SAFETY: Turn off power at the breaker or the outdoor unit’s disconnect before touching any thermostat wiring. Confirm the display is dark before pulling the old thermostat off the wall. The thermostat side is 24V AC control voltage. The outdoor unit’s compressor and fan run on 208/240V, and that side is not DIY: leave the disconnect alone and never open the outdoor cabinet.
A heat pump thermostat wires almost exactly like a standard heat-and-cool thermostat, with one addition: the O/B terminal, which tells the reversing valve which way to run. Get O/B right, and everything else (R, C, Y, G) is the same 24V logic covered in the full thermostat wiring guide. Get it wrong and the system heats when you ask for cooling, which is the single most common heat-pump wiring complaint.
This page covers the terminals unique to heat pumps: O/B and the brand-by-brand convention behind it, single-stage versus two-stage (Y2) compressors, aux and emergency heat wiring, and dual-fuel systems that pair a heat pump with a gas furnace. For brand-specific step-by-step diagrams, jump to Honeywell, Nest, or Amazon Smart Thermostat below.
TL;DR
| Terminal | What it does on a heat pump |
|---|---|
| R (or Rc/Rh) | 24V AC power from the transformer |
| C | Common: constant power return for smart thermostats |
| Y (Y1) | Compressor: runs for both heating and cooling on a heat pump |
| Y2 | Second-stage compressor capacity, on two-stage/variable equipment only |
| G | Indoor fan (blower) relay |
| O/B | Reversing valve. O energizes in cooling (most brands); B energizes in heating (Rheem/Ruud and a few others) |
| W2 / AUX | Backup electric-resistance heat, runs with the compressor |
| E | Emergency heat — manual override that disables the compressor and runs backup heat alone |
Wrong O/B setting is the #1 heat pump wiring complaint: blows cold air in heat mode, or warm air in cool mode. It’s a setting, not a wiring fix. Jump to O/B troubleshooting.
R, C, Y, G: the same four terminals as any thermostat
A heat pump’s base wiring is identical to a standard forced-air system: R for 24V power, C for common (constant power), Y for the compressor, G for the fan. If your old thermostat has two R wires (separate heating and cooling transformers), the jumper and dual-transformer rules are the same as any system. Full logic in the Rc/Rh jumper guide.
The difference on a heat pump: Y runs the compressor for heating and cooling both. There’s no separate W call for normal heat, because “heat” on a heat pump just means running the compressor with the reversing valve in the heating position. That’s what O/B does.
The O/B terminal and the O-vs-B convention
The reversing valve has two states, and only one wire tells it which: O/B. Whether that wire should be set to “O” or “B” in the thermostat’s installer menu depends on which way your specific heat pump’s valve is built to fail-safe:
- O — energizes in cooling. This is the default on most manufacturers.
- B — energizes in heating. Rheem, Ruud, Weathermaker, Ameristar, and Bosch Air Source (ducted, air-source models) build their valves the other way. If your equipment isn’t on this list, that’s a hint it’s probably O, not a confirmation — check the data plate either way.
Two wires with the same job, opposite polarity. Setting it wrong doesn’t damage anything, but it inverts your modes: ask for heat, get cold air; ask for cooling, get warm air. This is the single most common heat-pump-specific complaint, and it always traces back to one setting.
Find the correct setting on the outdoor unit’s data plate or install manual (it will say whether the reversing valve is energized in heat or cool), or in the wiring diagram taped inside the outdoor unit’s access panel. If you can’t find either, set O first (it’s the more common default) and check that cooling actually blows cold air.
Single-stage vs. two-stage: what Y2 adds
A single-stage heat pump has one compressor speed: on or off, wired to Y (or Y1). A two-stage or variable-capacity heat pump adds a Y2 terminal for a second capacity level: roughly 60-70% output on Y1 alone, stepping up to full capacity when Y2 also engages. Y2 doesn’t run independently; it adds capacity on top of Y1, not instead of it.
Wiring-wise, Y2 is just another 24V conductor from the outdoor unit’s control board to the thermostat’s Y2 terminal. The thermostat and the outdoor unit both need to support staging for it to do anything. A single-stage thermostat wired to two-stage equipment just never energizes Y2, and the system runs like a single-stage unit.
