Explainer: Why Leaving Your Thermostat The Same All Day Wastes Money

Monster Furnace (Image: 20th Century Fox)
Monster Furnace (Image: 20th Century Fox)

Keeping Your Thermostat The Same All Day Wastes Money

No one likes to waste money, right? Unfortunately, there are still some pernicious myths out there slowly draining our bank accounts. One of these myths is that leaving your thermostat at the same temperature all day somehow saves money. This is untrue for the vast majority of homes, yet the myth won’t die. Many sites on the internet will tell you that lowering your temperature by 10 °F during the day can save you 10% on heating costs, but they don’t explain why. This lack of explanation leads people to keep believing this myth and wasting money and energy. On my Money Saving New Year’s Resolutions post a commenter wrote the following:

I’ve heard that it’s more efficient to just leave the house at a constant temperature so the heat/AC can just work at a regular level instead of working overtime to “catch-up” in the morning or when people get home from work

This pretty much sums up the common misconception about thermostats. I’ve heard similar statements from many friends and relatives and have spent hours explaining why it’s wrong. In this article I hope to explain once and for all why leaving your thermostat the same all day wastes money. (Note: I’m only going to explain this in depth for heating, but the same basic principles apply to cooling with an air conditioner as well.)

A Furnace is Not a Car

To understand why it makes sense to turn down your heat during the day, you have to understand a bit about how your home heating system works. Most people have limited experience with furnaces and so they start thinking about car engines because they understand those more. People understand that flooring the gas pedal is not efficient for a car’s engine and they apply this logic to their furnaces.

Tachometer (Credit: Marcus Spiske)
Tachometer (Credit: Marcus Spiske Unsplash)

Car engine logic does not apply to furnaces. Car engines have to operate at many different speeds (RPMs) with different amounts of fuel flowing into them. An engine will naturally have different efficiencies across this range and you get the most out of the engine by keeping it at its most efficient combination of RPM and fuel flow by shifting gears appropriately and not slamming down on the gas pedal. Furnaces, in contrast, generally only have one speed, maybe two or three if they’re fancy (the technical term for furnace speeds is “stages”). The design for furnaces is much simpler and they’re built to be most efficient when running at full blast.

Furnaces are also much more efficient than car engines because they’re burning something to make heat. Car engines, on the other hand, are burning something to generate spinning motion in the wheels. As you might have guess it’s hard to turn a fire into spinning motion and car engines end up generating a lot of waste heat. For home furnaces this is not an issue, since generating heat is the goal. This is why most gas car engines are around 20% efficient while modern furnaces are 80-98% efficient. Older furnaces can be as low as 56% efficient, so if you own your home and have an old unit you should look into upgrading it.

How Furnaces Work

The inefficiency of modern furnaces is largely due to heat that escapes your house through the furnace exhaust. You could make a nearly 100% efficient gas furnace by simply piping the exhaust into your living space. Of course no one does this because the carbon monoxide in the exhaust would kill everyone inside. Instead, efficient gas furnaces run their exhaust through a heat exchanger that pulls heat out of the exhaust before it is vented outside.

Heat exchangers aren’t perfect. They can’t extract heat as effectively right when the furnace is starting up or shutting down. This is why changing your thermostat every minute or two or having an oversized furnace for your house wastes money. Running a furnace for a while means that a higher percentage of time is spent burning full blast when the heat exchanger is most efficient. This is why using your furnace to heat your home up from, say, 58 °F to 68 °F in a single shot is more efficient than going from 58 °F to 63 °F, pausing for a minute, then going from 63 °F to 68 °F. The furnace isn’t wasting energy working hard, it’s actually saving it.

What About Electric Heat?

So far we’ve only talked about heaters that burn fuel. This is because most U.S. homes are heated by burning fuel in a furnace (usually natural gas, but sometimes heating oil, propane, or kerosene), but a significant number use electricity to heat instead. Electric heating can be even more efficient than burning fuel. Electric resistive heating, like what you see in baseboard heaters, is around 100% efficient. This means every watt of electricity you put in produces a watt of heat. These aren’t more efficient when heating many degrees in a single run, but they aren’t less efficient either. The issue with resistive heaters is that electricity is a lot more expensive per watt than something like natural gas right now. So, while resistive heaters are technically more efficient, they can cost a lot more to operate.

Electric heat pumps are even more efficient than resistive heating. They work by stealing heat from the outdoors and putting it into your home. Because they don’t have to create all the heat that they’re putting in your house, they can actually be more than 100% efficient. For example, they can can pull two watts of heat into your home from the outdoors using just a single watt of electricity. This is pretty amazing when you think about it, and if you have a heat pump you are killing it efficiency wise. They’re even more amazing because they can cool your house as well as heat it, so you don’t need an air conditioner and a furnace, just a single heat pump.

