What is a thermostat?
Updated August 2026
A thermostat is the temperature sensor and low-voltage switch that decides when your furnace, air conditioner, or heat pump runs. It compares the air at one spot in the house against your setpoint, then closes a 24-volt circuit to call for heat, cooling, or fan. Scheduling, geofencing, humidity readouts, and phone alerts all sit on top of that single switching job. Four generations still hang on walls today: mechanical dials, digital non-programmable, 5-2 or 7-day programmable, and smart Wi-Fi models.
Where it is
Almost always on an interior wall of the main living space, 52 to 60 inches above the floor, deliberately away from sunlight, supply registers, lamps, and kitchen heat, because whatever it feels becomes the setting for the whole house. Lift the faceplate off the subbase and the wiring reads plainly: R or Rh and Rc for power, W for heat, Y for cooling, G for the fan, C for common, plus O/B for the reversing valve and W2 or AUX for backup heat on a heat pump. Check for that C wire before buying a smart thermostat, since most Wi-Fi models need one or an add-a-wire kit.
What it does
A dead thermostat impersonates a dead HVAC system, and plenty of $150 service calls end with a tech installing two AA batteries. The other everyday failures are just as mundane: a corroded wire at the subbase, dust on old contacts, a mercury unit knocked out of level so it reads wrong, or placement in a sunbeam that shuts the AC off while the rest of the house bakes. Heat pumps raise the stakes, because a thermostat wired or configured wrong can fire the electric auxiliary strips while the compressor is working fine, adding $80–200 to one winter bill. Before blaming the furnace, tape a separate thermometer beside the thermostat for 20 minutes and see whether they agree within 2 degrees.
Maintenance
Change the batteries once a year whether the screen complains or not, ideally the day you set the clocks back, and keep it as a fixed annual task, because a low-battery warning nobody reads is the classic way to lose heat overnight. Once a year, pull the cover, blow the dust out, and confirm the wires are seated. Test both modes each shoulder season. The rest of the routine lives on the thermostat checklist. Units last about 10–15 years, and replacing one sooner often pays for itself in scheduling alone.
What it costs to replace
A replacement thermostat runs $20–60 for a basic digital model, $120–300 for a smart Wi-Fi thermostat, plus $100–250 if a pro does the swap.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
Why is my thermostat blank or not turning on the heat?
Work the cheap causes first. Replace the batteries, confirm the mode switch reads Heat and the setpoint sits at least 3 degrees above room temperature, then check the furnace door switch, which cuts power whenever the access panel is loose. Next look at the breaker and the small inline fuse on the furnace control board, usually a 3 or 5 amp automotive blade fuse. If a replacement fuse blows again, a wire is shorting somewhere and that is a call for a tech.
Do smart thermostats actually save money?
They save money by making setbacks automatic, not because they are clever. The U.S. Department of Energy puts the savings from setting a thermostat back 7 to 10 degrees for 8 hours a day at around 10% of heating and cooling costs. If you already do that by hand, a smart model buys convenience and remote control rather than a smaller bill. If the house has been held at one temperature around the clock, a $150–250 unit typically pays back within two seasons.
Can I install a thermostat myself?
Most people can. This is 24-volt wiring, not line voltage, and the job takes 30 to 45 minutes: kill power at the furnace breaker, photograph the existing wires and terminal letters before touching anything, label each wire with the stickers in the box, mount the new base, and match letter to letter. Two situations are worth hiring out: heat pumps with auxiliary and emergency heat staging, and any install where no C wire exists and the workaround is unclear.
Where should a thermostat never be placed?
Bad locations create complaints that look like equipment problems. Avoid exterior walls, direct sun from any window, the wall above a lamp or television, hallways with no return air, kitchens, drafty stairwells, and anywhere within about 5 feet of a supply register. Relocating a thermostat costs $150–350 and frequently solves a house that has felt unevenly heated for years, which makes it far cheaper than the equipment change people usually consider first.