Honeywell thermostat wiring goes by terminal letter, not by wire color: R and Rc carry 24-volt power, C is the common wire, W runs heat, Y runs the compressor, G runs the fan, and O/B, AUX and E are used on heat pumps. Before you disconnect anything, turn off power to the HVAC system at the breaker or furnace switch, photograph the old wiring, and label every wire with the letter of the terminal it came from. Wire colors are only a habit of installers, so never wire by color alone.
Quick facts
| Task | Identify and connect the low-voltage wires on a Honeywell Home thermostat |
|---|---|
| Appliance | Honeywell Home 24-volt thermostats with the UWP wallplate (T4 Pro, T5, T6 Pro, T6 Pro Smart, T9) and similar models |
| Time needed | 30 to 60 minutes for a like-for-like swap |
| Difficulty | Moderate |
| Tools needed | Phillips and small flat screwdriver, phone camera, wire labels or tape, pencil, needle-nose pliers, wire stripper |
| Most likely mistake | Wiring by color instead of by the old terminal letter, or leaving the R slider in the wrong position |
This guide explains Honeywell thermostat wiring using the terminal definitions in Honeywell Home (Resideo) installation guides. We have no diagrams here; instead, the tables below show what each terminal does and which terminals common systems use. Your own setup may differ, and Honeywell’s guides say the same: “This wiring is just an example, yours may vary.” For reset and setup help after wiring, see our Honeywell thermostat reset guide for every model.
Why should you label wires by terminal, not by color?
Because the color of a thermostat wire does not decide what it does; the terminal it connects to at the furnace or air handler does. Honeywell’s T9 installation guide says plainly that “there is no universal color” for the C wire. The T5+ guide warns that wire colors can fade, or two terminals can have the same color.
The safe method is the one Honeywell’s guides use: photograph the old wallplate, label each wire with its old terminal letter, and move it to the matching terminal on the new thermostat. At the furnace or air handler, the control board uses the same letters, which is how you confirm where a wire really goes.
What does each Honeywell thermostat terminal do?
Each letter controls one job, such as power, heat, cooling or the fan. The table uses the descriptions in Honeywell’s T4 Pro, T6 Pro Smart, T5+ and T9 guides. The color column shows colors installers often use, but they are not guaranteed and are not set by Honeywell.
| Terminal | What it does | Color often seen (not guaranteed) | Notes |
|---|---|---|---|
| R | 24 VAC power from the heating transformer (or the only transformer) | Red | With one R wire, set the R slider up (1 wire) |
| Rc | 24 VAC power from the cooling transformer | Red | Two R wires (R and Rc): set the R slider down (2 wires) |
| Rh | Heating transformer power on some old thermostats | Red | Honeywell’s T5 and T9 guides put an R or Rh wire into the R terminal |
| C | 24 VAC common; gives constant power to Wi-Fi models | Often blue or black; Honeywell says no universal color | Required on T5, T6 Pro Smart and T9; on two-transformer systems it must come from the cooling transformer |
| W | Heat, stage 1 (conventional systems) | White | Honeywell says do not use W on heat pumps |
| W2 | Heat, stage 2 (conventional systems) | No common color | Some guides group it with AUX in their wire lists |
| Y | Compressor, stage 1 (cooling, and heating on heat pumps) | Yellow | Honeywell lists Y and G as the first wiring check for no cooling |
| Y2 | Compressor, stage 2 | No common color | Only on two-stage equipment |
| G | Fan relay (indoor blower) | Green | |
| O/B | Heat pump changeover (reversing) valve | Orange is common for O; no common color for B | In installer setup, choose O (energize in cool) or B (energize in heat) |
| AUX | Heat pump auxiliary or backup heat | No common color | Used instead of W on heat pumps |
| E | Heat pump emergency heat relay | No common color | If you have only one wire for AUX and E, Honeywell says use AUX |
| K | Combined Y and G signal for the THP9045 C-wire adapter | Not applicable | Only used with the adapter |
| L/A | Heat pump fault input (L) on some models | No common color | Not used on every model |
| S, U | Wired sensors (S) and ventilation relay (U) | No common color | T9 cannot control U1 or U2; T5 does not support S or U |
A jumper is a short wire or metal staple linking two terminals, often R and Rc. Honeywell’s T5 and T9 guides say the new thermostat does not need jumpers; the built-in R slider does that job. Keep the jumper with the old wallplate, and do not label it as a wire.
