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 line voltage (208/240V); leave the disconnect and the outdoor cabinet alone.
A Honeywell heat pump thermostat wires like any Honeywell thermostat with one addition: the O/B terminal, plus (depending on your system) an L fault-input wire, a second Y2 for two-stage compressors, and AUX/E for backup heat. For the universal terminal set behind every 24V system, see the thermostat wiring guide. For Honeywell’s own generational quirks (jumpers, the T10’s R Slider Tab, the K terminal), start with the Honeywell thermostat wiring guide. For the O/B logic that applies to every brand, not just Honeywell, see heat pump thermostat wiring. This page is the Honeywell-specific wire-count walkthrough: what changes as you go from 6 wires to 10.
TL;DR: which table applies to you
| Wire count | Configuration | What’s added over the base 5-wire set |
|---|---|---|
| 6 | Single-stage heat pump, no backup heat | O/B (reversing valve) |
| 7 | Single-stage heat pump + aux/emergency heat | O/B, AUX/E |
| 8 | Two-stage heat pump (2 compressor stages), no backup heat | O/B, Y2 |
| 10 | Two-stage heat pump + aux/emergency heat | O/B, Y2, AUX/E |
| any | Fault indicator present on the old thermostat | add L (heat pump fault input) |
Two rules apply at every wire count. First, never land a wire on the plain W terminal on a heat pump system. Honeywell’s own FocusPRO installation documentation states this in capital letters: “DO NOT CONNECT ANY WIRE TO W FOR HEAT PUMP APPLICATIONS! THIS CAN CAUSE HEAT TO RUN CONTINUOUSLY.” Backup heat goes on AUX/E or W2, never plain W, on a heat-pump system. Second, wire colors are a convention, not a standard. Go by the terminal letter stamped next to the screw, not the color of the wire.
6-wire: single-stage heat pump, no backup heat
The base heat-pump configuration. One compressor stage, one reversing valve, no electric backup strips.
| Terminal | Function |
|---|---|
| R (or Rc) | 24V power |
| C | Common |
| Y | Compressor contactor |
| G | Fan relay |
| O/B | Reversing valve; pick O or B in installer setup |
| (none) | W stays empty; see the caution above |
If your old thermostat had a single R wire, keep the factory jumper in place (retail RTH models) or set the T10’s R Slider Tab UP. Two R wires (Rh/Rc): remove the jumper or set the slider DOWN.
7-wire: single-stage heat pump with aux/emergency heat
Adds one wire for backup heat. This is the most common full configuration on a single-compressor heat pump with electric strips.
| Terminal | Function |
|---|---|
| R, C, Y, G, O/B | Same as the 6-wire set above |
| AUX/E | Auxiliary or emergency heat relay |
| L (if present) | Heat pump fault input; connects only if your specific heat pump has a fault-output terminal, leave it unconnected otherwise |
AUX and E are related but not identical: auxiliary heat stages in automatically when the compressor can’t keep up in cold weather; emergency heat is the manual override that runs backup heat alone, with the compressor locked out. On some control boards they share one terminal; on others they’re separate. If your old thermostat distinguished them, wire to match. Don’t guess.
8-wire: two-stage heat pump, no backup heat
Two compressor stages (two Y wires) instead of one, still no electric backup. This wire count shows up on Honeywell’s T6 Pro / TH6320U-family thermostats, which support a 2H/2C (two heat, two cool stage) heat-pump mode; confirm your exact model supports it before wiring, since the full model list wasn’t independently verified for this page.
| Terminal | Function |
|---|---|
| R, C, G, O/B | Same as above |
| Y | Compressor contactor, stage 1 |
| Y2 | Compressor contactor, stage 2; energizes in addition to Y1, not instead of it |
| L (if present) | Heat pump fault input |
10-wire: two-stage heat pump with aux/emergency heat
The full configuration: two compressor stages plus electric backup.
| Terminal | Function |
|---|---|
| R, C, G, O/B, Y, Y2 | Same as the 8-wire set |
| AUX/E | Auxiliary or emergency heat relay |
| L (if present) | Heat pump fault input |
The L terminal: heat pump fault input
L is not universal. It only gets wired if your specific heat pump exposes a fault-output terminal at the outdoor unit. Honeywell’s own installation documentation is direct about this: the L wire connects “only if heat pump has a fault terminal,” and it drives the thermostat’s fault indicator light so you know when the outdoor unit is reporting a problem (a stuck contactor, a lockout, a control-board fault) rather than a simple loss of power. If your old thermostat’s sub-base had no L terminal, don’t add one; there’s nothing on your equipment to wire it to.