Aux and emergency heat: W2/AUX vs. E
Below a certain outdoor temperature (or when the compressor can’t keep up), a heat pump needs backup heat. That backup lands on one of two terminals, and they are not the same thing:
- W2 (or AUX): backup heat that runs alongside the compressor. The thermostat energizes Y and W2 together: the heat pump keeps contributing, and the strips (or a dual-fuel furnace) fill the gap. This is automatic and normal on cold days.
- E (or Em): emergency heat, a manual mode that disables the compressor entirely and runs backup heat alone. It exists for a broken compressor, not routine cold weather. On some systems W2 and E are wired to the same physical heat source and only differ in whether the compressor also runs.
Why running the wrong one by mistake is expensive: electric-resistance backup heat runs at roughly a 1:1 ratio of electricity to heat. A heat pump compressor moves 2-4 units of heat per unit of electricity in normal conditions. Landing a wire on the plain W terminal instead of O/B and W2 on a heat pump system is a known field cause of the backup heat running continuously instead of only when the compressor needs help, because the thermostat treats a W call as first-stage heat on a system that has no such stage. Land backup heat on W2/AUX, not W.
Most smart thermostats also expose when aux/backup heat is allowed to fire, as an outdoor-temperature lockout:
- ecobee: Compressor Min Outdoor Temperature (default 35°F) locks out the compressor below that point; Aux Heat Max Outdoor Temperature (default 70°F) blocks aux above it.
- Honeywell T10 Pro: balance-point lockout (installer code ISU 3120) plus a Backup Heat Differential (ISU 3090, 2-15°F) that sets how long the compressor runs before backup stages in.
- Nest: Heat Pump Balance modes (Max Comfort through Max Savings) control how aggressively aux is avoided; turning it off exposes a manual lockout temperature.
Dual-fuel: heat pump plus gas furnace
A dual-fuel (hybrid) system pairs a heat pump with a gas furnace as the backup instead of electric strips. The thermostat still needs O/B for the reversing valve and Y for the compressor, but the backup heat call goes to the furnace’s W terminal through a compatible dual-fuel or hybrid-heat thermostat, which enforces two rules in hardware or firmware:
- Heat pump and furnace don’t run at the same time. Simultaneous operation wastes energy and can stress both systems; every dual-fuel-capable thermostat locks this out.
- The switchover happens at an outdoor temperature you set (the “balance point”), below which the thermostat stops calling for the compressor and hands heating to the furnace. That setpoint is a thermostat setting, not a wiring change. Full comparison of how different thermostats implement it: dual-fuel switchover test.
If you’re wiring a dual-fuel retrofit and the old thermostat wasn’t dual-fuel aware, don’t assume the furnace’s W wire can just land next to the heat pump’s Y and O/B. Confirm the new thermostat has an explicit dual-fuel/hybrid-heat mode in its installer setup before powering it up.
The reversing valve direction is a setting, not a wire
Worth repeating because it’s the error people try to fix with a screwdriver: you never move the O/B wire to change heating/cooling direction. The wire’s physical landing on the O/B terminal doesn’t change. What changes is a menu setting, O or B, in the thermostat’s installer or equipment configuration. If a heat pump is blowing the wrong temperature, the fix is in the settings menu, not at the wall plate.
How to wire a heat pump thermostat, step by step
- Kill the power at the breaker and the outdoor unit’s disconnect. Confirm the old thermostat’s display is dead.
- Photograph the old wiring. One wide shot of the sub-base, one close enough to read every terminal letter.
- Label each wire by letter with tape, not by color. Include O or B specifically, and note which letter the reversing valve wire was actually on.
- Disconnect and secure the cable so it can’t slide back into the wall.
- Mount the new thermostat and land each wire on the matching letter: R to R (or Rc/Rh), C to C, Y to Y (and Y2 to Y2 if present), G to G, the reversing valve wire to O/B, and backup heat to W2/AUX (never plain W on a heat-pump-only system).
- Run installer/equipment setup. Select heat pump system type and single- or two-stage, and, critically, set the O or B orientation for your specific equipment.
- Restore power and test cooling first. Cold air on a cooling call means O/B is correct. Warm air means flip the setting, not the wire.
- Test heat, then force aux/emergency heat if your thermostat allows a test mode, to confirm the backup circuit works before you need it on the coldest night of the year.
