Wiring guide
Rh and Rc Wires (Dual Transformer) Thermostat Wiring
Wires on both Rh and Rc (not a short jumper between them) mean separate heating and cooling transformers, common with a boiler plus a separate AC air handler. Bridging them can destroy equipment.
Smart thermostat compatibility
| Thermostat | Verdict | What to know |
|---|---|---|
| Nest Learning (4th gen) | Pro install recommended | Google recommends a professional install for dual-transformer (Rh + Rc) systems. |
| ecobee Premium | Pro install recommended | Dual transformer: keep Rh and Rc on separate terminals and never jumper them together. |
| ecobee Enhanced | Pro install recommended | Dual transformer: keep Rh and Rc on separate terminals and never jumper them together. |
| Honeywell T9 | Pro install recommended | Dual transformer: keep Rh and Rc on separate terminals and never jumper them together. |
| Amazon Smart Thermostat | Not compatible | Separate Rh and Rc wires mean two transformers, which the Amazon Smart Thermostat doesn’t support. |
Where each wire goes, thermostat by thermostat
| Old wire | Nest Learning (4th gen) | ecobee Premium | ecobee Enhanced | Honeywell T9 | Amazon Smart Thermostat |
|---|---|---|---|---|---|
| RH (red) | Rh | Rh | Rh | Rh | None |
| W (white) | W | W1 | W1 | W | W |
| RC (black) | Rc | Rc | Rc | Rc | None |
| Y (yellow) | Y | Y1 | Y1 | Y | Y |
| G (green) | G | G | G | G | G |
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
Separate wires on Rh and Rc, with no metal jumper between them. That means two transformers: one in the heating equipment (often a boiler) and one in the cooling equipment (an air handler). A short jumper between Rh and Rc is different: that is a single transformer, and the jumper is just a link.
Mistakes to avoid
- Joining Rh and Rc together, or leaving a new thermostat’s internal jumper in place. That connects two transformers and can damage both.
- Adding a C wire from the wrong transformer. If you run one later, it must come from the same transformer as the R wire the thermostat takes power from.
Our advice
Pick a thermostat that isolates Rh from Rc. ecobee and Honeywell T9 handle it, and Google recommends a professional install for this setup on the Nest Learning (4th gen). The Amazon Smart Thermostat doesn’t support dual transformers.