Wiring guide
K Terminal Wiring (Honeywell Wire Saver)
A wire on K means a Honeywell wire saver module is installed at the furnace, combining fan and cooling on one wire to free up a C wire. Only some thermostats understand K.
Smart thermostat compatibility
| Thermostat | Verdict | What to know |
|---|---|---|
| Nest Learning (4th gen) | Pro install recommended | No K terminal. A pro must remove the wire saver and restore G and Y. |
| ecobee Premium | Pro install recommended | No K terminal. A pro must remove the wire saver and restore G and Y. |
| ecobee Enhanced | Pro install recommended | No K terminal. A pro must remove the wire saver and restore G and Y. |
| Honeywell T9 | Works: direct swap | Direct swap: every wire has a matching terminal. |
| Amazon Smart Thermostat | Works: direct swap | Direct swap: every wire has a matching terminal. |
Where each wire goes, thermostat by thermostat
| Old wire | Nest Learning (4th gen) | ecobee Premium | ecobee Enhanced | Honeywell T9 | Amazon Smart Thermostat |
|---|---|---|---|---|---|
| R (red) | Rc or Rh | Rc | Rc | R | R |
| C (green) | C | C | C | C | C |
| W (white) | W | W1 | W1 | W | W |
| K (yellow) | None | None | None | K | K |
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
A wire on K means a Honeywell wire saver module (THP9045 or similar) is installed at the furnace. It combines the fan and cooling calls on one wire so another wire in the cable can become C. You’ll find a small module wired into the furnace control board.
Mistakes to avoid
- Moving K to Y or G on a thermostat that has no K terminal. The fan and cooling won’t work properly without removing the wire saver and rewiring the furnace.
- Removing the wire saver without reconnecting Y and G at the furnace board.
Our advice
Honeywell T9 and the Amazon Smart Thermostat have a K terminal, so the wire saver stays in place. For Nest or ecobee, remove the wire saver, reconnect Y and G directly at the furnace, and then use that brand’s own power option.