Thermostat guide
Honeywell Home T9 Smart Thermostat wiring guide
Room-by-room sensors and the widest label support: accepts K (wire saver), L/A, Rh/Rc and E terminals. Needs a C wire or wire saver.
What it supports
| Conventional systems | Up to 2 heat / 2 cool stages |
|---|---|
| Heat pumps | 2 compressor stages, 1 aux stage, separate E terminal |
| Dual fuel | Supported |
| Dual transformer (Rh/Rc) | Supported |
| Power / C wire | Needs a C wire, or a Honeywell wire saver with 4 wires (Y moves to K). |
| Line voltage / millivolt | Not compatible |
Where your old wires go
| Old terminal | Honeywell T9 terminal |
|---|---|
| R (single transformer) | R |
| Rh + Rc (dual transformer) | Rh |
| C (common) | C |
| W / W1 (heat) | W |
| W2 (2nd-stage heat) | W2 / Aux |
| AUX / W2 on a heat pump | W2 / Aux |
| E (emergency heat) | E |
| Y / Y1 (cool) | Y |
| Y2 (2nd-stage cool) | Y2 |
| G (fan) | G |
| O, B or O/B (reversing valve) | O/B |
| K (wire saver) | K |
| L / L/A (service light) | L/A |
B, X and K depend on your system. Use the tool to check. Confirm with the official guide.
What’s different about wiring the Honeywell T9
The T9 accepts the widest range of labels on this site, including K for a Honeywell wire saver, L/A for a heat pump service light, E for emergency heat, and separate Rh/Rc. If your wiring already has a wire saver installed, the T9 is usually the least work.
It needs a C wire. Without one, a wire saver module at the furnace provides it using 4 wires. The T9 runs one aux heat stage on heat pumps, which covers most homes.
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Honeywell T9 with common wiring setups
| Setup | Verdict |
|---|---|
| 2-Wire Thermostat (Heat Only) | Works with an adapter |
| 3-Wire Thermostat (Heat + Fan, No C Wire) | Works with an adapter |
| 4-Wire Thermostat Wiring (No C Wire) | Works with an adapter |
| 5-Wire Thermostat Wiring (With C Wire) | Works: direct swap |
| Cooling-Only Thermostat Wiring (AC + Fan) | Works: direct swap |
| 2-Stage Heat & 2-Stage Cool Thermostat Wiring | Works: direct swap |
| Heat Pump Thermostat Wiring (5 Wires, No C Wire) | Works with an adapter |
| Heat Pump Thermostat Wiring (6 Wires With C Wire) | Works: direct swap |
| Heat Pump Wiring With AUX and E (Emergency Heat) | Works: direct swap |
| Heat Pump With a B Wire (Rheem / Ruud Style) | Works: direct swap |
| B Wire as Common (Trane / American Standard Style) | Works: direct swap |
| Rh and Rc Wires (Dual Transformer) Thermostat Wiring | Pro install recommended |
| K Terminal Wiring (Honeywell Wire Saver) | Works: direct swap |