The main problem with heat pumps is that they can’t pull in heat super fast. Heating a house from 55 °F to 70 °F with a heat pump might take an hour instead of a few minutes. Lots of people can’t handle waiting an hour, so many heat pump installations include backup resistive heating strips to heat things up quickly when needed. If you have a heat pump with a backup resistive unit, you will be more efficient by using the resistive part less. The heat pump is two to three times more efficient than the resistive heat strips so you basically want to avoid using the heat strips as much as possible.

A smart programmable thermostat, like a Nest, can help avoid using resistive heat by turning your heat pump on in advance. This way your house will be at the temperature you want by the time you want it there. Without this type of thermostat, you could still just toughen up and live through your house heating a bit slowly. But you might be weak and end up turning on the backup resistive heating unit to warm your home faster.

Overuse of backup resistive heat strips in a house that has a heat pump is basically the only case where leaving your heat at the same setting all day might be more efficient. For example, if your heat pump is able to maintain your home at 68 °F, but you would turn on the resistive heat to push it from 55 °F to 68 °F, then you might not want to turn it down to 55 °F during the day. Even this case isn’t a slam dunk, though, because your house loses soooo much more heat when it is warmer and lost heat is lost money.

How Heat Gets Out of Your House

Okay, so now you get how running most heaters for longer periods of time saves energy and money. This alone is just a small part of why turning your thermostat down for some of the day is super efficient. The heat escaping from your house is responsible for most of the money wasted by leaving a thermostat at one setting all day.

Basic physics explains how heat transfers faster when there is a greater difference in temperature between two systems. That means that when your house is 70 °F it is transferring heat to the world outside faster than when it is only 60 °F. Exactly how fast depends on how big your house is and how well insulated it is. Larger houses have more surface area to transfer out energy so they lose heat faster.  Well insulated houses have a better barrier that helps trap heat inside. Your furnace has to generate heat to replace every watt of energy that is transferred out of your house. So, the more heat you lose the more your furnace runs and the more your furnace runs the less money you have.

Your House is Like a Cup of Coffee

Coffee (Credit: Worthy of Elegance)
Coffee (Credit: Worthy of Elegance)

Don’t worry if you’re not all the way on board yet; heat transfer can be hard for people to wrap their heads around. Try thinking of your morning cup of coffee. It comes out really, really hot but quickly cools to a drinkable temperature. This quick initial cooling is because there is a big temperature difference between the coffee and the air, so the air takes heat from the coffee faster. Similarly, if you keep your house hotter then the outside air takes heat from it faster.

If a cup of coffee has been sitting around for a while it will still be a little warm and it will take a really long time for it to cool down to the actual temperature of the room. This is because the air takes heat from the coffee more slowly as the coffee gets closer in temperature to the air. Similarly keeping your house cooler in winter means that the outside air will take heat from it more slowly.

How quickly the coffee cools also depends on how well insulated a cup it is in. If you put coffee in a thin paper cup it will cool more quickly than if you put it in a styrofoam cup because styrofoam is a better insulator. Scientists can calculate just how well a material insulates and they call this the R-value of that material. When you buy insulation for your house the R-value will be prominently advertised. Adding insulation is a great home improvement project that can save you hundreds of dollars a year. I added R-60 insulation to the attic of my old house in Ithaca over a weekend for less than $400. It was a couple days of hard work, but it saved me tons of money over time and helped my house sell when it was time for me to move. This is like putting a nice thick lid over the coffee cup that is your house; it’ll keep it warmer far longer.

Coffee Schmoffee, I Demand Math!!!

Okay, I can hear some of you screaming that a simple coffee analogy isn’t good enough! Yes, you won’t be satisfied until you see some real equations (if that isn’t you, feel free to skip to the next section). Let’s do some simple math to see how this works. Imagine you have a 1,000 square foot house that has 10 foot ceilings. It’s basically a 50 foot by 20 foot by 10 foot box. The walls have an R-value of 10, the floor has an R-value of 20 and the ceiling has an R-value of 25. Let’s imagine that the outside air temperature is 40 °F and the ground temperature is 50 °F. How much more energy does your house lose every hour when your thermostat is set at 70 °F instead of 60 °F?

