Most smart thermostats need 24-volt low-voltage HVAC wiring and constant power from a C (common) wire. Heat pumps need O/B support, and two-stage systems need W2 or Y2 terminals. Electric baseboard heaters run on 120 or 240 volts and need a line-voltage thermostat instead. Photograph your current wiring, with the power off, before you buy.
A smart thermostat that cannot power itself, or does not understand your heat pump, is a return waiting to happen. Five minutes with a flashlight and your phone camera prevents that.
Read your current wiring
Turn off power to the HVAC system at the breaker or switch. Remove the thermostat face and photograph the terminals and the letters next to each wire.
| Terminal | Usually means |
|---|---|
| R, Rh, Rc | 24-volt power (Rh for heat, Rc for cooling if separate) |
| C | Common wire, constant power for the thermostat |
| W, W1 | Heat, stage 1 |
| W2 | Heat, stage 2 or auxiliary heat |
| Y, Y1 | Cooling or heat pump compressor, stage 1 |
| Y2 | Compressor stage 2 |
| G | Fan |
| O/B | Heat pump reversing valve |
Colors are a convention, not a rule. Go by the terminal letters.
The C-wire question
Thermostats with Wi-Fi and screens need constant power. Without a C-wire, some models borrow power through other wires, which can cause problems on some systems. Look for an unused wire tucked behind the wall; if you find one, it may be connectable at the furnace control board. Otherwise use the manufacturer’s adapter kit.
Heat pumps and stages
A heat pump thermostat must control the O/B wire and, where fitted, auxiliary or emergency heat. Two-stage furnaces and compressors need W2 or Y2 support. Dual-fuel systems (heat pump plus furnace) need explicit support.
Line voltage is different
Electric baseboard and some wall heaters connect thermostats directly to 120 or 240 volts, with thick wires and wire nuts. Low-voltage thermostats will not work and can be dangerous. Use a line-voltage smart thermostat or hire an electrician.
Platform fit
Decide whether you want Matter support or a specific ecosystem; see Matter and the smart-home standards. Browse climate and air.
Common questions
What is a C-wire on a thermostat?
The common wire completes the 24-volt circuit so the thermostat gets constant power. Most Wi-Fi thermostats need it or an adapter.
What if I don't have a C-wire?
Check for an unused wire behind the wall plate. If there is none, use the maker's power adapter kit or have an HVAC technician run a wire.
Do smart thermostats work with heat pumps?
Most do, but they must support the O/B reversing valve wire, and auxiliary heat if you have it. Check the compatibility tool.
Can I use a smart thermostat on baseboard heaters?
Only a line-voltage model built for 120 or 240 volt heaters. Standard low-voltage smart thermostats are not compatible.