Quick answer
A thermostat is the wall control that senses room temperature and switches your heating and cooling equipment on and off to hold the temperature you set.
Most home thermostats send low-voltage (24-volt) signals over thin wires to the furnace, air handler or heat pump. Programmable and smart models add schedules, remote control and energy reports.
Quick facts
The key facts about Thermostat, with sources:
- What it controls
- Heat, cool, fan, and on heat pumps the reversing valve 3
- Typical signal
- 24-volt control wiring; 18- to 22-gauge wire in one maker's guide 3
- Mounting height
- About 5 feet (1.5 m) above the floor 3
- DOE setback figure
- Up to about 10% a year by setting back 7–10°F for 8 hours a day 1
- ENERGY STAR smart thermostat spec
- Certified on field data; effective date January 1, 2017 2
- Heat pump caveat
- Use a model designed for heat pumps to avoid backup heat 1
Key takeaways
- A thermostat is a switch with a thermometer: it decides when your system runs.
- DOE says setting back 7–10°F for 8 hours a day can save up to about 10% a year 1.
- Placement matters: an interior wall, away from sun, drafts and supply vents 1.
- Heat pumps need a thermostat built for them, or setbacks can trigger costly backup heat 1.
- ENERGY STAR certifies smart thermostats on measured field savings 2.
What a thermostat does
A thermostat does two things: it measures the room, and it decides. When the room drifts from your setting by a small amount, it closes a circuit that starts the system. When the room reaches the setting, it opens the circuit and the system stops.
The wires behind it are labeled by letter. R is power, C is the common return, W calls for heat, Y calls for cooling, G runs the fan, and O/B switches a heat pump's reversing valve 3. A smart thermostat usually needs the C wire for steady power.
The thermostat is also where most energy decisions get made. The equipment can only be as efficient as the schedule and settings you give it.
From room to equipment
- Room temperature → sensor in the thermostat
- Below heat setting → W signal → furnace or heat pump heats
- Above cool setting → Y and G signals → outdoor unit and blower cool
- Setting reached → signals drop → equipment stops
How cycle settings work
- Most thermostats wait for a small swing past the setpoint before starting, so the system does not flick on and off.
- Many let the installer set a cycle rate in cycles per hour (CPH).
- Honeywell's FocusPRO guide recommends 3 CPH for most compressors 3.
- It also builds in compressor protection, a minimum off time between cycles 3.
- Smart models may "learn" how long your home takes to warm up and start early.
Types of thermostats
| Type | What it does | Best for |
|---|---|---|
| Manual / non-programmable | Holds one setting until you change it | Homes where someone adjusts it daily |
| Programmable | Follows a daily or weekly schedule | Regular routines |
| Smart / Wi-Fi | Schedules, remote control, learning, geofencing and reports 2 | Irregular schedules, remote homes |
| Heat pump thermostat | Controls the reversing valve and limits backup heat | Heat pump homes |
| Communicating thermostat | Talks digitally to matched variable-speed equipment | Brand-matched high-end systems |
| Line-voltage thermostat | Switches 120/240-volt baseboard heaters directly | Electric baseboard (not 24-volt systems) |
Example: what a setback schedule looks like
DOE's figure is that turning the thermostat back 7–10°F from its normal setting for 8 hours a day can save as much as 10% a year on heating and cooling 1. Here is a heating schedule that delivers those hours. Settings are illustrative.
| Period | Hours | Setting | Setback hours |
|---|---|---|---|
| Wake, 6:00–8:00 | 2 | 68°F | 0 |
| Away, 8:00–17:00 | 9 | 60°F (8°F back) | 9 |
| Evening, 17:00–22:00 | 5 | 68°F | 0 |
| Sleep, 22:00–6:00 | 8 | 62°F (6°F back) | 8 |
That gives 17 hours of setback, more than DOE's 8-hour example. DOE adds that savings are larger in milder climates 1. Your result depends on your home and fuel, so run your own numbers in the thermostat savings calculator. On a heat pump, use smaller setbacks or a heat-pump-aware thermostat.
Where you will see the term
- On installation quotes, as the thermostat model included with a new system.
- On utility rebate lists, which often require an ENERGY STAR certified smart thermostat 2.
- In demand response programs, where the utility may adjust your setpoint on peak days.
- In the installer setup menu: system type, cycle rate and changeover settings 3.
Smart thermostats: pros and cons
Pros
Cons
- Often needs a C wire; some homes need wiring added.
- Savings depend on how you used your old thermostat.
- Wi-Fi and app dependence; keep the router close 2.
- Some features need accounts and data sharing.
What a thermostat cannot fix
A thermostat cannot make an undersized system keep up or a leaky duct system deliver air. DOE's review notes that thermostat and fan settings "greatly impact overall system performance," but alongside duct condition and static pressure 4.
It also reads only one spot. Rooms far from it, upstairs rooms, and rooms with big windows may run warmer or cooler. Zoning or duct balancing fixes that, not a new thermostat. See zoning systems.
Sources: [4]
Cost drivers
Thermostat prices vary widely by type, so HyreHVAC does not list them. Install cost depends on whether a C wire exists, the number of stages, heat pump or dual-fuel setup, communicating equipment, and whether the old wiring is damaged. Many utilities offer rebates on certified smart models; check the rebate finder.
Thermostats rarely fail, but batteries and old mercury-switch models wear out. If the screen is blank, try fresh batteries before calling.