The Heat Transfer Equation

BTUs of Heat Transferred per Hour = Surface Area in Square Feet × Temperature Difference in Farenheit / R-value

Scenario 1: Thermostat set to 70 °F

House Side Length (in feet) Width (in feet) Area (in square feet) R-Value Inside Temp (in °F) Outside Temp (in °F) Temperature Difference (in °F) Heat Transfer Per Hour (in BTUs)
Ceiling 20 50 1,000 25 70 40 30 1,200
Floor 20 50 1,000 20 70 50 20 1,000
Front Wall 10 50 500 10 70 40 30 1,500
Back Wall 10 50 500 10 70 40 30 1,500
Left Wall 10 20 200 10 70 40 30 600
Right Wall 10 20 200 10 70 40 30 600
BTUs lost per hour 6,400

Scenario 2: Thermostat set to 60 °F

House Side Length (in feet) Width (in feet) Area (in square feet) R-Value Inside Temp (in °F) Outside Temp (in °F) Temperature Difference (in °F) Heat Transfer Per Hour (in BTUs)
Ceiling 20 50 1,000 25 60 40 20 800
Floor 20 50 1,000 20 60 50 10 500
Front Wall 10 50 500 10 60 40 20 1,000
Back Wall 10 50 500 10 60 40 20 1,000
Left Wall 10 20 200 10 60 40 20 400
Right Wall 10 20 200 10 60 40 20 400
BTUs lost per hour 4,100

As you can see, our fantasy house loses an extra 6,400 – 4,100 = 2,300 BTUs of heat every hour when its thermostat is set to 70 °F instead of 60 °F. If you turn your thermostat down to 60 °F when you leave to go do stuff during the day and turn it back up to 70 °F when you get back eight hours later, you save 8 × 2,300 = 18,400 BTUs per day. Setting the thermostat back again during the eight hours you’re asleep doubles this to 36,400 BTUs per day. If you did this every day for a whole month you would save over 1 million BTUs!!!

Real houses are certainly more complicated than our fantasy house. For one, they’re likely much larger with the average American house being more than double the size of our fantasy home, and thus losing a lot more heat. They are also probably not perfect cubes, but instead have additions that jut off in different directions, increasing the surface area they lose heat from. The heat inside them also isn’t evenly distributed. Since hot air rises, the air touching the ceiling will be a few degrees warmer than air on the floor. This is why the most important place to insulate your house is your attic (this is true for AC as well, but for a different reason).

It’s not all bad news, though. Many Americans live in townhouses and apartments that share walls with other living spaces. Because the temperature on the other side of a shared wall is close to that of your own home, you likely don’t lose much heat through it. If you keep your heat significantly colder than your neighbors’, your home will actually gain heat through shared walls based on the same principles of heat transfer explained above.

Understanding the heat transfer equation also helps you understand how important each degree you drop your home temperature is. In the above example, dropping it by 10 °F while the house is unoccupied saved 2,300 BTUs per hour. Dropping it by 20 °F (all the way down to 50 °F) would have saved twice as much (4,600 BTUs per hour). Dropping a thermostat down to 50°F may risk pipe freezing if it’s very cold outside and a house has poor insulation. It should be okay in most houses though and 55 °F should be fine for pretty much everyone. Here’s an online heat transfer calculator if you want to explore this more for your own house.

How Does This Hit My Bank Account?

Home Alone House (Credit: 20th Century Fox)
Home Alone House (Credit: 20th Century Fox)

Now you understand that leaving your thermostat set high all the time causes a bunch of BTUs of heat to flow from your house to the outside world, but maybe you’re still wondering how exactly this affects your bank account. Well, your furnace has to burn extra fuel to replace every BTU of energy that flows out of your house, so you end up paying for those lost BTUs. If you have a furnace that costs a lot to operate, like resistive baseboards or an old gas furnace, then this can cost a lot. Having a bigger house and worse insulation make wasting heat cost even more. Setting your thermostat from 70 °F down to 55 °F during the day when you’re at work and down to 60 °F while you’re asleep will save a ton of dough. For example, if you lived in that huge house from Home Alone with the rickety old furnace, you could probably save well over $200 each month and over $1,000 a year! Even a small apartment can easily save $20/month by simply turning the thermostat down when no one is home or awake.

Seriously, why are you heating your house when you’re not able to appreciate it anyways? Your dishes will not shatter if they get below 70 °F. Your pets evolved to live through the rigors of winter, and unless they’re cold blooded or some tropical bird they will be just fine at 55 °F. When you’re asleep under your blankets you won’t notice that the temperature has dropped a bit. If you have a programmable thermostat, now is the time to figure it out (if you need help, please drop a note in the comments). If you don’t have a programmable thermostat, then just make it a point to remember to turn your thermostat down every time you leave the house. Tell your friends! Tell your family! If all of us start turning down our thermostats when we’re not able to enjoy the heat, we’ll save a ton of money and a ton of energy too.

