 70-72^o. Meanwhile, I bet other households have different stories to tell that involve completely different sets of numbers.

As the article states, outside conditions are also factors. We tend to set the A/C to a lower temperature when it’s cooler outside but raise it to 76-77^o on very hot days to achieve the same comfort level. This is simply because the unit works much harder on hot days, so it dehumidifies more. And lower humidity tends to make us feel more comfortable at higher temperatures. However, on mild days and in the off-seasons, I’ll nudge the thermostat down to ~ 72^o to get our central A/C to run enough to hold the humidity down to an acceptable level.

A similar situation exists in the winter in that we need to raise the temperature slightly on colder days (around 73^o) because 1) more heating lowers the humidity, and less humidity feels cooler as I mentioned earlier; 2) radiational heat loss from our bodies. When the outside walls and windows run colder during a cold snap outside, it literally draws more heat out of our bodies via radiation. Hence, we sometimes need to bump up the air temperature by a couple of degrees to compensate.

A lot of people may not know about the radiational heat loss (AKA radiational cooling) or might find it hard to believe. Just think about what happens when you leave your car parked in the driveway or street overnight on a clear night. Usually, come the next morning (at or shortly before dawn), the car is covered with condensation and, when you step inside, it feels noticeably cooler than outside. This is because the radiational cooling effect that outer space has on your car during the overnight hours. It is usually more pronounced at higher altitudes due to a thinner atmospheric layer.

Cost Considerations and Tips On Saving

Let’s admit it, energy prices have skyrocketed lately, and many of us are cutting back on energy use in response. Again, I don’t believe this should translate into a hard-set rule on the temperature at which we set our thermostats; however, it could/should put an upward bias on our A/C settings and a downward bias on heat settings. In general, it makes the most sense to adjust your A/C setpoint higher during the hottest points of the day and likewise with heat at night. This is when the inside-versus-outside temperature gradients are the greatest, as it takes more energy to fight these higher gradients. Also, heat pumps work more efficiently with lesser gradients. IOW, you get more BTU of cooling per fewer kilowatt-hours at night when the outside temperature is closer to that indoors. The reverse is true with electric heat pumps in that they provide less efficient heating at night when it’s much colder outside than inside. This does not apply to natural gas and oil furnaces, as their efficiency doesn’t change with the inside/outside temperature gradient. (However, they still have to generate more heat in total as the gradient increases.)

For households whose occupants are at work/school during the day, it definitely helps to set back your heating and A/C while you are gone. I found this to be particularly helpful with A/C in the summertime. I had my thermostat programmed to 78^o in the daytime (just enough A/C to keep the cats comfortable) and back to 70^ostarting around dusk. My KWH usage was ½ to 2/3 of what it would have been had I kept the house at 70^o 24/7. The energy/cost-savings of setting back your A/C during the day is so pronounced because of the double benefit of more efficient cooling at night coupled with lessening the aforementioned temperature gradient during the day. Meanwhile, I still found it advantageous to set back the heat during day. It still saved money, but the cost-savings was much less pronounced.

Disproportionately high energy prices seem to come in waves. The last big wave was in the 1970s when I was growing up. I remember the mantra in Michigan was “dial down (your heat) and slow down (your car)”. I am also fully confident that this too shall pass, especially once we further our transition from fossil fuels to clean/renewable energy. In fact, we’re seeing signs of it happening already. A friend of mine has rooftop solar and a nearly all-electric home. He and his wife both charge their EVs at home, yet their electric costs are negative (i.e., the power company pays them because their solar panels generate more electricity than they use on average).

So, hang in there, as I am confident that better times lie ahead, and we might very well reach a point in a few years that energy costs are barely a consideration in our household budgets, our thermostat settings, how much we drive our cars, etc.