Wiring guide
Cooling-Only Thermostat Wiring (AC + Fan)
Cooling-only systems (central AC with an air handler and no heat) are common in the South. With a C wire, most smart thermostats install directly.
Smart thermostat compatibility
| Thermostat | Verdict | What to know |
|---|---|---|
| Nest Learning (4th gen) | Works: direct swap | Direct swap: every wire has a matching terminal. |
| ecobee Premium | Works: direct swap | Direct swap: every wire has a matching terminal. |
| ecobee Enhanced | Works: direct swap | Direct swap: every wire has a matching terminal. |
| Honeywell T9 | Works: direct swap | Direct swap: every wire has a matching terminal. |
| Amazon Smart Thermostat | Works: direct swap | Direct swap: every wire has a matching terminal. |
Where each wire goes, thermostat by thermostat
| Old wire | Nest Learning (4th gen) | ecobee Premium | ecobee Enhanced | Honeywell T9 | Amazon Smart Thermostat |
|---|---|---|---|---|---|
| R (red) | Rc or Rh | Rc | Rc | R | R |
| Y (yellow) | Y | Y1 | Y1 | Y | Y |
| G (green) | G | G | G | G | G |
| C (blue) | C | C | C | C | C |
Colors are typical, not a rule: always go by the letter the wire was connected to. Click a thermostat for its full diagram, parts and app settings.
How to confirm this is your setup
R, Y, G and C with no W. You have an air conditioner and air handler and no heat, which is common in warm climates and in some apartments. If your home does have heat from a separate system such as a boiler, that system is usually controlled by its own thermostat.
Mistakes to avoid
- Choosing “heat pump” in the app because the outdoor unit looks like one. With no O/B wire it is cooling only: set it up as AC.
- Wiring the fan wire to W because the new thermostat’s wiring screen expects W. Leave W empty.
Our advice
Set the system type to cooling only during setup so the thermostat doesn’t offer a heat mode that can’t do anything. Most thermostats also let you set a compressor minimum off-time, which protects the outdoor unit.