How it reads your wires
Thermostandard mathO/B present, or heat pump picked: heat pump. W2 present: two-stage heat. Y2 present: two-stage cooling (on a heat pump, a two-stage compressor). AUX/E, or W on a heat pump: backup heat. C present: constant 24-volt power for the thermostat.
What counts as a matchA thermostat is left out only when its maker's listing rules out your setup, for example a model the maker says doesn't support heat pumps. Where a listing doesn't say, the thermostat goes under 'check with the maker' rather than being dropped or promised.
No C wire?Thermostats that need a C wire still appear when you don't have one, in their own group. Most can be powered with the maker's adapter or a spare conductor in the cable; What a C wire is walks through the options.
SafetySwitch off power at the breaker or the furnace switch before you remove the old thermostat, and photograph the wiring first. Anything inside the furnace or air handler is a job for an HVAC technician if you're unsure.
Questions shoppers ask
How do I know if I have a C wire?
Look for a wire on the terminal marked C. A spare wire folded back into the wall may also be usable as a C wire, but only if it's connected to C at the furnace end too.
What does the O/B wire do?
It switches a heat pump's reversing valve between heating and cooling. If you have one, you need a thermostat that supports heat pumps.
Can I use a smart thermostat with only two wires?
Two wires (R and W) usually means heat only, often a boiler or an older furnace. Some thermostats run on batteries and work with two wires; others need a C-wire adapter.
My wires are 120 or 240 volts. What now?
That's line voltage, used by electric baseboard heaters. You need a line-voltage thermostat rated for the heater load, not a 24-volt model.