Which terminals does your system use?
Most homes fall into one of a few common setups. These examples come from the wiring lists in Honeywell’s T4 Pro and T6 Pro Smart guides.
| System | Terminals used |
|---|---|
| Heat only (furnace or boiler, no fan control) | R, W, and C if available |
| Heat only with fan | R, W, G, and C |
| Cool only | R, Y, G, and C |
| 1 heat / 1 cool (furnace with AC), one transformer | R (slider up), W, Y, G, C |
| 1 heat / 1 cool, two transformers | R (heating), Rc (cooling), W, Y, G, C from the cooling transformer |
| 2 heat / 2 cool conventional | R, W, W2, Y, Y2, G, C |
| Heat pump, no backup heat | R, Y, O/B, G, C |
| Heat pump with backup heat | R, Y, O/B, G, C, AUX, plus E if present |
| Two-stage heat pump with backup heat | R, Y, Y2, O/B, G, C, AUX, E (and L on some models) |
If you see thick black wires with wire nuts, or a label of 120 volts or higher, you have a line-voltage system. Honeywell’s T9 guide says those are not compatible, and the T6 Pro Smart guide says it does not work with electric baseboard heat (120-240V) or millivolt systems.
How do you wire a Honeywell thermostat? (step by step)
These steps follow the order in Honeywell’s T9 and T5+ installation guides. Work slowly, and check each wire against your photo.
Total time: 45 minutes
Turn off power to the HVAC system
Switch off the breaker or the furnace switch that controls your heating and cooling. Honeywell notes that the thermostat’s own Off setting does not cut power to the equipment.
Confirm the system is off
Turn the old thermostat up (or down) so it should call for heat (or cool). Honeywell’s T9 guide says if nothing starts within 5 minutes, the power is off.
Photograph the old wiring
Pull the old thermostat off its wallplate and take several photos from different angles so every terminal letter shows.
Label every wire by its terminal
Put a label with the terminal letter (R, C, W, Y, G and so on) on each wire before you disconnect it. Do not label jumpers, and write down any wires you find in terminals not listed in your new guide.
Remove the old wallplate
Release the wires with a screwdriver and remove the old plate. Wrap the wires around a pencil so they cannot fall back into the wall.
Mount the new wallplate
Feed the wires through the UWP wallplate, level it, and fasten it with the supplied anchors and screws. Leave about 1/4 inch of bare copper on each wire.
Set the R slider and connect R
With one R wire (R, Rc or Rh), set the R slider up and insert it into R. With two R wires, set the slider down, put Rc into Rc and R or Rh into R.
Connect the rest, one wire per terminal
Push each labeled wire into the matching terminal until it locks, then tug gently to check it. On a heat pump, put backup heat on AUX or E, never on W.
Restore power and finish setup
Snap the thermostat onto the wallplate, turn the power back on, and complete installer setup so the system type matches your equipment.
How is heat pump wiring different?
Heat pumps use Y for both heating and cooling, O/B to switch between them, and AUX or E for backup heat. That is why Honeywell’s guides warn against putting a wire on W for heat pump systems.
- O/B setting: After wiring, set the reversing valve in installer setup to O (energized in cooling) or B (energized in heating). The T4 Pro guide calls this “Reversing Valve O/B.”
- Backup heat: The T6 Pro Smart guide says, “Do NOT use W for heat pump applications. Auxiliary heat must wire to AUX or E.”
- Symptoms of a mistake: Honeywell’s troubleshooting lists aux heat running in cooling, or cooling running on a call for heat, when a wire is attached to W on a heat pump.
Other brands label things differently; ecobee, for example, puts the heat pump AUX or W2 wire on its W1 terminal. Follow the guide for the thermostat you are installing.
What if you do not have a C wire?
Look for an unused wire first, then use a C-wire adapter if there is none. Honeywell’s T5, T6 Pro Smart and T9 require 24 VAC power from a C wire.
- Check the wire bundle in the wall for a spare wire that was folded back or not connected.
- If you find one, connect it to C at the thermostat and to the C terminal on the furnace or air handler control board, keeping any existing C wires connected.
- If there is no spare wire and you have Y and G wires, Honeywell’s guides say the THP9045A C-wire adapter can be used. It uses the K terminal in place of Y and G, which frees one wire to become C.