Matt Herndon
Environmental Blogger at Rampant Discourse
Earnest pragmatist. Non-theist ascetic. Data aficionado. Amicable skeptic. Matt is a new father who's spent too much time debating whether the plastic box his spinach came in is the perfect first birthday present for his baby, or just a good one.

This article has 9 Comments

  1. I know you mentioned the Nest thermostat as being good in terms of electrical heat pumps, but is it also good for gas furnaces? We currently have (and use) a programmable thermostat. Would we gain anything by switching to a Nest?

    1. The Nest’s big feature is automatically programming itself, but if you’ve already programmed your thermostat that doesn’t really matter. It still has a few features that can help you if you have a gas furnace. I think the best of these is that it can automatically detect when you leave the house (either by motion sensor or by your phone’s gps) and then turn down the heat. If your family always remembers to turn down the heat when they leave then this won’t be needed, but I’m pretty sure everyone forgets every once in a while so this could save some money. The Nest also has a phone app so you can remotely change your thermostat if it’s auto detection isn’t working well or if you turned it off because it was creeping you out. On top of this Nest can also remind you when to change you furnace filters (though a calendar could do the same) and claims to be able to detect when your furnace is having issues and notify you of that. I haven’t seen that last feature in action, but if it works it could save a lot of money because a slightly malfunctioning furnace can just bleed money away for months before you notice it. Check out this link to read more about Nest’s features https://nest.com/thermostat/install-and-explore/ . Also note that there are other smart thermostats out there, but Nest is still probably the best http://thewirecutter.com/reviews/the-best-thermostat/

  2. Ok now that summer is here, we want a summer version!

    Here are some things you could help us on: “the same basic principles apply to cooling with an air conditioner as well” except when those things aren’t true:

    1. Please explain thermal mass and how that will affect our decisions. If I have the temperature at 85 during the day when I’m away, could it take more energy to get the house down to 77 at one of the hottest parts of the day, when I get home at 5:30?

    2. Pretend we don’t live in Arizona. Pretend we live in Virginia, where a 50% internal humidity is apparently bad for mold and dust-mite propagation. I’m not sure how to get my house down to 50% humidity without running the A/C even when I’m not in the house or running the 400W dehumidifier practically non-stop.

    Thanks! Your post (this one, the post you made on facebook, and the one about biking) has definitely affected my lifestyle.

  3. Hey Adrian, great to hear that the posts have been helpful. I will have to write a summer version of this post, but right now I’m working on one about LED bulbs (spoiler: they heat up your house a lot less so it makes a ton of sense to install them now in Spring). Since the summer version might take a while I’ll give you a little info now.

    Thermal mass does mean that your home will take longer to get cool, but you won’t save energy cooling by leaving your AC on during the day. The heat transfer equation still governs how quickly heat is conducting into your home, thermal mass just allows your home to store more of this heat in total. What this means in practice is that in the morning the heavy cement walls of my home absorb a bunch of heat before the air inside gets it keeping it cooler for longer. Then at night the cement walls radiate this heat back inside for a while meaning my home cools slower. If this causes your home to be too warm when you get home from work you should simply move the time you program you AC to come on a little earlier. This will give your house the least aggregate temperature differential from the outside air over the course of the day, and that will lead to the least waste.

    In my case I actually stay home every day, but that lets me use my home’s thermal mass to my advantage. Every night I open my windows as soon as the outside air gets cooler than my house. I actually run a fan facing out one of them to help push all the hot air out of my house for a few hours most nights. This cools my house down all night long for almost no electricity/money. In the morning once it gets warmer outside the house I shut all the windows and rely on my homes insulation and thermal mass. Last night the low here was 72 degrees and the high today was 99. My house was around 75 when I shut the windows in the morning and maxed out at 83 when I opened them tonight. If it had less thermal mass it would have gone much higher and I would have had to turn on the AC. Once the night time temperatures get above 80 this won’t really help any more and my homes thermal mass will basically do nothing to affect my cooling, but as long as it gets cool at night the mass can allow me to store that coolness daytime enjoyment. VA has less temperature difference between day and night, but this tactic will still work there.