Heat-pump-specific troubleshooting
Wiring by brand
Every heat pump follows the O/B, Y, G, W2/AUX, E logic above, but each brand’s connectors and setup menus differ enough to earn a dedicated page:
- Honeywell heat pump thermostat wiring: terminal-by-terminal for the T-series and Pro-series lines.
- Nest thermostat wiring for heat pump: the single OB connector and app-based O/B setting.
- Amazon Smart Thermostat wiring diagram for heat pump: connector layout and compatibility notes.
If your old thermostat shows two R wires or you’re unsure whether you have one or two transformers, resolve that first: Rc/Rh jumper wiring.
FAQ
What wires does a heat pump thermostat need?
At minimum R (power), Y (compressor), G (fan), and O/B (reversing valve). Most installs also carry C (common, for constant power) and W2/AUX (backup heat). Two-stage systems add Y2; dual-fuel and emergency-heat setups add E.
How do I know if my heat pump uses O or B?
Check the outdoor unit’s data plate, install manual, or the wiring diagram inside its access panel; it states whether the reversing valve energizes in heat or cool. As a brand shortcut, Rheem, Ruud, Weathermaker, Ameristar, and Bosch Air Source (air-source models) typically use B; most other brands default to O, but that’s a rule of thumb, not a guarantee. When in doubt, set O first and confirm cooling actually blows cold air.
Why is my heat pump blowing cold air when it’s set to heat?
The O/B setting is almost certainly backwards for your equipment. Go into the thermostat’s installer or equipment menu and flip O to B (or B to O). This is a setting change, never a wiring change.
What’s the difference between aux heat and emergency heat wiring?
Aux (W2) runs backup heat alongside the compressor automatically. Emergency heat (E) disables the compressor and runs backup heat alone, and you turn it on manually; it’s meant for a broken compressor, not routine cold weather.
Can I wire a heat pump thermostat without a C wire?
Many can run on power sharing without one, but heat pump systems draw more relay current than simple heat/cool systems, which makes power-related symptoms (Wi-Fi drops, relay chatter) more likely without a real C wire. Options for adding one: installing a thermostat without a C wire.
Does a dual-fuel system need special wiring?
It needs a thermostat with an explicit dual-fuel or hybrid-heat mode, which enforces that the heat pump and furnace never run together and lets you set the outdoor-temperature switchover point. The physical terminals (O/B, Y, W) are the same; the logic that keeps them from fighting each other lives in the thermostat’s setup, not extra wires.
| Wire color (typical) | Terminal | What it does |
|---|---|---|
| Red | R (or Rc / Rh) | 24 V AC power from the transformer. Rc = cooling power, Rh = heating power; one system uses a jumper between them. |
| White | W (W1, W2) | Heat call. W2 is second-stage or auxiliary heat. |
| Yellow | Y (Y1, Y2) | Cooling call — energizes the compressor contactor. On heat pumps, Y runs the compressor for heating too. |
| Green | G | Indoor fan (blower) relay. |
| Blue | C | Common — completes the 24 V circuit and gives smart thermostats constant power. Sometimes black. |
| Orange | O (or O/B) | Heat-pump reversing valve, energized in cooling (most brands). |
| Dark blue | B | Heat-pump reversing valve, energized in heating (Rheem/Ruud). Occasionally used as C — verify. |
| Brown | E | Emergency heat — manual backup that locks out the compressor. |
| Varies | AUX | Auxiliary heat — automatic electric backup on heat pumps. Often shares a wire with W2. |
⚠️ Know when to stop
Thermostat wiring is 24 V AC low voltage — safe to handle once you cut power at the breaker (do it anyway; a short can kill the transformer or blow the board fuse). But stop and call an HVAC pro or electrician if you hit any of these:
- Line voltage (120/240 V): thick wires with wire nuts, often on baseboard or wall-heater thermostats. Miswiring these can injure you or start a fire.
- Gas components: anything past the furnace control board — gas valves, ignitors, pilot assemblies.
- Refrigerant lines or sealed-system parts on heat pumps and AC units.
- No idea which system you have: if the wiring doesn’t match any diagram here, photograph it and get a pro’s eyes on it before guessing.
Last reviewed: July 2026. Wiring conventions verified against Honeywell Home, Google Nest, ecobee, and Resideo/Honeywell T10 support documentation, plus HVAC Know It All and HVACR School for reversing-valve and staging conventions; always confirm against your equipment’s own manual before wiring.