The S terminal: two different jobs depending on the model family
S shows up on more than one Honeywell line, and it does different things on each. Check which product family’s install sheet applies to your thermostat before wiring it:
- FocusPRO N100 series: S pairs with a specific outdoor-sensor accessory (the C7089U1006) used to lock out auxiliary heat above a set outdoor temperature, so backup strips don’t run on a mild day when the compressor alone could handle the load.
- T6 Pro / TH6320U family: the S terminals on the mounting plate connect either a wired outdoor sensor or a wired indoor remote sensor, and you tell the thermostat which one it is in installer setup.
If your sub-base has an S terminal and you’re not sure which job it does, leave it unconnected and confirm against your specific model’s install card. Wiring the wrong sensor type to it won’t damage anything, but it won’t do what you expect either.
O/B setup: T-series and Pro-series
The reversing-valve wire always lands on the O/B terminal; there’s no separate O screw and B screw to choose between on the wall unit. What you choose is a setting, in installer setup, not a terminal:
- O (valve energized in cooling) covers most brands.
- B (valve energized in heating) covers Rheem and Ruud.
Get the setting backwards and the tell is unmistakable: the system heats when you call for cooling, or vice versa. Fix it in the installer menu, not by moving the wire. On T10/T10+ Pro thermostats specifically, two related setup codes matter for heat-pump balance: ISU 3120 sets the compressor’s outdoor-temperature lockout point, and ISU 3090 (Backup Heat Differential, adjustable 2-15°F) controls how long the compressor runs before backup heat stages in. Both live in the same installer setup menu as the O/B selection.
W2/AUX on Equipment Interface Module (EIM) systems
On VisionPRO IAQ and some Pro-series installs, the wall thermostat doesn’t talk directly to W2/AUX. A module at the air handler (the Equipment Interface Module) does the staging, and the wall unit just tells the module what to do. If your system has an EIM, the W2/AUX wiring and terminal labels live on the module’s install sheet, not the generic wall-plate diagram, and they vary by module generation. Locate the EIM’s own documentation before wiring backup heat on these systems.
How to wire it: 7 steps
- Kill power at the breaker and at the outdoor unit’s disconnect. Confirm the display is dark.
- Photograph the old wiring, wide and close enough to read the terminal letters.
- Label every wire by terminal letter, not color.
- Count your wires and match to the tables above; that tells you whether you’re working with a 6-, 7-, 8-, or 10-wire heat pump.
- Set the jumper or R Slider Tab to match your transformer count before landing any wires.
- Land each wire on its matching letter. Leave W empty on a heat-pump-only system; if you have a separate furnace (dual-fuel), see below.
- Run installer setup: system type, O or B, stages, and (on T10/T10+ Pro) the ISU 3120/3090 balance-point codes if you want to tune when backup heat kicks in. Restore power and test heat, cool, fan, and aux/emergency.
Dual-fuel systems (heat pump + gas furnace)
Dual-fuel systems pair a heat pump with a gas furnace as backup instead of electric strips, and some Honeywell/Resideo thermostats use an outdoor sensor on the S terminals to control the switchover temperature between the two. The exact switchover logic varies by thermostat generation and isn’t confirmed here for every model. Check your specific equipment’s install sheet; if you’re replacing an older dual-fuel Trane or similar setup, old Trane thermostat wiring to new thermostat covers a comparable rewire.
| 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. |
FAQ
How do I wire a Honeywell thermostat for a heat pump?
Reversing valve to O/B, compressor to Y (and Y2 on two-stage systems), fan to G, power to R and C, backup heat to AUX/E (never plain W), then select O or B in installer setup. Wire count (6, 7, 8, or 10) depends on whether you have backup heat and a two-stage compressor.
What is the L terminal on a Honeywell heat pump thermostat?
The heat pump fault input. It only gets wired if your outdoor unit has a fault-output terminal, and it drives the thermostat’s fault indicator light.
Can I use the W terminal on a heat pump system?
No. Honeywell’s own installation documentation warns that connecting a wire to W on a heat-pump system can cause heat to run continuously. Backup heat belongs on AUX/E or W2.
What’s the difference between O and B on a Honeywell thermostat?
Both control the same reversing valve; O energizes it in cooling (most brands), B energizes it in heating (Rheem/Ruud). You pick O or B in installer setup, not by choosing a different terminal. There’s only one O/B terminal on the wall unit.
Does the T10 Pro have a jumper for heat pump wiring?
No jumper. The T10/T10+ Pro uses the R Slider Tab (UP for one R wire, DOWN for two) for the R/Rc connection. The O/B, aux, and balance-point settings are separate, in installer setup codes ISU 3120 and ISU 3090.
⚠️ 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