Honeywell’s T9 guide says adapter installs are more complicated on zoned systems (several thermostats on one furnace) and suggests using a pro. Battery-capable models such as the T4 Pro can run on AA batteries without a C wire; see our Honeywell thermostat battery guide.
⚠️ Safety first
- Turn off power to the HVAC system at the breaker or furnace switch before wiring work. The thermostat’s Off mode does not cut power.
- Disconnect power at the breaker before removing panels on the furnace or air handler.
- Do not connect a Honeywell 24-volt thermostat to line-voltage (120-240V) wiring; it can cause shock or equipment damage.
- Some furnaces will not power up if the door is not fully closed. Close it securely when you finish.
- If you remove an old thermostat with a sealed mercury tube, do not put it in the trash. Honeywell says to contact your local waste authority for recycling.
- If you have a gas furnace and you smell gas, leave the home and call your gas utility or 911 from outside. Do not operate switches.
When should you call a technician?
Call an HVAC technician when the wiring does not match a simple setup or the system does not run after a careful install. Clear signs include:
- You find thick black wires with wire nuts, or the old thermostat is labeled 120V or higher.
- There are wires on terminals your new thermostat does not list.
- You have a zoning panel, or need to run a new wire through walls.
- A fuse on the furnace control board blows, or the breaker trips, after you restore power.
- You see burned, corroded or brittle wires.
- The thermostat powers on but the furnace, AC or heat pump does not respond after 5 minutes.
If “Cool On” just flashes after power returns, that is normally the compressor delay; see our guide to the Honeywell thermostat blinking Cool On.
How can you avoid Honeywell thermostat wiring mistakes?
A few habits prevent almost every wiring problem.
- Always label wires by their old terminal letter, never by color.
- Take photos before you remove a single wire, and keep them.
- Check the R slider: up for one R wire, down for R plus Rc.
- Use 18- to 22-gauge thermostat wire if you add or replace wire, as Honeywell’s guides specify.
- Run installer setup right away so system type, heat pump settings and stages match your equipment. Our Honeywell T6 thermostat setup guide walks through those settings.
Frequently asked questions
What color is the C wire on a Honeywell thermostat?
There is no fixed color. Honeywell’s T9 guide states there is no universal color for the C wire. Blue or black are common, but always confirm by the terminal it connects to on the furnace or air handler control board.
Where does the red wire go on a Honeywell thermostat?
A red wire is usually the R (power) wire, but check the terminal it came from. With one R, Rc or Rh wire, set the R slider up and insert it into R. With separate R and Rc wires, set the slider down and connect each to its matching terminal.
Can I wire a Honeywell thermostat without a C wire?
Some models, like the T4 Pro and T6 Pro, can run on AA batteries. The T5, T6 Pro Smart and T9 need a C wire, or the THP9045A C-wire adapter when you have Y and G wires but no spare wire.
What is the O/B wire on a Honeywell thermostat?
O/B controls the reversing valve on a heat pump, which switches it between heating and cooling. After wiring it, set installer setup to O (energized in cooling) or B (energized in heating) to match your heat pump.
Where does the AUX or W2 wire go on a heat pump?
On Honeywell T-series thermostats set up for a heat pump, backup heat goes to AUX or E, not W. If you only have one wire for both, Honeywell says to connect it to AUX.
Related error codes
- How to reset a Honeywell thermostat (every model)
- Honeywell T6 thermostat: setup, settings and troubleshooting
- Honeywell thermostat blinking “Cool On”: what it means and how to fix it
- How to change a Honeywell thermostat battery
- How to set a Honeywell thermostat (heat, cool and schedule)
- Ecobee aux heat: what it means and when it turns on
- All thermostat guides
Sources
- Resideo: T6 Pro Smart Thermostat Installation Guide (33-00392)
- Resideo: T4 Pro Thermostat Installation Guide (33-00187EFS)
- Resideo: T9 Smart Thermostat Installation Guide (33-00421EF)
- Resideo: T5+ Smart Thermostat Product Data (33-00474)
- Resideo: T6 Pro Programmable Thermostat Installation Instructions (33-00181EFS)
- Honeywell Home: My Thermostat Is Set to Cool but My Cooling System Isn’t Running. Why?
- ecobee: Smart Thermostat Premium Installation, Setup and User Guide