How a thermostat is replaced
Many homeowners swap thermostats, but turn power off at the furnace or air handler first. Line-voltage thermostats and any wiring changes at the equipment are electrical work for a pro.
- Confirm the system type: gas, heat pump, dual fuel, number of stages.
- Turn off power to the HVAC equipment.
- Photograph the old wiring and label each wire by terminal letter.
- Mount the new base, about 5 feet up on an interior wall 3 1.
- Connect wires to matching terminals and set the installer menu for your system type 3.
- Restore power and test heat, cool and fan.
Keeping it accurate
- Replace batteries once a year, or when a low-battery icon shows.
- Keep lamps, TVs and other heat sources away from it.
- Do not put furniture in front of or below it, since that blocks natural air movement 1.
- Update the schedule when routines change.
- Check the reading against a separate thermometer now and then.
Warning signs and when to call a pro
- The system runs even when the thermostat is off.
- It calls for heat but nothing starts after a minute or two.
- Frequent short cycles. See short cycling.
- Heat pump runs on "aux" or "emergency heat" most of the winter.
- A blank screen after new batteries.
Standards and rules
ENERGY STAR. ENERGY STAR labels smart thermostats that are independently certified, based on field data, to deliver energy savings. The current specification lists an effective date of January 1, 2017 2.
Mercury. Older thermostats with a glass bulb contain mercury. EPA says old mercury thermostats should go to a state or local household hazardous waste collection center for recycling, not the trash 5.
Energy codes. Many state energy codes require programmable or setback-capable controls on new systems. Adoption varies, so check locally.
Thermostat vs related controls
| Control | What it does |
|---|---|
| Thermostat | Controls heating and cooling by temperature |
| Humidistat | Controls a humidifier or dehumidifier by humidity |
| Zone control panel | Opens and closes duct dampers for zones |
| Building automation system | Controls many systems in larger buildings |
Common misconceptions
- Myth The furnace works harder to recover, so setbacks save nothing.
- Reality DOE calls this a misconception: a cooler house loses heat more slowly, so it uses less energy overall 1.
- Myth Cranking the setting warms the house faster.
- Reality Most systems deliver heat at one rate. A higher setting just overshoots.
- Myth Any smart thermostat suits a heat pump.
- Reality Pick one designed for heat pumps to limit backup electric heat 1.
- Myth A smart thermostat saves energy by itself.
- Reality Savings come from the schedule and settings it runs.
Climate changes the savings
DOE says setback savings are a larger share of the bill in milder climates 1. Cold-climate heat pump homes should use smaller setbacks, because recovery on very cold mornings can bring on backup heat. Look up your heating and cooling degree days to see how much your climate leans each way.
When it matters to you
It matters when your schedule is regular enough to set back, when you have a heat pump, or when your utility offers a rebate or a demand response credit.
Next step: estimate savings with the energy savings calculator, and check whether your thermostat supports your equipment before buying.
Questions about Thermostat
What temperature should I set my thermostat to save money?
DOE says you can save as much as 10% a year by setting the thermostat back 7–10°F from its normal setting for 8 hours a day. Pick a comfortable setting when you are home and a setback when you are asleep or away. Savings are larger in milder climates.
Are smart thermostats worth it?
They can be if your schedule varies or you forget to adjust a programmable model. ENERGY STAR certifies smart thermostats based on field data showing savings. If you already set back faithfully, the extra savings may be small. Utility rebates can lower the cost.
Do I need a C wire for a smart thermostat?
Most smart thermostats need steady 24-volt power, which the C (common) wire provides. Some use adapters or power-stealing designs. If you have no C wire, an HVAC tech or electrician can add one or install an adapter at the equipment.
Where should a thermostat be placed?
On an interior wall about 5 feet above the floor, in a room you use, away from sunlight, drafts, doorways, windows, supply vents and heat sources. Avoid outside walls and spots behind doors. Poor placement causes false readings and extra cycling.
Should I set back a heat pump thermostat?
Use small setbacks, or a thermostat designed for heat pumps. A large setback can make the system call for backup electric heat to recover, which can cost more than it saved. DOE notes that heat-pump-specific thermostats use special logic to limit backup heat.
What do the letters on thermostat wires mean?
R is 24-volt power, C is common, W is heat, Y is cooling, G is the fan, and O/B controls a heat pump's reversing valve. Systems with stages add W2 or Y2. Always label wires before removing an old thermostat.
Why does my thermostat say "aux heat"?
On a heat pump, aux heat means backup heat, often electric strips, is running to help. It is normal in very cold weather and during defrost. If it shows most of the time in mild weather, have a technician check the heat pump and thermostat settings.
Is it better to leave the AC at one temperature all day?
DOE's guidance is that setting the temperature up when you are away saves energy, because the house gains heat more slowly when it is warmer inside. Use a moderate setback so the system can recover in time.
Sources
- U.S. DOE Energy Saver, Programmable Thermostats (archived copy, 2025), retrieved .
- ENERGY STAR, Smart Thermostats, retrieved .
- Honeywell, TH6110D/TH6220D FocusPRO Installation Guide, retrieved .
- U.S. DOE Building Technologies Office, Residential HVAC Installation Practices: A Review of Research Findings (June 2018), retrieved .
- U.S. EPA, Mercury in Consumer Products, retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: DOE's setback figure and placement rule are quoted from an archived copy of the Energy Saver page (the live page now returns 404). Wiring and cycle-rate facts are Honeywell's installation guide. The schedule example counts hours only; no savings figure is computed.
Suggest a correction. We fix errors and say what changed.