Wiring guide
Honeywell Home T9 Smart Thermostat wiring for heat pump, no C wire
The Honeywell Home T9 Smart Thermostat works with this setup, but you’ll need an adapter or extra wire (details below).
A heat pump with R, Y, G, O/B and AUX has a reversing valve and backup electric heat but no C wire. Heat pumps are exactly where smart thermostats get picky about power. Read the full heat pump, no C wire guide for how to confirm your setup and mistakes to avoid.
Wiring diagram
Detected system: Heat pump, 1-stage compressor, 1-stage aux heat, fan, no C wire · 5 wires
Honeywell Home T9 Smart Thermostat
Works with an adapterNo C wire: install a Honeywell wire saver at the furnace. It frees up a wire to become C, with the Y wire moving to K.
| Wire | Color | Job | Connect to | Note |
|---|---|---|---|---|
| R | red | 24 V power | R | |
| Y | yellow | Cooling, stage 1 | K | Adapter carries cooling + fan on this wire |
| G | green | Fan | C | Moves to C with the adapter |
| O/B | orange | Reversing valve (O/B) | O/B | |
| AUX | white | Auxiliary heat | W2 / Aux |
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 using the settings below.
- Test heat, cool and fan. If a breaker trips or equipment runs nonstop, power off and call a pro.
App settings to choose
- Reversing valve (O/B): set to O or B (O for most brands; B for Rheem, Ruud and some others).
- Equipment type: heat pump with auxiliary (electric strip) heat.
What you need
- Honeywell Home T9 Smart ThermostatThe thermostat itselfCheck price
- Honeywell Home wire saver moduleCreates a C wire from 4 wiresCheck 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 → R; Y → K; G → C; O/B → O/B; AUX → W2 / Aux.
Do I need a C wire for the Honeywell T9?
No C wire: install a Honeywell wire saver at the furnace. It frees up a wire to become C, with the Y wire moving to K.