Wiring guide
Honeywell Home T9 Smart Thermostat wiring for rh + Rc dual transformer
This setup is possible with the Honeywell Home T9 Smart Thermostat, but a professional install is recommended.
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. Read the full rh + Rc dual transformer guide for how to confirm your setup and mistakes to avoid.
Wiring diagram
Honeywell Home T9 Smart Thermostat
Pro install recommended| Wire | Color | Job | Connect to | Note |
|---|---|---|---|---|
| RH | red | Heating power | Rh | |
| W | white | Heat, stage 1 | W | |
| RC | black | Cooling power | Rc | |
| Y | yellow | Cooling, stage 1 | Y | |
| G | green | Fan | G |
Install steps
- Turn off power to the furnace/air handler (switch or breaker) and the AC condenser. Confirm the old thermostat is dead.
- Photograph the old wiring, then label each wire with the letter it came off. Ignore short jumpers.
- Connect each wire to the Honeywell T9 terminal shown above. Cap unused wires one at a time.
- Restore power and finish setup in the app.
- Test heat, cool and fan. If a breaker trips or equipment runs nonstop, power off and call a pro.
What you need
- Honeywell Home T9 Smart ThermostatThe thermostat itselfCheck price
- 18/5 thermostat wireAdds a C wireCheck price
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Confirm against the official Honeywell Home install guide. The app also checks your wiring during setup.
Frequently asked questions
Where does each wire go on the Honeywell T9?
RH → Rh; W → W; RC → Rc; Y → Y; G → G.
Do I need a C wire for the Honeywell T9?
Dual transformer with no C wire: C-wire adapter kits assume one transformer. Have a pro confirm which transformer the thermostat should draw power from, or run new 18/5 cable.