Smart Thermostats: What They Do and Whether One Fits Your System

What a smart thermostat is, and what it adds

ENERGY STAR lists common features: control from your phone, geofencing, learning your preferences, and software updates. Geofencing means the thermostat uses your phone, with your permission, to tell whether you are home or away. The Department of Energy says smart thermostats are generally easier to use and have more reporting and interactive features than programmable ones. For what any thermostat does underneath, see how a thermostat works.

What the ENERGY STAR label checks, and what it leaves out

ENERGY STAR requires a certified smart thermostat to work as a basic thermostat when it cannot reach the service provider. It also requires a schedule, feedback on the energy effect of your settings, and information such as monthly run time. The accuracy test uses a lab chamber set to 70 degrees Fahrenheit, and the pass mark is a deviation of no more than 2 degrees. Our reading: a 2 degree pass mark means a certified thermostat can still read a degree or two off. A thermostat that reads far off is covered in signs a thermostat is going bad.

To earn the label, the maker must also show energy savings, using a year of data from hundreds of customers' homes. The criteria set separate minimum run-time reductions for heating and for cooling. Our reading: those pass marks apply to a national sample, not to one house, so this page does not repeat them. ENERGY STAR lists the current specification as effective January 1, 2017. None of the four ENERGY STAR documents read for this page mentions humidity, including the criteria and the specification. Our reading: the label tells you about a model's basics and a national savings test, not whether it fits your equipment or handles humidity.

Systems a standard smart thermostat may not suit

ENERGY STAR says most smart thermostats sold in the United States are not compatible with baseboard heating, biofuels or in-wall heaters. Its specification leaves out line-voltage thermostats, which are powered by or switch more than 30 volts of alternating current. Most smart thermostats use thin, low-voltage wires. Our reading: line-voltage wiring is a different matter, so checking for it is work for a licensed professional.

For the very highest-efficiency equipment, ENERGY STAR says you may want a controller from the same company. A 2021 market scan for the Northwest Energy Efficiency Alliance, a regional nonprofit, found that only a thermostat from the heat pump's own maker would control variable-capacity heat pumps as intended. It said the data wires that tell the heat pump how hard to run and how fast to move air are absent in conventional and third-party smart thermostats. ENERGY STAR's 2017 specification lists thermostats for variable-capacity equipment as a topic for later revisions. Our reading: if your equipment is variable-speed, ask its maker or your contractor which thermostat it accepts before you shop for a general smart model. The equipment itself is covered in variable-speed air handler.

Power is the other fit question. ENERGY STAR says most, but not all, smart thermostats get their power through a C-wire, also called the common wire. What the C-wire does, and the options when a house has none, are covered in the C-wire on a thermostat.

If you have a heat pump

A heat pump is an air conditioner that can also run in reverse to heat the house, and it may have electric-resistance coils as backup heat. The Department of Energy says a heat pump owner should not turn the thermostat down for 8 hours a day without a smart or programmable thermostat designed for heat pumps. It also says not to set back a heat pump manually if that makes backup electric-resistance heating come on, because that heating is more expensive.

ENERGY STAR makes certified thermostats report how much electric-resistance heat a heat pump uses, but only as a reporting requirement. Its 2017 specification says EPA may later add requirements to minimize that heat if the data show wide variance among certified models. Our reading: the label alone does not tell you how a model limits backup heat, so ask how it handles it. What backup heat is, and when it should and should not run, is covered in auxiliary heat and emergency heat.

The thermostat tells a heat pump's reversing valve which way to run. Matching a thermostat's heat pump settings to the equipment is covered in thermostat installation and problems.

Humidity: what a smart thermostat can and cannot do in Florida

EPA advises keeping indoor humidity below 60 percent, ideally between 30 and 50 percent relative humidity. Relative humidity is how much moisture is in the air compared with the most the air could hold at that temperature. So a smart thermostat helps with humidity only if it can sense humidity and act on it.

Help pages from three thermostat makers describe overcooling: running the cooling system until the air is up to 3 degrees Fahrenheit below the set point. One maker says its feature can turn the air conditioner on when humidity passes 75 percent and cooling is off. That maker also says it will not cool the home below 75 degrees, even if humidity stays high. A different maker says its overcooling cannot start a cooling cycle and only extends one that is already running. Our reading: the makers' humidity features are not the same, so a claim of humidity control can mean different things from one model to the next. That second maker also says real humidity control needs a wired connection that a contractor should install.

UF/IFAS Extension notes that when a system does not dehumidify enough, the typical owner response is to lower the thermostat setting. Our reading: overcooling automates that response for a few degrees. It is a workaround, and it does not change the equipment's size or the ducts. The causes of a humid house with the air conditioner running are covered in why your house feels humid with the AC running. A separate dehumidifier is covered in whole-home dehumidifiers in Florida. The Department of Energy advises setting the thermostat fan switch to Auto unless an indoor air quality professional has set the system to run continuously for health reasons.

Away and vacation settings in a Florida summer

ENERGY STAR describes vacation mode as a way to manage the temperature during a long absence, avoiding frozen pipes in winter and extreme heat in summer. The Department of Energy advises raising the summer temperature when you are sleeping or away. A 2005 study by the Florida Solar Energy Center, done for a utility, tested humidity control in three vacant homes in Brevard County, on Florida's east coast. Setting the air conditioner to 85 degrees did not work in most homes, because its run time was too short. In some test periods it did not run at all at 85 or at 83 degrees. Running it at 74 degrees for a few early-morning hours appeared to work well in a majority of the homes. Our reading: that was three vacant homes in 2005, so it is not a rule for your house. It does show why a large away setting needs a second look in a Florida summer.

One maker's page says that after 3 days on its away temperatures, its humidity feature will turn the air conditioner on if humidity rises above 65 percent. Our reading: ask any model you consider what its away or vacation mode does with the cooling setting, and whether it watches humidity while you are gone.

Utility programs in the Tampa Bay area

ENERGY STAR says its certified smart thermostats are designed to be compatible with the programs that some local utilities offer. Its specification requires that customers can override their thermostat's response to a utility signal. Tampa Electric's Prime Time Plus program, in a release dated July 21, 2026, says eligible participants receive monthly bill credits and a professionally installed ENERGY STAR smart thermostat. The release says the program temporarily pauses power only to the equipment a customer chose to enroll, during rare periods of exceptionally high demand. It lists centrally ducted electric heating and cooling systems among the eligible equipment. Our reading: if Tampa Electric is your utility, read the program's current terms before you buy a thermostat, because the program supplies its own. If another utility serves you, ask whether it runs a similar program.

What to check before you buy a smart thermostat

These checks come from the sources above and apply to any model you consider.

1. Does the equipment accept a general smart thermostat? Ask its maker or your contractor, especially if it is variable-speed, multi-stage or zoned.
2. Is there a C-wire or another power plan? See the C-wire on a thermostat.
3. If you have a heat pump, does the model list heat pump support, including backup heat? Ask what it does when backup heat could come on.
4. What does its humidity feature do, and does it need extra wiring? Ask whether it overcools, can start the air conditioner, or needs a contractor.
5. What do its away and vacation settings do to the cooling setting? Ask whether it watches humidity while you are gone.
6. Is it ENERGY STAR certified, and does your utility run a program that supplies a model? ENERGY STAR's Product Finder lists certified models.
7. Who will install it? ENERGY STAR says that depending on your comfort level you may install it yourself or prefer a professional's help.

A licensed contractor can confirm the equipment questions on site, and you can answer the rest from the maker's documents and your utility.