Heat pump thermostat wiring
Heat pumps wire differently from a furnace + AC. Here is what each thermostat terminal does and the typical color code.
Heat pump thermostat terminals
| Terminal | What it controls |
|---|---|
| R / Rc / Rh | 24V power from the transformer |
| C | Common (return) — powers smart thermostats |
| Y | Compressor (cooling and heat-pump heating) |
| O/B | Reversing valve — the key heat-pump terminal (O = energized in cooling, B = energized in heating) |
| G | Indoor fan/blower |
| Aux / W2 / E | Auxiliary and emergency (backup) heat |
Typical wire color code
Common (not universal) colors: R red, C blue/black, Y yellow, G green, O/B orange (O) or dark blue (B), aux/W2 white, E emergency often brown/pink. Always label your existing wires before disconnecting — colors vary by installer.
Why heat pumps wire differently
A furnace + AC has separate heat (W) and cool (Y) calls. A heat pump uses the same compressor (Y) for both and flips the reversing valve (O/B) to switch modes, plus separate aux/emergency heat. That is why a heat pump needs a compatible thermostat. See our best thermostat guide. For anything beyond a like-for-like swap, use a licensed technician.
Frequently asked questions
What is the O/B terminal on a heat pump thermostat?
It controls the reversing valve that switches the heat pump between heating and cooling. "O" energizes it in cooling; "B" energizes it in heating — set this to match your equipment.
What wires does a heat pump thermostat need?
Typically R (power), C (common), Y (compressor), O/B (reversing valve), G (fan), and aux/W2 plus E for backup/emergency heat.
Can I replace a heat pump thermostat myself?
A like-for-like swap is doable if you label wires carefully and set heat-pump mode + O/B correctly. For rewiring or aux-heat setup, use a professional.
Related
Sources & further reading
Educational guide, reviewed against US DOE & ENERGY STAR guidance and updated June 2026. Estimates only — not a substitute for a professional assessment or Manual J load calculation.