SAFETY: Turn off power at the breaker or furnace switch before touching any thermostat wiring. Confirm the old thermostat’s display is dark before pulling it off the wall. The control side is 24V AC, but a shorted wire can blow the board fuse or kill the transformer.
A heat pump adds exactly one thing to Nest wiring: the reversing-valve wire, and it goes on the thermostat’s single OB connector no matter which letter it used on your old sub-base. Everything else (R, C, Y, G) wires the same way covered in the full Nest thermostat wiring guide, which is itself an offshoot of the site’s thermostat wiring guide. This page covers only what’s different on a heat pump: the OB connector, the O-vs-B setting, which Nest models handle aux and emergency heat, and two-stage compressors.
TL;DR
| Your situation | What to do |
|---|---|
| Old thermostat has an O or a B wire (or both) | Land it on the Nest’s single OB connector |
| Heat runs when you ask for cooling (or vice versa) | Flip O to B (or B to O) in the app’s equipment settings. Never move the wire. |
| You need emergency heat (E) | Only the Nest Learning Thermostat (3rd/4th gen) has an E connector. The standard Nest Thermostat and Nest Thermostat E don’t. |
| Two-stage compressor | Land the second wire on Y2; the thermostat and outdoor unit both need to support staging |
| No C wire | Same power-sharing rules as any Nest (see below); symptoms show up faster on a heat pump |
The OB connector: one terminal, whatever letter it used to be
Old heat-pump thermostats label the reversing-valve wire O, B, or O/B depending on the brand and installer. A Nest doesn’t care which letter it arrives as: the wire lands on the single OB connector every time. What determines whether the valve energizes in heating or cooling isn’t the wire position. It’s a setting.
- O: energizes in cooling. The default on most manufacturers (Carrier, Trane, Goodman, Lennox).
- B: energizes in heating. Rheem and Ruud build their valves the other way.
Set the wrong one and the system heats when you ask for cooling, or blows cold air in heat mode. That’s the single most common heat-pump complaint on a Nest, and the fix is always in the equipment settings menu, never at the wall plate. If you inherited an old thermostat with both an O wire and a B wire landed on separate terminals, that’s unusual. Check which one is actually connected to the reversing valve at the outdoor unit before guessing, or treat the OB connector as taking whichever wire your compatibility check flagged as the reversing-valve wire.
Which Nest models support aux and emergency heat
This is where heat-pump capability actually splits by model, and it’s worth checking before you buy:
| Model | Second stage (W2/Y2) | Emergency heat (E) |
|---|---|---|
| Nest Learning Thermostat (3rd and 4th gen) | Yes | Yes |
| Nest Thermostat (standard) | Yes | No |
| Nest Thermostat E | Yes | No |
Only the Learning Thermostat lists an E connector in Google’s own spec sheet. If your heat pump has backup strips that need a manual emergency-heat override (for when the compressor itself fails and you still need heat), a Learning model is the one that can wire it. The standard Nest Thermostat and Nest Thermostat E both support a second stage of heating or cooling (W2 or Y2), which is enough for staged aux heat that runs with the compressor automatically, just not the manual emergency-only mode.
Single-stage vs. two-stage compressors
A single-stage heat pump wires one compressor wire to Y1. A two-stage or variable-capacity unit adds a second wire to Y2, which adds capacity on top of Y1 rather than running independently. Wiring-wise this is no different from any Nest install: land each wire on its matching connector. Whether Y2 actually does anything depends on both the thermostat and the outdoor unit’s control board supporting staged operation. A single-stage-only Nest wired to two-stage equipment just runs the system as single-stage.
Aux heat: automatic backup vs. manual override
Two different terminals cover backup heat, and they’re not interchangeable:
- W2 or AUX: backup heat that runs with the compressor automatically when the heat pump can’t keep up. This is normal cold-weather operation on every Nest model.
- E: emergency heat, a manual mode that disables the compressor and runs backup alone. Learning models only (see table above). Use it when the compressor itself is down, not routinely.
If your equipment has strip heat but your Nest model has no E connector, land the backup wire on W2/AUX and skip emergency heat entirely. The system will still run staged backup automatically when the compressor can’t keep up.
Coming from an old thermostat: the B-or-C trap
Heat pump sub-bases are where the blue-wire ambiguity bites hardest, because heat pumps use both a C wire and (on Rheem/Ruud systems) a B wire, and both are sometimes blue. Google Nest’s own wire-identification guidance is explicit: check which terminal the wire occupied on the old thermostat, don’t assume by color. If your old thermostat shows both an O wire and a B wire, or both a B and a C wire, land the reversing-valve wire on OB and the common wire on C. Never guess from wire color alone. The full color breakdown for every terminal, not just heat pumps, is in the thermostat wiring color code guide.
Testing after you wire it
- Restore power and complete setup in the app. Confirm the app’s detected-wire list matches what you physically connected, including OB.
- Test cooling first. Set a call for cooling and wait about five minutes. Cold air confirms O/B is set correctly.
- Test heat. Warm air on a heat call confirms the other direction. If either test gives you the opposite temperature, flip the O/B setting in the equipment menu. Don’t touch the wire.
- Test aux/emergency heat if your model has it. On a cold day (or using a test mode if your thermostat offers one), confirm backup heat actually engages before you need it.
FAQ
Which connector does the O or B wire go on for a Nest heat pump?
The single OB connector, regardless of which letter the wire used on your old thermostat. Then set O or B in the app: O for most brands, B for Rheem and Ruud.
Why is my Nest heat pump blowing cold air in heat mode?
The O/B orientation is set backwards for your equipment. Change it in the app’s equipment settings; this is always a setting fix, never a rewiring fix.
Does every Nest support emergency heat on a heat pump?
No. Only the Nest Learning Thermostat (3rd and 4th gen) has an E connector. The standard Nest Thermostat and Nest Thermostat E support a second heating/cooling stage (W2/Y2) but not a manual emergency-heat override.
Do I need a C wire for a Nest on a heat pump?
Not always — power sharing works on many systems. But heat pumps pull more relay current than a simple heat/cool system, so Wi-Fi drops, clicking relays, or a battery warning show up faster without one. Options for adding a C wire: installing a thermostat without a C wire.
What’s the difference between the general Nest wiring guide and this page?
The Nest thermostat wiring guide covers every terminal, every model’s connector count, and the no-jumper rule for any system. This page covers only what a heat pump adds on top of that: OB, aux, and emergency heat. For the non-Nest fundamentals of heat pump wiring (O/B conventions across brands, dual-fuel systems), see the heat pump thermostat wiring guide.
| 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
