The first blockbuster smart-home device was a thermostat, of all things—the undisputed champ for Least Sexy Home Appliance. The Nest Thermostat, released in 2011, was a revelation, with striking good looks and built-in AI to program itself. But the real hook with the original Nest was its vaunted ability to knock as much as 30% off your home heating and cooling bills. If your home is located where extreme temperatures in either direction can result in a steep bill of hundreds of bucks a month, the current, $250 Nest (or any of the other picks in our guide to smart thermostats) is a purchase that could pay for itself in savings pretty quickly.
However, smart thermostats produce the best results only if your home’s climate-control system relies on a furnace and/or central air conditioning. If your home doesn’t have central air, or if you use steam or hot water radiators or radiant floors, a smart thermostat isn’t likely to save you much energy or money.
What makes a smart thermostat smart
A smart thermostat is considered smart because it can proactively manage your climate-control system in a way that traditional thermostats can’t. An old-timey dial thermostat lets you set a desired temperature and then tells your heat or AC to turn on until your home reaches that temperature from either heating or cooling. That means you typically have to turn the thermostat up and down throughout the day to manage your home’s comfort. A programmable thermostat adds the ability to create a schedule for automatic adjustments, so that you can have your set temperature shift throughout the day and night, but these adjustments are limited next to what a smart thermostat can do.
Some smart models can learn your household’s schedule and anticipate what your preferred setting will be throughout the day, and they can even adjust themselves automatically. Smart thermostats can also calculate how long it takes for your specific system to heat and cool your home and then use that for scheduling. In addition, smart thermostats can detect when members of your household are and aren’t around by employing some combination of presence sensors and geolocation, using your smartphone’s wireless signals to determine where you are located. And a smart thermostat allows you to make adjustments on the fly through the device’s app or a voice assistant like Alexa.
How smart thermostats save energy and money
Dan Holohan, founder of HeatingHelp.com and author of several books on home heating, says that the most important tool for smart thermostats is their awareness of where you and your household are. “For smart thermostats, the big breakthrough is that they know you’re not home and so can shut down when you aren’t around,” he says. Most smart thermostats contain a presence sensor, so if you don’t walk past them they may assume you aren’t home and turn off. They also may have access to little remote temperature and presence sensors placed around your home. And finally, smart thermostats connect to the internet and so can be alerted when you (actually, your smartphone) leave your home’s Wi-Fi network and go off to school or work or the park, and then again when you return home. During your absence, your system turns off instead of wasting energy. “Nothing saves energy more than something that’s not running,” Holohan says.
Aside from turning down the heat or reducing the AC when you’re not around to use it, smart thermostats save energy by changing the set temperature in small ways at times when you don’t notice it as much, so you don’t use as much energy. Climate-control pros call this process “performing a setback.”
The best models perform setbacks whenever possible to eke out a few percentage points of savings here and a few more there, and overall are able to radically transform your climate-control bill. For instance, a learning thermostat like the Nest figures out how long it takes to cool or heat your home, as well as how much the outside temperature affects that, and as a result the thermostat turns your system on at the last possible moment and turns it off at the earliest possible moment. In contrast, a standard programmable thermostat just changes the temperature based on an interval timer.
Why some homes don’t benefit
The process of making small adjustments, or setbacks, works well with central air conditioning systems, as well as with furnace-based heating systems that heat with hot air. Radiant-heating systems are another matter.
The vast majority of homes in the US—Holohan says somewhere around 90%—have furnaces that heat air up and then blow it around your house to make it feel warmer. And as Holohan points out, that’s terribly inefficient. “Air is a really lousy thing to use to transfer heat—that’s why insulation has so much air in it,” he says. Furnaces make a space feel warm very quickly but need to run often to make the home continue to feel warm. As a result, a smart thermostat can easily find ways to save energy by turning off the heat until it’s needed.
In contrast, homes that use hydronic, or water-based, radiant heating rely on a boiler to heat up water and then send it around a home variously to heavy cast-iron radiators, baseboard heaters, or radiant flooring. Radiant heat is vastly more efficient than forced-air systems. “Radiant isn’t about heating the air,” Holohan says. “It’s about controlling the heat loss of people—you know, we’re all radiators. Most of our body heat is lost through giving up radiant energy to things that are cooler than us. That’s why they put those silver Mylar blankets around marathon runners at the end of a race.” Whereas forced-air systems heat up air, radiant systems primarily heat up people, as well as the furniture and carpets and other objects in a room, via infrared heat.
The problem for smart thermostats is that radiant systems heat and cool slowly, so there’s much less of an opportunity to use setbacks. For instance, for a radiant floor, it might take a day or even several days to warm up a floor to the right temperature, and so after dropping the temperature for a few hours it would take many hours again to recover. In comparison, a forced-hot-air system can heat a room in 20 minutes. Some smart thermostats do have built-in software to theoretically anticipate the time to heat and cool, after which they adjust accordingly; however, since many radiant systems run continuously, there’s just not much opportunity to gain efficiency with such systems. Also, because radiant heat works by warming people and furniture, not the air, and thermostats measure only the air temperature, thermostats just aren’t as effective in that situation anyway. “With a radiant system, it’s really questionable whether you’re going to get any massive savings,” Holohan says.
Why you might opt for a smart thermostat anyway
Smart thermostats first and foremost are about saving energy. But depending on your home, they might not make sense—such as if you install $1,000 worth of thermostats and sensors only to save a cumulative $100 a year on your bills.
But for some people, the monetary concern may be less important than the convenience a smart thermostat brings. For some folks, the ability to use an app to keep tabs on a vacation or rental home, to monitor just a specific room in the house, or to control the temperature in advance of arriving somewhere may be worth the price of admission. We’ve also found that the apps are far easier to use to program a schedule than jabbing at the buttons on a programmable thermostat. And when your smart thermostat is properly configured, your home can be more consistently comfortable, too.
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