Wiring guide
Honeywell Home T9 Smart Thermostat wiring for 3 wires, heat + fan
The Honeywell Home T9 Smart Thermostat works with this setup, but you’ll need an adapter or extra wire (details below).
Three wires on R, W and G control a furnace’s heat and blower but not AC, and there is no C wire. Some smart thermostats cope; others need extra wire. Read the full 3 wires, heat + fan guide for how to confirm your setup and mistakes to avoid.
Wiring diagram
Honeywell Home T9 Smart Thermostat
Works with an adapter| Wire | Color | Job | Connect to | Note |
|---|---|---|---|---|
| R | red | 24 V power | Rh | Heat circuit power; adapter goes to Rc + C |
| W | white | Heat, stage 1 | W | |
| 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.
- Install the adapter at your furnace or air handler following its instructions, with power still off.
- 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 wire (best fix)Check price
- Plug-in 24 VAC thermostat adapterPowers the thermostat without new 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?
R → Rh; W → W; G → G.
Do I need a C wire for the Honeywell T9?
No C wire and too few wires for a furnace adapter. Best fix: run a new 18/5 thermostat cable. Alternative: a plug-in 24 VAC adapter wired to Rc and C, with your existing heat circuit moved to Rh (the Honeywell T9 supports split power).