Unless you’re old-school enough to completely heat your house with your fireplace, you’ve almost certainly got at least one thermostat in your house controlling your HVAC system. Whether you’ve got central air conditioning, a forced-air furnace, radiators, a heat pump, or you-name-it: that simple little box on your wall is your connection to all that technology. And like a lot of simple things, it can be easy to overlook its importance. Choosing a good thermostat, using it intelligently, and thinking about how your old home interacts with it are all simple ways to make sure your old home is optimized for savings and comfort.
We just turned on our heat for the season here, so it seemed like a good time to dig into the topic.
How Does a Thermostat Work?
Silly question, right? You select the temperature you want, and the thermostat tells your heat or air conditioning to adjust to make the house the temperature you asked for.
Well, sort of. Let’s drill down on this a little bit. There are two important aspects of how a thermostat works that the sentence above totally glosses over. And misunderstanding these factors is responsible for probably the two biggest myths about thermostats.
(For simplicity’s sake, we’re going to focus on a house with a single “zone,” controlled by a single thermostat. If you’ve got a house with multiple zones controlled by multiple thermostats, the same principles apply, it’s just a little more complicated to think about.)
First of all, when you turn the dial or punch in a number on your thermostat, you are indeed setting a target for your HVAC system. But the first key thing to remember is that your heating or cooling system works simply by on-off cycling. When the thermostat is happy with the temperature, it’s going to shut your system off. When your thermostat isn’t happy with the temperature, it’s going to turn your system on. And repeat. It’s a total binary.
This brings us to Myth #1: If setting the temperature to 68 will warm up the room, setting the temperature to 78 will warm up the room faster.
When you set the temperature higher, your heating system isn’t going to get hotter. It’s going to pump out the same heat for longer before it shuts off. If your goal is to reach 68 degrees, you’ll reach 68 degrees at the same time regardless of whether you choose 68 or 78 on the thermostat. The same is true for cooling: setting a lower number isn’t going to make your air conditioning colder. It’s going to pump out the same conditioned air for longer until it shuts off.
On. Off. On. Off. On. Off. It’s all your thermostat does.
Second, it’s definitely true that the goal of a thermostat is to get your house to the temperature you ask for. However, you need to remember that a thermostat can’t technically do that! You’ve been in enough houses to know that every one of them has rooms and parts of rooms that are warmer or colder than others. This is true in the greenest of new construction and in the draftiest of old homes. So what does it mean to say that your “house” is a certain temperature?
Let’s talk about Myth #2: If the thermostat is set for 68 degrees and shuts off, it means my house has reached 68 degrees.
A thermostat can’t tell you the temperature your house is. It can only tell you the temperature the thermostat is. That is, at best, the air or the room around your thermostat. Your thermostat has no idea what the temperature in the next room is, let alone the temperature on the other side of the house.
This is why thermostat placement is important. Due to stack effect, a unit installed on the bottom floor of a house might keep sending the signal to pump out heat long after the top floor is warm enough. At the other extreme, a thermostat on the top floor of a house might not turn on often enough to warm the lower floors, as warm air sits at the top and keeps the thermostat happy. Hopefully your thermostat was installed in a location that makes sense for your home.
Combining these two simple facts about thermostats and keeping them at the front of your mind will give you a powerful tool for diagnosing problems in your home, operating your system efficiently, and keeping your comfort high and costs low:
Your HVAC system cycles on and off based on the temperature at the thermostat and nothing else.
Choosing a Thermostat
I’m going to keep this section short because I don’t think it’s worth your time to do a lot of in-depth analysis of thermostat models. As discussed above, a thermostat is at its heart a super simple on-off switch. Differences in models primarily have to do with levels of control and programmability. Here’s how I’d sum it all up:
- If you have a manual thermostat (such as one with a dial), you should replace it with a programmable model ASAP.
- If you have a programmable model, you should take some time to be thoughtful about your program. And use it.
- I do not recommend buying a smart thermostat. As I’ve said elsewhere, I want you to be thinking more about your home and how it operates, not less. A programmable model lets you take an active role in the automation process, whereas a smart thermostat is a black box that you outsource your thinking to. (Disagree? Tell me why here or in the comments.)
I have a simple programmable model from Honeywell. It’s enough for my house, which has only one zone. The model I have is discontinued, but this is the current equivalent (affiliate link).
Programming Your Thermostat
A programmable thermostat should give you the option to create a schedule of at least four events per day, and also allow you to differentiate between weekdays and weekends (5-2 day programming). Many models will also allow you to program each day separately (7-day programming). If you’re buying a new thermostat, avoid “one-week” models, which don’t even differentiate between weekdays and weekends and therefore don’t offer much real flexibility.
(As I write this, I’m aware of how much easier it is to use a programmable thermostat if you have a “normal” schedule. You know, where you work or go to school during the day, come home at night, and are home a lot more on the weekends. If that doesn’t describe you—say you work irregular shifts—I’d still recommend a programmable model. You’ll just have to think more about your program and override it more often. Unfortunately, a “smart” thermostat isn’t going to help you, because it probably isn’t going to figure out your weird schedule!)
You’re going to need to play around with your thermostat to figure out how exactly to program your particular model. (Or read the owner’s manual, if you’re no fun. Lost it? Search on manualslib.com.) I also obviously can’t tell you what hours you should use to program your thermostat. Those will depend on you and your family’s schedule.
However, what I can tell you is that you should probably have three different temperature settings: let’s call them high, low, and off.
