A heat pump thermostat looks like any other thermostat on the wall. The difference is behind it: one extra wire decides whether the system heats or cools, and the settings menu has to know it's talking to a heat pump, not a furnace. Get either one wrong and the system can blow cold air in heat mode or run its expensive backup heat all winter.
If you're still deciding whether to hire this out, smart thermostat installation costs run higher for heat pumps than for standard swaps, for exactly the reasons below: more wires, more settings, more testing.
Standard furnace wiring has R for power, W for heat, Y for cool, and G for the fan. A heat pump adds two wires a furnace never has: O/B for the reversing valve, and W2/Aux for auxiliary heat. Some systems add E for emergency heat, which is a separate terminal on many thermostats.
The reversing valve is the heart of the whole thing. It's a valve in the outdoor unit that flips the system between heating and cooling. The thermostat controls it with the O/B wire, and the setup menu decides whether that wire energizes in cooling mode or heating mode. Most systems energize in cooling — that's the O setting — but some brands do the opposite, so check your heat pump's manual rather than guessing.
Aux heat is the backup: electric heat strips, or a gas furnace on dual-fuel systems, that kick in when the heat pump can't keep up or goes into defrost. The thermostat has to know aux heat exists, or it can't call for it — and on a cold night, that's the difference between a warm house and a cold one.
Shut the system down at the breaker or the furnace switch before you touch any wire. Thermostat circuits are low voltage, but a slipped wire on a live circuit can blow the small fuse on the control board or damage the board itself.
Photograph the old wiring at the thermostat and at the indoor unit, and label every wire by its terminal letter. Colors are not standardized — the letter is the fact, the color is a guess. If there's no wire on C, sort that out before you go further: the no-C-wire install guide covers the six fixes, cheapest first.
| Terminal | What it does | Heat pump note |
|---|---|---|
| R / Rc | 24V power | Some systems split Rh and Rc — jumper them if your thermostat calls for it |
| C | Common, completes the power circuit | Needed for steady smart thermostat power |
| Y | Compressor | Runs the heat pump in both modes |
| G | Indoor fan | Same as any system |
| O/B | Reversing valve | The critical wire — set O or B per your heat pump's manual |
| W2 / Aux | Auxiliary heat | Heat strips or dual-fuel furnace backup |
| E | Emergency heat | Some thermostats share this with Aux; check the manual |
If your home runs radiators off a boiler instead of a heat pump, the wiring logic is different altogether — the boiler vs furnace comparison explains the system side, and a boiler thermostat job follows different rules.
Mounting the thermostat is the easy half. The settings menu is where heat pump installs go wrong. Work through it slowly, with the manual open:
System type: heat pump, not conventional. This one setting changes what the rest of the menu asks. Pick the wrong type and the thermostat treats O/B as a second-stage furnace wire.
O/B orientation. Choose whether the reversing valve energizes on cool (O, the common case) or on heat (B, some brands). Wrong choice here means the system heats when you ask for cooling and cools when you ask for heat.
Aux heat type. Tell the thermostat whether backup heat is electric strips or a gas furnace. On dual-fuel systems, this setting also controls the switchover point where the system stops using the heat pump and fires the furnace instead. Dual-fuel setups have more settings than straight electric-aux systems — if the menu asks questions your manual doesn't answer, that's a good time to call a pro.
Compressor lockout and aux lockout temperatures. Many thermostats let you set outdoor temperatures below which the compressor stops and aux takes over, and above which aux stays off. Defaults are usually fine for a first install; tune them later once the system has run a few weeks.
Kill the power. Breaker off, or the furnace switch off. Confirm the old thermostat screen is dead before you touch a wire.
Photograph and label. Photo the old wiring, then tag each wire with its terminal letter. Pay special attention to O/B and W2/Aux — those two are the whole job.
Land the wires on the new thermostat. Match letters, not colors. O/B goes to the O/B terminal, W2/Aux to W2/Aux. Tug each conductor lightly after landing it — a wire that slips out now would have slipped out later anyway.
Mount and power up. Level the wall plate, attach the thermostat, restore power, and let it start up fully before you touch the settings.
Run the setup menu. System type: heat pump. O/B orientation per your heat pump's manual. Aux heat type per your backup heat. Don't rush this screen — it's the one that decides whether the system runs right.
Test heat, cool, and fan. Run heat and confirm warm air. Run cool and confirm cool air — this is the test that catches a backwards O/B setting. Run the fan on its own. For mounting and app setup details, the general smart thermostat install guide covers the parts that are the same on every system.
Test emergency heat. Switch to emergency heat and confirm the backup heat runs. Then switch back to normal heat. If emergency heat doesn't run, the Aux/E wiring or settings need another look before you call the job done.
O/B set backwards. The classic symptom: the system blows cold air when you ask for heat, and warm air when you ask for cool. Flip the O/B setting in the menu — don't rewire anything.
Aux wired as plain W. On a conventional setup menu, W means furnace heat. On a heat pump, the backup wire belongs on W2/Aux with the system type set to heat pump. Wired as W on a conventional profile, the system may never call aux when it needs it.
Skipping the cool test. Plenty of installs heat fine and fail the cool test, because the O/B error only shows in one mode. Test both, every time.
Skipping emergency heat. A system that heats normally but can't run emergency heat has a wiring or settings gap you'll only discover on the coldest night of the year. Test it now, in daylight.
A heat pump thermostat install is finished when all four modes run right: heat, cool, fan, and emergency heat. If any one of them misbehaves, the answer is in the wiring letters or the setup menu, not in the thermostat itself. And if the system is dual-fuel, or the manual's O/B guidance doesn't match what you see on the wall, a pro's hour is cheaper than a winter of backup-heat bills.
They need a thermostat built for heat pumps — one with an O/B terminal and a heat pump system type in its settings. A standard furnace-only thermostat can't control the reversing valve or aux heat.
It's the wire that controls the reversing valve, the part that flips a heat pump between heating and cooling. The thermostat setting decides whether it energizes on cool (O, the common case) or on heat (B, some brands).
The usual cause after a thermostat swap is the O/B setting: it's backwards. Flip it in the thermostat's settings menu. If it was fine before the swap and nothing else changed, the setting is the first thing to check.
Auxiliary heat is backup heat — usually electric heat strips, or a gas furnace on dual-fuel systems — that runs when the heat pump can't keep up on its own or goes into defrost. The thermostat calls for it through the W2/Aux wire.
No. Check the model's stated compatibility: it needs heat pump support, an O/B terminal, and aux heat handling. Most major smart thermostats offer this, but budget models sometimes don't.