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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.

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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:

  1. The smaller your home’s U, the slower heat will transfer.
  2. The smaller your home’s A, the slower heat will transfer.
  3. 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.

Image Credit: Will Markusen via Shutterstock

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 7 Nest Automation Tricks to Cut Your Heating Bill in Half 7 Nest Automation Tricks to Cut Your Heating Bill in Half If there were a Nest thermostat in every home, it would have the biggest single impact on energy consumption in history; and it could save you more money than you might imagine. Here's how. Read More .

Which Temperatures Are Ideal for Saving Energy?

According to Energy.gov, general thermostat guidelines include:

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 How to Beat the Winter Cold With These 8 Home Gadgets How to Beat the Winter Cold With These 8 Home Gadgets If you're feeling extra cold this winter - or you just want to save on heating costs - there are lots of small gadgets and devices that may come in handy. Read More .

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).

For more tips, check out these home energy efficiency mistakes Is Your Home Energy Efficient? 7 Things You've Overlooked Is Your Home Energy Efficient? 7 Things You've Overlooked The true cost of a smart home is far cheaper than you think. In fact, there are many home automation devices that everyone can afford, and many of them will feature in this article. Read More as well as these helpful energy-saving tips from Reddit Be An Energy Star: 20+ Reddit Tips That Help To Slash Your Utility Bill Be An Energy Star: 20+ Reddit Tips That Help To Slash Your Utility Bill Is your utility bill killing you? Who would have thought that Reddit is a source of wisdom for saving energy & money? Reddit's crowd wisdom can help you solve everyday problems, like gigantic utility bills. Read More .

Tell us about your biggest home energy woes in the comments below!

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  1. Eric Thomas
    July 5, 2017 at 11:29 pm

    You should add the Celcius equivalent next to the farenheit.

  2. Gene
    February 4, 2017 at 9:57 pm

    First Q = UA (delta T) is explained wrong. The area (A) is not the area of the house. It's the area of exterior surfaces or area of unconditioned surfaces to conditioned surfaces. So what this tells us is the greater the temperature difference between the outside (usually) or an attic space and your conditioned space the greater the heat transfer. Therefore the lower your summertime temperature setpoint the greater the heat transfer and the more the AC must run to offset.

    But this equation doesn't really apply as well to the AC supply air temperature. Also the emboldened statement "Heaters and air conditioners actually pump out a constant temperature no matter where your thermostat is set." is wrong. The delta T between the return air (or room temp) and the supply air temperature or vent temperature actually stays the same NOT the supply air temperature. This is true for virtually all home AC systems in USA. Variable speed fans or staged compressors can change this. So if the delta T for your AC is normally around 18 F then take the air temp near return air grille or thermostat and subtract 18 F and that should be close to the temp coming out the vents. So in this case the vent air temp with a set point of 75 F should be around 57 F. Its always good to know what this Delta is when your system is working correctly. So if you setback the stat to say 82 F the supply air temp is only 64 F.

    This is true of heaters as well. After all these a somewhat dumb devices only generally being only on /off control with nothing variable. In any case your main point that turning the thermostat setpoint up or down depending on season saves energy I agree with and feel is worthwhile esp if the house is unoccupied for several hours.

  3. Rob de koter
    January 30, 2017 at 12:05 pm

    that is the case when you have a heater but not when you have a heat-pump.
    when the heat-pump has to heat the house after you let the temperature drop 10 degrees in you absence it will switch on the auxiliary heat since the difference between target temperature and present temperature is mere then 2 degrees.
    The Auxiliary heath is a set of coils consuming between 10,000 and 20,000 watt and 3 times less efficient then your heath-pump, in other words: changing the temperature is going to cost you.
    In general, if you have an heater on oil gas or electricity turn the temperature down when leaving. When you have a heath-pump do not touch the thermostat or only mak slow changes in increments of 1 degree.