Wiring guide
Heat Pump Thermostat Wiring (5 Wires, No C Wire)
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.
Detected system: Heat pump, 1-stage compressor, 1-stage aux heat, fan, no C wire · 5 wires
Smart thermostat compatibility
| Thermostat | Verdict | What to know |
|---|---|---|
| Nest Learning (4th gen) | Works with an adapter | Google says heat-only, cool-only and heat pump systems without a C wire need a C wire or the Nest Power Connector. |
| ecobee Premium | Works with an adapter | No C wire: use the Power Extender Kit that comes in the box. It installs at your furnace/air handler. |
| ecobee Enhanced | Works with an adapter | No C wire: use the Power Extender Kit that comes in the box. It installs at your furnace/air handler. |
| Honeywell T9 | Works with an adapter | 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. |
| Amazon Smart Thermostat | Works with an adapter | No C wire: use Amazon’s C-wire power adapter. Per Amazon’s guide, the G wire moves to C and the Y wire moves to K. |
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 |
| Y (yellow) | Y | PEK | PEK | K | K |
| G (green) | G | C | C | C | C |
| O/B (orange) | *OB | O/B | O/B | O/B | O/B |
| AUX (white) | AUX | W1 | W1 | W2 / Aux | W |
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
R, Y, G, O/B and AUX with no C wire. The outdoor unit both heats and cools, and backup electric heat (AUX, sometimes W2) runs when it can’t keep up. The old thermostat probably has an “EM HEAT” or “AUX” indicator.
Mistakes to avoid
- Setting the reversing valve as O when the equipment uses B, or the other way round. Heat and cool come out backwards. Most heat pumps use O, and Rheem/Ruud use B.
- Relying on power stealing with a heat pump. Heat pump contactors and valves are often less tolerant of it, which is why the brands ask for a C wire here.
- Connecting AUX to W on a thermostat that has a separate AUX terminal. Check which one the new thermostat expects.
Our advice
Plan for a C wire from the start. A spare conductor in the cable, a new cable, or the brand’s adapter will be more reliable than running without one on a heat pump.