Off is going to be your setting for the hours when you’re not usually home. And it isn’t really going to mean “off”—after all, you don’t want your pipes to freeze. Instead, “off” is going to mean the lowest temperature you can safely get away with. 55 °F seems to be the generally agreed-upon number for keeping your pipes safe; keep in mind that if the air around your thermostat is 55 °F, the air around some of your pipes in a basement or crawlspace could be a lot lower. I don’t know where the number 55 comes from, but I’ve seen it a million times and have always abided by it. (Definitely open to other opinions—or better yet, evidence—about the subject.) The point is, keeping the temperature as low as possible is going to keep your heating system off as much as possible. Remember what your thermostat does!
High is going to be your setting for your waking hours at home. Let’s define it as the lowest temperature you can get your family to live with—without constantly fiddling with it. In my house, we’ve gotten used to a setting of 65 °F by dressing like it’s winter when it’s winter, and in particular the strategic use of long underwear. Remember that this “high” setting represents keeping your heating system on the most, so try to use it as little as possible.
Low is going to be your setting for times when you’re at home, but you don’t need the heat pumping for your comfort. The obvious example of this is when you’re asleep. However, you might also want to use it if there’s a time in the day when you regularly exercise, for example. In my house, we use 60 °F. Totally warm enough when you’re under the covers, and tolerable when you get up to use the bathroom. If you can get by using your “off” setting here, more power to you!
Let’s look at an example schedule. Again, I can’t tell you what times of day to use, but let’s pretend that on a typical weekday everyone in your house wakes up at 6:30AM, heads out the door at 8:30AM, returns home at 5:30PM, and goes to bed at 10:30PM.
The simplest schedule would be to have your thermostat set to “high” from 6:30AM to 8:30AM and 5:30PM to 10:30PM, “low” between 8:30AM and 5:30PM, and “off” between 10:30PM and 6:30AM.
Realistically, however, you might want to set your heat to turn up to “high” 15 minutes before you wake up or return home so that your house has a chance to reach temperature. But you might also realize that you can tolerate chipping away at the opposite end of each period—for example, scheduling your heat to turn down to “low” or “off” 15 or even 30 minutes before you leave the house or go to bed.
That’s more or less what we do in our house. Translating my family’s schedule to the example above, we essentially have the heat on “high” from 6:15AM to 8AM and 5:15PM to 10PM, switching to “low” or “off” at the corresponding times. But experiment with your own house and your own family! They’ll love and appreciate you for it.
Finally, don’t allow your programmable thermostat to shut your brain off completely. Weekends, unfortunately, you may have the heat on “high” most of the day. But if you’re heading out of the house for more than a couple minutes? Override your program and turn down to “off”! Doing something active like house projects or exercising? Turn down to “low!” And as always, if you’re cold, add a layer before touching that thermostat.
(We used heating season as our example above, but the principles are more or less the same for cooling season if you have central air. 78 °F is setting generally recommended by utility companies and the Department of Energy, so I’d make that your “high” setting—meaning maximum cooling for when you’re home. Probably the biggest difference is that your “off” setting for when you’re away from home can probably really mean “off.” You also might not need a “low” setting. Again, experiment and focus on trying to find ways to have your system spend as much time not actively cooling as possible.)
Thinking about Your Thermostat
The main point of this article is not to remind you that using less heat and cooling is a Good Thing, although a programmable thermostat is a simple and powerful tool to help you do that.
On a deeper level, remembering to taking a moment to think about how your thermostat works can help you make smart decisions about how your house and its HVAC system are operating. It can also help keep you focused on your air sealing journey. For example, if you’ve got one freezing cold room on the second floor of your house, you can remind yourself that your first-floor thermostat has no idea that is happening. You’d clearly be better off trying to figure out where all that cold air is coming from than adjusting your temperature setting.
Or is your thermostat satisfied when it shouldn’t be? Take an example from my house this weekend. My wife came into the kitchen and said she’d been freezing upstairs. Well, I’d been cooking and using the oven and had some doors shut to contain the sound. Shutting those doors and running the oven had kept the thermostat in that part of the house happy—meaning it wasn’t sending the signal to heat the rest of the house, much of which had dropped below our 65 °F target.
Or think about the opposite case. Is something triggering your thermostat too often? In my house, like a lot of houses, my thermostat is mounted near the door to the basement (where the furnace is, and therefore where all the control wires run). Well, my basement stays pretty cold, especially now that I’ve sealed my ductwork and installed a heat pump water heater. And that cold air was being drawn up through my basement door (as a side-effect of stack effect) and triggering my thermostat to kick the heat on. It didn’t matter that the temperature in the rest of the house was basically fine—my energy usage was being dictated by what was going on in a tiny fraction of my house.
See what I mean? Do yourself a favor and spend some time thinking about where you have thermostats in your house, and how representative they are of the areas they control. Will your thermostat be satisfied before or after the rest of its “zone” during heating season? What about during cooling season? Is it near drafty windows or doors that could be triggering it unnecessarily? Is it in direct sunlight, or in a room that gets a lot of sunlight?
Fixing any issues that could be affecting your thermostat’s ability to get a representative read on your home’s temperature could have a noticeable impact on your comfort and energy usage. In some cases, you could see a big benefit to relocating your thermostat. Let us know what issues you find, or what improvements have worked for you!
Update 12/6/22: My three-year-old went over to a heating vent in the bathroom yesterday to try to warm up. I pointed out that no air was coming out. In possibly my proudest moment as a father, my kid responded “That must mean the thermostat is happy!” If my toddler can understand this stuff, so can you!