In the dead of winter and peak of summer, there’s one question that’s bound to pop up at least once throughout each season: how should you set your thermostat so that both comfort and savings are maximized?
There are basically two schools of thought.
The first group says that you should set your thermostat to your target temperature and leave it there all the time — even when you’re away from home for extended periods. The reasoning is that it takes more energy to heat up a cold house or cool down a hot house than it takes to maintain a steady temperature.
The second group says that you should reduce the heating or cooling when you expect to be gone for a long time. The reasoning is that the energy you save during those off-times will offset whatever extra costs you incur by trying to heat up a cold house or cool down a hot house. Leaving it on all the time actually costs you more in the long run.
Only one of these can be true. In this article, you’ll learn which one is actually right (and why) according to the physics of heat.
The Basics of Heat Transfer
Before we can understand why one method is better than the other, we have to understand the basics of heat transfer. Whether it’s winter or summer, whether you’re trying to heat up or cool down your home, the principles are the same.
When talking about home temperatures, we need to look at two things: the current temperature and the target temperature. More specifically, we care about the difference between these two temperatures, which is called the delta temperature (ΔT).
Here’s the basic equation for heat flow (Q):
U (pronounced “U-value”) is a rating number that represents how quickly your home loses heat. Another way to think of U is as the reciprocal of your home’s insulation rating, so the better your home is insulated, the smaller the U.
A is the area of your home. Pretty simple.
So this equation tells us three basic truths about heat transfer speed:
- The smaller your home’s U, the slower heat will transfer.
- The smaller your home’s A, the slower heat will transfer.
- The higher your home’s ΔT, the faster heat will transfer.
That last point is most important for understanding thermostats! In layman’s terms: the greater the difference between your home’s current temperature and your home’s target temperature, the more quickly it will heat up when subjected to a heater (and the faster it will cool down when subjected to an air conditioner). As your home gets closer to the target temperature, the rate at which the temperature changes will slow down.
For example, let’s say it’s 50F in your home and your target temperature is 70F. According to this equation, it will take a lot less time for your home to go from 50F to 60F than it will take to go from 60F to 70F. In fact, the first half will be twice as fast!
The Science Behind Thermostat Setback
There’s one more thing we need to know, and that’s how modern heaters work.
You’ve probably heard people say that your heater needs to “work harder” when your home temperature is cold and it “eases up” as the temperature gets warmer, almost like pressing the gas pedal to go from 0 MPH to 60 MPH. You’ve probably heard the same about air conditioners as well. This is called “valve theory” and is absolutely wrong.
Heaters and air conditioners actually pump out a constant temperature no matter where your thermostat is set. Once your home reaches the target temperature, the system cycles between on and off to maintain that temperature. (Unless you have a manual heater or air conditioner, in which case you’ll have to turn it off when you feel comfortable.)
For example, whether your home is currently 40F or 50F or 60F, your heater will emit heat at 100F no matter what (arbitrary value for illustration’s sake). Thinking that your heater will pump out 120F heat when your home is 40F, then 110F at 50F, then 100F at 60F, and so on is wrong.
Now combine this with the basics of heat transfer from above.
The truth is that your home heats up much faster than you think it does. Not only that, you have to think about the difference in temperature between your heated-up home and the cold winter outside: because the difference is so great, the heat is quickly transferred out, which prompts your heater to kick back on, only to repeat the cycle.
On the flipside, turning off the heat will cause your home to quickly drop in temperature, but as the temperature falls, so will the rate of heat loss. This is why it actually takes a lot of energy to maintain an indoor temperature that’s significantly different from the outside temperature, and this is true in both the summer and winter.
Don’t just take my word for it. Here’s what ENERGY STAR said on the matter:
The evidence is quite clear that, in the winter, letting the house cool down when you are not home for several hours during the day and while you sleep at night saves the most energy.
Ultimately, you should turn down your heater or air conditioner when your home is empty for hours at a time and turn it back up when you come home. This technique, called thermostat setback, is the right answer and is the reason why smart thermostats like the Nest can save you so much money.
Which Temperatures Are Ideal for Saving Energy?
According to Energy.gov, general thermostat guidelines include:
- 68F in the winter when you’re at home and awake.
- 78F in the summer when you’re at home and awake.
- Set back the thermostat by 7–10F when you’re asleep or away. A smart programmable thermostat is extremely effective for this!
Feel free to tweak these to suit your own comfort levels, but remember that every single degree matters. Even though the difference between 68F and 69F may not seem like much, you’ll definitely notice a bump in your energy bill. If 68F is too uncomfortable, use these nifty tricks for staying warm.
Note that you should never drop your thermostat below 55F in the winter as this could cause areas of your home to become so cold that pipes freeze and burst. So if you go on vacation, for example, keep your thermostat on at that temperature or higher.
But the most effective way to slash your energy bill is to make sure your home is well-insulated and to reduce the amount of air that needs to be heated or cooled (which makes the U-value and the Area smaller in the heat flow equation from earlier in the article).
Tell us about your biggest home energy woes in the comments below!