    If you like the idea of harvesting cold like this, but don’t have a lot of thermal mass in your home there are some solutions. One is to get a fill your home with big heavy objects like cement tables and big decorative boulders. Another is to add phase change materials to your home. These are things like waxes that melt/freeze at 71 degrees F. The phase change allows them to absorb far more energy per pound that regular materials so you can get a lot more thermal mass for the same weight. Here’s a fun table that has this https://www.wired.com/2015/05/table-sucks-heat-lower-ac-bills/ but it’s probably too small to make a big difference in a house. In Australia they’ve gone big and put PCMs in drywall to give lightweight homes huge apparent thermal mass http://www.knaufplasterboard.com.au/comfortboard .

    Okay enough thermal mass talk. The next question you had was on humidity. I haven’t done a ton of research on mold growth so I can’t give you as much info on this. I can say two things though. One, I lived in Ithaca NY for years and only used my window AC units a few days each summer and didn’t have mold problems. Running the fans with open windows at night helped cycle a bunch of fresh air into my house every day though so maybe that helped? Based on that experience I think you’re fine without AC in VA through May (because VA in May is no more humid than Ithaca in August), though it’s also possible that your house traps humidity more than mine did. After May you will probably be running AC every day because the night time temps are just too high. I’d say it’s worth experimenting to find the highest temperature you can keep your home at during the day that keeps its humidity down and to see how the energy use of that compares to the dehumidifier. My theory is that the AC will use more energy, but real testing will show for sure. The final thing to note is that showering and cooking contribute a ton of humidity to a house. Running the bathroom exhaust fan while showering and for another ten minutes after (with the bathroom door shut) should exhaust the majority of this outside. For cooking you can use the range hood fan, assuming it’s properly vented. If you want to be really hard core you could also try a solar oven type cooker outside. I’m actually looking for a free old projection to build this crazy solar cooker with https://www.youtube.com/watch?v=XFw7U7V1Hok

    Hope this helps!

  4. > testing will show for sure

    Love your post! Soaking it all in, but I need help with this one. Central HVAC can’t easily be “tested” for their energy consumption since they’re hard-wired. My utility company gives me hourly (!) updates on my overall usage, but it can be hard discerning one thing from the overall usage of a large home. Thoughts? Thanks!

    1. Hourly usage stats make it much easier to figure this out. Basically you just need to find back to back days that are predicted to have very similar weather and ensure you’re other electronics will have very similar usage across those days. Looking at NOVA’s weather I think next Tuesday and Wednesday could work because they’re only a couple degrees apart. The high isn’t above 70 though so you may need to wait for warmer weather though, because who turns on an AC at 65 degrees? Whenever you do find your two comparison days just make sure you try and keep the other electric use in the house the same across them. I’d do this by avoiding running the dishwasher and clothes washer, and making sure my lighting, TV and computer use was consistent between the days. If you’re careful about making all of those things the same and the only difference is AC vs dehumidifier then you should be able to see the energy use difference between those two in your hourly graph. You could use this coming Tuesday/Wednesday as a test run to see if you can get the same energy consumption both days when not using the AC or dehumidifier.

      If you really want to get detailed data you could buy a home energy monitor that hooks onto the wires going into your breaker panel. This can provide usage graphs of the energy being consumed through specific breakers and can go down to Watts per minute and sometimes even per second.. This Eyedro (https://www.amazon.com/Eyedro-EHWEM1-Wireless-Electricity-Connection/dp/B00EP774LM) wired system seems like the cheapest option for this at $129. If you’re into open source the emonPi could be good too (https://shop.openenergymonitor.com/emonPi-3). If you want to be super fancy you could drop $299 on Sense https://sense.com/buy.html . I’ve always thought that these were overkill, but the energy dork in me has wanted to get one. It might make sense to buy one with a few friends and share it around, since once you’ve monitored your energy usage for a few months and changed what you can the data stops being as useful.

  5. I am using Nest Learning . The potential to save energy and costs with this thermostat is very much higher than many of its competitors due to some of the features and the information that you are supplied with.

    The ‘Auto Away’ function, which detects when there has been no movement in the area for 2 hours and switches the heating of cooling down, is a real energy saving benefit if you have a changeable schedule.

    The small green leaf at the bottom of the display lights up as you adjust the temperature, the green leaf shows when you are at the lower energy use settings.

    The 10 report can help you see where your energy usage has been high and could be reduced in the future. This does rely on positive action by the user to benefit from this, though.

    In some areas energy suppliers offer rebates or discounts for having an approved programmable thermostat, like the Nest, installed.

    1. Glad to hear you’re having success with the Nest Ryan. I certainly like it’s features, but if you’re good about remembering to turn the thermostat down when you leave the house you can get similar performance from a regular thermostat. Of course in reality people forget stuff like that all the time (or just don’t care enough to do it) so the Nest can be great for them.

Continue the discourse