Honeywell Thermostat Wiring: From the Round T87 to the T10, Every Terminal

SAFETY: Turn off power at the breaker or furnace switch before touching any thermostat wiring. Confirm the display is dark (or the system is verifiably dead) before pulling the thermostat off the wall. The control side is 24V AC, but a short can blow the board fuse or cook the transformer.

Honeywell has kept the same terminal letters for 70 years: R is 24V power, W is heat, Y is cooling, G is fan, C is common. Wire a 1965 round T87 or a 2026 T10 Pro and the letters mean the same thing, so the job is always the same: photograph the old wiring, note which color sits on which letter, and land each wire on the matching terminal of the new thermostat.

What changes between generations is the hardware around those letters: jumpers on the retail models, a slider tab on the T10, and a K terminal that carries two signals on one wire. This page covers all of it; for the terminal system itself (every letter, every wire count), start with the full thermostat wiring guide.

TL;DR

Honeywell terminal conventions by generation

The letters are stable; the quirks are generational. Find your model family:

Two universal rules across every family. First, wire colors are a convention, not a standard: Honeywell’s own support docs say so, and a previous installer’s “white” can land anywhere. Go by the terminal letter stamped next to the screw. Second, a healthy system reads roughly 24-28V AC across R and C; that one measurement separates wiring problems from equipment problems.

Reading an old Honeywell sub-base

Replacing an old Honeywell means decoding what’s there before you disconnect anything:

1. Kill power at the breaker or furnace switch.
2. Pull the cover. On round models the wiring is on the sub-base behind the body (the body twists or snaps off). On rectangular models, the faceplate pulls straight off the wall plate.
3. Photograph twice: one wide, one close enough to read the letter stamped beside each screw.
4. Label each wire with tape by letter, not color. A red wire on W is a heat wire, whatever its color says.
5. Note every jumper: a bare staple or short wire connecting two terminals (commonly R to Rc, or Rh to Rc). Write down which terminals it joined; the new thermostat replaces its function with its own jumper, slider, or internal bridge.
6. Ancient boiler stats: some heat-only sub-bases show two unlabeled terminals (or T-T). Those are functionally R and W: the thermostat is just a switch in a 24V loop.

If the sub-base letters don’t match modern conventions at all, the thermostat wiring guide has the full terminal glossary.

Wiring by system type

Standard heat/cool: 4 or 5 wires

R, C, W, Y, G is the modern standard and every current Honeywell installs on it directly: R to R (jumper in place or slider UP), C to C, W to W, Y to Y, G to G.

With 4 wires (no C), a Wi-Fi Honeywell needs constant power. The RTH6580WF, for instance, requires a C wire outright. Your options: a spare conductor in the wall, the K terminal trick below (T-series only), or an add-a-wire module.

Heat pump: O/B, plus W2/AUX for backup heat

Heat pump wiring adds two things to the standard set. The reversing valve wire lands on O/B, and you tell the thermostat which convention your equipment uses in installer setup: O (valve energized in cooling) for most brands, B (energized in heating) for Rheem/Ruud. Wrong setting = heat when you asked for cooling; the fix is in the menu, not the wiring.

Backup heat strips land on W2/AUX. On IAQ/EIM systems the staging logic and the W2 landing live on the Equipment Interface Module at the air handler, so wire to the module’s diagram, not the generic one. Model-by-model walkthrough: Honeywell heat pump thermostat wiring.

Full-house systems: 7 and 8 wires

Seven wires usually means a heat pump with aux and emergency heat (R, C, Y, G, O/B, AUX, E); eight adds a second compressor stage (Y2). The terminal-by-terminal map is in the 7-wire Honeywell thermostat wiring diagram, and every wire-count diagram is collected on the thermostat wiring diagrams page.

Rc, Rh, and the T10’s R Slider Tab

Most homes have one 24V transformer and one R wire; some older homes run separate heating and cooling transformers, giving two R wires (Rh and Rc). The two cases must be wired differently, and this is where Honeywell’s generations diverge:

• Retail RTH models: a physical metal jumper joins R and Rc from the factory. One R wire: leave the jumper in and land the wire on R. Two R wires: remove the jumper, Rh wire to R/Rh, Rc wire to Rc.
• T10/T10+ Pro: no jumper at all. A built-in R Slider Tab on the back does the job: slider UP for one R wire (R and Rc internally joined), slider DOWN for two (kept separate). Set it before landing wires.
• Pro series: conventional jumper wire between Rh and Rc; same leave-in/remove logic as retail models.

