Posted By Profit Greenly on January 31, 2017

Monster Furnace (Image: 20th Century Fox)

Note: I originally wrote this post 5 years ago for my friend’s blog. That site is seeing a lot less use these days and loading slowly so I figured I should copy the content over here to ensure it is still accessible and open it up to new readers. I’ve made a few minor updates to bring it up to date.

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. The main difference for AC is that an AC unit actually becomes more efficient after the sun goes down and the outside air temperature drops, so there’s even more reason to avoid running it during the heat of the day)

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 resistance 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 resistance heaters is that electricity is a lot more expensive per watt than something like natural gas right now. So, while resistance heaters are technically more efficient, they can cost a lot more to operate. If the resistance heater is powered by a natural gas generator that is only 50% efficient then its net efficiency is worse than burning that gas directly in your home for heat, but if its powered by a renewable source like hydro or wind it is better.

Electric heat pumps are even more efficient than resistance 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 four watts of heat into your home from the outdoors using just a single watt of electricity. This is incredibly amazing when you think about it. A 400% efficient heat pump powed by a 50% efficent natural gas generator puts more heat in your home than a 95% efficient gas furnace was burning 2 times as much gas (see this video for thorough details)! Heat pumps are 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 units installed on homes often can’t pull in heat super fast. Heating a house from 55 °F to 70 °F with a heat pump might take over an hour instead of a few minutes. Lots of people can’t handle waiting an hour, so many heat pump installations include backup resistance heating strips to heat things up quickly when needed. If you have a heat pump with a backup resistance unit, you will be more efficient by using the resistance part less. The heat pump is two to four times more efficient than the resistance heat strips, depending on outdoor air temperature, 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 resistance 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 resistance heating unit to warm your home faster.

Overuse of backup resistance 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 resistance 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. The outdoor air temperature during the day will also usually be warmer than it is at night so running the heat pump then will be slightly more efficient. 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

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. If you insulate don’t forget to air seal as well because air moving through cracks and holes in your ceiling will transfer heat right through your insulation. Using caulk or spray foam to seal the holes in your ceiling can massively reduce this major source of heat loss.

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

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)

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 resistance 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.

If you have a super efficient heat pump like me, then you won’t save as much money by turning down your thermostat during the day, but you’ll still likely save. This is even more true for cooling in the summer time because the efficiency of an AC increases at night when the outdoor air is cooler. My HVAC costs only around $350/year to power, but it’s still a good idea to turn it down a bit when I won’t notice a difference.

Seriously though, 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.

Profit Greenly

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