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 runs on line voltage and is not DIY.
Seven wires on a Honeywell thermostat almost always means a heat pump with both automatic backup heat and a manual emergency-heat switch: R, C, Y, G, O/B, AUX, and E. That’s one more terminal than the heat pump thermostat wiring baseline (R, C, Y, G, O/B), split into two distinct backup-heat circuits instead of one. This page covers that configuration terminal by terminal, plus the two-stage furnace/AC seven-wire pattern that shows up less often. If your color code doesn’t match, the full thermostat wiring guide covers every standard wire and how to trace a non-standard setup.
TL;DR
| Terminal | Wire (typical color) | What it does |
|---|---|---|
| R | Red | 24V AC power from the transformer |
| C | Blue | Common; constant power for the thermostat |
| Y | Yellow | Compressor (heating and cooling on a heat pump) |
| G | Green | Indoor fan relay |
| O/B | Orange | Reversing valve: O energizes in cooling (most brands), B in heating (Rheem/Ruud) |
| AUX | White | Automatic backup heat, stages in with the compressor in cold weather |
| E | Brown | Emergency heat: manual override that runs backup heat alone, compressor off |
Match the terminal letters, not the wire colors. Photograph the old thermostat before disconnecting anything, and label each wire by the letter it occupied.
The 7 terminals, one at a time
R — 24V power. The transformer feed. On systems with a single transformer this is often labeled Rc on Honeywell sub-bases; a jumper (or the T10 Pro’s slider tab) bridges Rc to Rh so one wire can power both circuits.
C — common. Gives the thermostat constant 24V power. Without it, battery-only operation or power-stealing is the fallback, and both ecobee and most Honeywell Wi-Fi models either require C or strongly recommend it.
Y — compressor. On a heat pump, Y runs the compressor in both heating and cooling; the O/B terminal decides which direction the refrigerant flows, not Y.
G — fan. Runs the indoor blower relay independent of a heat or cool call, which is what the thermostat’s FAN ON setting does.
O/B — reversing valve. The terminal that makes a heat pump a heat pump. O energizes the valve in cooling mode on most manufacturers; B energizes it in heating mode on Rheem, Ruud, Weathermaker, and Ameristar equipment. You select O or B in the thermostat’s installer setup; you never move a wire to change this. Wiring it right but setting it wrong is the single most common heat-pump complaint: the system heats when you ask for cooling, or the reverse.
AUX — automatic backup heat. Electric heat strips (or a furnace, on a dual-fuel system) that stage in automatically when the compressor alone can’t keep the house at temperature in cold weather. This is different from emergency heat: AUX runs with the compressor, adding capacity, not replacing it.
E — emergency heat. A manual override, selected on the thermostat itself (usually labeled EM HEAT), that shuts the compressor off entirely and runs backup heat alone. Use it only when the heat pump itself is malfunctioning: running EM HEAT routinely because “it’s cold outside” costs far more to operate than the compressor.
7-wire Honeywell color code
| 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. |
The color-to-terminal mapping above is the widely followed convention, and Honeywell’s own support documentation says explicitly that it isn’t a rule: colors vary by installer and system. Verify by terminal letter on the old thermostat or at the air-handler control board, not by wire color.
Which 7-wire system do you have?
Honeywell heat pump installs land on seven wires a few different ways, depending on staging:
| Configuration | Terminals used | What’s different |
|---|---|---|
| Single-stage heat pump, AUX + E | R, C, Y, G, O/B, AUX, E | The configuration this page leads with |
| Two-stage heat pump, single backup circuit | R, C, Y1, Y2, G, O/B, W2/AUX | No separate E; W2 does both automatic and manual backup |
| Two-stage furnace, two-stage AC (no heat pump) | R, C, W1, W2, Y1, Y2, G | No O/B at all; see heat pump thermostat wiring to confirm you don’t have this pattern instead |
If your old thermostat shows W1/W2 and Y1/Y2 with no O or B anywhere, you likely have the third pattern: a two-stage conventional system, not a heat pump. The AUX/E terminals on this page don’t apply. Check for O or B specifically before assuming heat-pump wiring.