Never jumper Rc to Rh when two live transformers are present: out-of-phase transformers can put up to 48V across the short and blow the board fuse or the transformers themselves. Quick test: measure between the two R wires; more than about 12V AC means two live, out-of-phase feeds, so keep them separate. The full decision tree (including zoning panels) is in the Rc/Rh jumper guide.

The K terminal: Honeywell’s one-wire C fix

T-series thermostats (T5, T6, T9, T10) carry a terminal you won’t see on other brands: K. Paired with the THP9045A1023 adapter (Honeywell calls it the WireSaver), it solves the missing-C-wire problem without new cable:

1. At the air handler or furnace board, the WireSaver module connects to the C and G terminals.
2. The two conductors that used to serve G and C at the thermostat (or the single G conductor on a 4-wire system) connect to the module’s thermostat side.
3. At the thermostat, the freed-up conductor lands on K. One wire now carries both fan control and common power; the thermostat separates the signals internally.

The trade-off: independent fan control passes through the module, and the module must match the thermostat (K only works on models that have the terminal). On a 4-wire system this converts “no C wire” into “full-time power plus fan” with 20 minutes at the equipment. If your thermostat has no K terminal, use a standard add-a-wire kit or plug-in transformer instead.

How to replace a Honeywell thermostat: 8 steps

1. Kill the power at the breaker or furnace switch. Confirm the display (if any) is dead.
2. Photograph the old wiring, wide and close.
3. Label each wire by terminal letter with masking tape.
4. Note jumpers and sliders. Write down what the old unit bridged. Set the new T10’s slider (or jumper) to match your transformer count before wiring.
5. Unscrew the wires and clip the cable to something (clothespin, tape) so it can’t fall into the wall.
6. Mount the new wall plate and land each wire on the matching letter. Seat screws firmly; a loose W is an intermittent no-heat call in January.
7. Run installer setup: system type, fuel, stages, O or B for heat pumps. On IAQ/EIM systems the module handles most of this.
8. Restore power and test every mode: heat, cool, fan, and aux/emergency if present. A heat pump that heats on a cooling call needs the O/B setting flipped.

FAQ

Are Honeywell thermostat wires color coded?
By convention only: red to R, white to W, yellow to Y, green to G, blue to C. Honeywell’s own documentation says colors vary by installer. The terminal letter is the truth; the color is a guess.

Does a Honeywell Wi-Fi thermostat need a C wire?
Yes, plan on it. The RTH6580WF requires one outright, and the T-series models want constant power too. T-series owners can use the K terminal with the THP9045 WireSaver instead of pulling new cable.

What is the K terminal on my Honeywell thermostat?
A combined fan-plus-common terminal used with the THP9045A1023 WireSaver module. The module installs at the equipment’s C and G terminals and lets one existing conductor deliver both signals, which frees up a wire on 4-wire systems that lack a C.

Where does the jumper go on a Honeywell thermostat?
Between R and Rc (factory-installed on retail models), and only when you have a single R wire. Two R wires: remove it. On the T10 Pro there is no jumper; set the R Slider Tab UP for one R wire, DOWN for two. Full logic: Rc/Rh jumper wiring.

How do I wire a Honeywell thermostat for a heat pump?
Reversing valve to O/B, compressor to Y, fan to G, power to R and C, backup strips to W2/AUX, then select O or B in installer setup (O for most brands, B for Rheem/Ruud). Step-by-step: Honeywell heat pump thermostat wiring.

Can I replace a round T87 myself?
Usually, yes: it’s a 2-wire (R, W) or 4-wire job and any modern thermostat handles it. Two cautions: old T87s contain a mercury bulb (take it to hazardous-waste disposal, not the trash), and if the wires are thick with wire nuts, that’s line voltage, not a T87 system, and it’s an electrician’s job.

Last reviewed: July 2026