SAFETY: Turn off power to your heating and cooling system at the circuit breaker or fuse box before installing or touching any wiring. Verify the power is off. Treat every wire as if it could carry current, regardless of what it’s labeled.
An Amazon Smart Thermostat needs a C-wire (or Amazon’s own C-Wire Adapter) to power on at all, no exceptions, and that’s straight from Amazon’s own quick start guide, not a third-party assumption. On a heat pump, it also needs the reversing-valve wire landed on O/B. Everything else follows the standard thermostat wiring guide; this page covers the two things a heat pump adds: the O/B terminal and (if your equipment has backup strips) W2 and E.
TL;DR
| Your situation | What to do |
|---|---|
| No C wire | Required. Use a spare conductor in the wall, or Amazon’s C-Wire Adapter (repurposes G and Y as C and K) |
| Heat pump, no backup strips | R, Y, G, O/B, C (five terminals) |
| Heat pump with backup electric heat | Add W2 |
| Heat pump with backup and emergency heat | Add E as well |
| System blows the wrong temperature | Switch the O/B setting in the Alexa app; it’s a setting, not a wire |
| Old thermostat’s C wire lands on a terminal labeled “B” | Normal on Trane/American Standard boards, same wire, different label |
The terminal set for a heat pump, from Amazon’s own diagram
Amazon’s printed quick start guide includes a “Most Common HVAC System Types” reference showing exactly which terminals a heat pump uses, building up as backup heat is added:
| System | Terminals |
|---|---|
| Heat pump (no backup) | R, Y, G, O/B, C |
| Heat pump with backup electric heat | R, Y, G, O/B, W2, C |
| Heat pump with backup electric heat and emergency heat | R, Y, G, O/B, W2, E, C |
R is 24V power, Y runs the compressor (for both heating and cooling on a heat pump), G is the fan, and O/B is the reversing valve. That’s the whole heat-pump-specific set: two more terminals than the base thermostat needs, W2 and E, and only if your equipment has backup strips.
The C wire is not optional
Amazon’s documentation is unambiguous: the thermostat “requires a C-wire or C-wire power adapter,” and if the C wire is missing, the display stays blank. This is a harder requirement than some smart thermostats, which can run on battery-backed power sharing. The Amazon Smart Thermostat does not have that fallback.
If your wall cable has no spare conductor to press into service as C, Amazon’s own C-Wire Adapter installs at the HVAC control board instead of pulling new cable. The mechanism is specific: at the thermostat, the adapter relabels the G terminal as C and the Y terminal as K. Once it’s wired, the C and K terminals carry wires and neither Y nor G should have anything landed on them. The adapter uses the fan and cooling legs as a data path to synthesize a common wire, so those terminals aren’t available for their normal jobs while the adapter is active. Follow the adapter’s own included instructions for the control-board side; don’t wire it from memory.
One more note worth checking before you assume you have no C wire: on Trane and American Standard equipment, the terminal block on the furnace or air handler board that other manuals call “C” is sometimes printed “B” instead. Same wire, different silkscreen. Confirm at the control board rather than ruling out a spare conductor because its printed letter doesn’t match.
O/B: the reversing valve, and the one-setting fix
The reversing valve wire (orange on most installs, though color varies) lands on the O/B terminal. Whether the thermostat should treat it as O or B depends on which way your specific heat pump’s valve energizes:
- O: energizes in cooling. The common default, though Amazon doesn’t publish a brand-by-brand list; check your equipment’s data plate to be certain.
- B: energizes in heating. More common on Rheem and Ruud equipment.
If the system heats when you ask for cooling (or the reverse), Amazon’s own troubleshooting guidance is to switch the O/B setting in the Alexa app. It is never a wiring fix. The wire stays on O/B either way; only the app setting changes which direction it fires.
Backup and emergency heat: W2 vs. E
If your heat pump has electric-resistance backup strips, that wire lands on W2, and it runs automatically alongside the compressor when the heat pump alone can’t keep up. E is a separate, manual emergency-heat mode that disables the compressor and runs backup heat alone, for a failed compressor, not routine cold weather. Wire each to its own terminal; don’t combine them onto a single W terminal, which the thermostat won’t interpret as heat-pump-aware backup logic.
Coming from an old mercury or heat-pump-specific thermostat
Older heat-pump sub-bases, especially round mercury-switch thermostats, sometimes use an X2 terminal for auxiliary or backup heat instead of a plain W. On a like-for-like swap, X2 commonly maps to the Amazon Smart Thermostat’s W2/AUX terminal, but this is a field pattern, not something Amazon’s own documentation addresses directly. Confirm against your equipment’s control-board wiring diagram before landing it, and when in doubt, photograph the old thermostat’s wiring and label each wire by terminal letter before disconnecting anything.
Testing after you wire it
- Restore power at the breaker and complete setup in the Alexa app.
- Test cooling first. A cooling call should blow cold air within a few minutes. That confirms O/B is set correctly.
- Test heat. A heat call should blow warm air. If either test gives you the opposite temperature, switch O/B in the app, not the wire.
- Test backup heat if wired. Force a cold-weather scenario or use a test mode if your system offers one, so you know the strips work before you need them.
- Check the display stays on. If it goes dark or resets, the C-wire connection (or adapter wiring) needs another look. Recheck that C and K (if using the adapter) are seated and that Y and G are empty.
FAQ
Does the Amazon Smart Thermostat work with a heat pump?
Yes. Amazon’s own installation material documents heat pump wiring directly: R, Y, G, O/B, C as the base set, with W2 and E added for backup and emergency heat.
Does the Amazon Smart Thermostat need a C wire?
Yes, always: a C wire or Amazon’s C-Wire Adapter. Without one, the display doesn’t power on. This is a firmer requirement than some competing smart thermostats.
What does the O/B terminal do?
It controls the heat-pump reversing valve. O energizes it in cooling on most brands; B energizes it in heating on Rheem/Ruud and similar equipment. If heating and cooling are swapped, fix it in the Alexa app’s O/B setting, never at the terminal.
My old thermostat’s common wire is on a terminal labeled “B,” not “C.” Is that a problem?
No. Trane and American Standard equipment sometimes print “B” on the terminal block for what every other brand calls “C.” It’s the same common wire; wire it to the Amazon thermostat’s C terminal as usual.
Can I install it without a C wire using a spare conductor?
If there’s an unused wire in your existing cable bundle, yes: run it to the C terminal on both the HVAC board and the thermostat. If there’s no spare conductor, use Amazon’s C-Wire Adapter instead of skipping the requirement. General options (not Amazon-specific) are covered in installing a thermostat without a C wire.
Where can I find the non-heat-pump wiring basics?
The heat pump thermostat wiring guide covers O/B conventions across brands, two-stage compressors, and dual-fuel systems in more depth than any single-brand page. The thermostat wiring guide covers every terminal for any system, heat pump or not.
| 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 and C-wire requirements verified against Amazon’s own Amazon Smart Thermostat Quick Start Guide; always confirm against your equipment’s own wiring diagram and the current guide at amazon.com/amazonsmartthermostat/help before installing.