Diagnostic: is the problem the thermostat or the equipment?
If the system doesn’t respond to a heat call, you can isolate the thermostat from the equipment without any special tools. With power off, bridge R (or Rh), Y, and G together directly at the equipment’s control board terminals, not at the thermostat. If the compressor and fan start, the equipment is fine and the problem is upstream at the thermostat or its wiring. If nothing happens, the fault is at the equipment, not the thermostat. Restore the wiring exactly as photographed afterward; this is a diagnostic bridge, not a permanent wiring change.
Step-by-step: wiring a 7-wire Honeywell thermostat
- Kill the power at the breaker or the outdoor unit’s disconnect. Confirm the display is dead.
- Photograph the old wiring, close enough to read every terminal letter.
- Label each wire by letter, not color: R, C, Y, G, O/B, AUX, E.
- Disconnect and secure the cable so it can’t slide back into the wall.
- Mount the new Honeywell base, leveled, and land each wire on its matching letter. Confirm the R/Rc jumper matches your system: leave it in place for a single transformer, remove it if you have separate Rh and Rc wires.
- Set the installer menu: system type (heat pump), O or B reversing-valve logic for your brand, and number of backup-heat stages.
- Restore power and test all functions in order: cooling, heating, then force emergency heat briefly to confirm the strips respond and the compressor stays off.
Troubleshooting 7-wire heat pump problems
| Symptom | Most likely cause | Check |
|---|---|---|
| Heats when set to cool, or the reverse | O/B set to the wrong logic for your brand | Installer menu O/B setting; Rheem/Ruud use B, most others use O |
| Backup strips run constantly, even in mild weather | AUX staging threshold set too high, or system stuck in EM HEAT | Confirm EM HEAT is off; check the aux/backup-heat lockout setting |
| No heat at all, compressor silent | Y or O/B loose, or a low-ambient compressor lockout | Tug-test Y and O/B; if it’s genuinely cold outside, a lockout may be normal, so check the equipment manual |
| Emergency heat does nothing when selected | E wire loose or not landed at the air handler | Trace E to the air-handler terminal strip; confirm it lands on the emergency-heat relay, not AUX |
| Smart thermostat reboots or drops Wi-Fi | Weak or missing common power | Confirm C reads a steady ~24-28V AC against R |
FAQ
What do the 7 wires on a Honeywell thermostat mean?
On a heat pump: R (power), C (common), Y (compressor), G (fan), O/B (reversing valve), AUX (automatic backup heat), and E (manual emergency heat). If your system uses W1/W2 and Y1/Y2 instead of O/B, you likely have a two-stage furnace and AC, not a heat pump. See the 7-wire configuration table above.
What’s the difference between AUX and E on a Honeywell thermostat?
AUX is automatic: the thermostat brings on backup heat alongside the compressor when it can’t keep up on its own. E is manual: selecting EM HEAT on the thermostat shuts the compressor off entirely and runs backup heat alone. Use EM HEAT only when the heat pump itself is broken.
Do I need both O and B wires, or just one?
Just one physical wire, usually labeled O/B on the terminal strip; you tell the thermostat in the installer menu whether your equipment uses O-energized-in-cooling (most brands) or B-energized-in-heating (Rheem, Ruud). There’s no separate O and B wire to land.
Is 7-wire Honeywell wiring different from other brands?
The terminal letters are universal: R, C, Y, G, O/B, AUX, E mean the same thing on Nest, ecobee, and Honeywell. What’s Honeywell-specific is the hardware around R: retail models often ship with a physical jumper between R and Rc, and the T10 Pro replaces it with a slider tab. See Honeywell thermostat wiring for every generation’s quirks.
Can I install a 7-wire heat pump thermostat myself?
The wiring itself is 24V low-voltage and DIY-safe once the power is off. Where to stop: anything at the outdoor unit’s disconnect, refrigerant lines, or line-voltage wiring to the air handler. If the wiring doesn’t match any diagram here, photograph it and get a pro’s eyes on it before guessing.
⚠️ 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 HVACR School documentation; always confirm against your equipment’s own manual.
