Honeywell T5 Smart Thermostat Wiring Diagram
Heat only
Forced-air furnace: gas, oil, electric or fan coil.
Terminals and components · 4
- R · 24 V
- Common
- Heat
- Heat 2
- Fan
How to set it
With a single R wire (it may have been on R, Rh or Rc on the old thermostat), set the switch up and insert that wire into R. With two wires, set it down: the Rc wire goes into Rc and the R or Rh wire into R.
Jumpers
The T5 needs no jumpers: remove any from the old thermostat.
What it does
Brings 24 V AC to the thermostat, which sends it back through W, Y, G… to call for heat, cooling or fan.
How to connect it
A single R wire goes into R with the switch up, even if it was on Rh or Rc on the old thermostat.
What it does
Completes the 24 V circuit and powers the thermostat continuously.
Required
The T5 does not work without a C wire. If the old thermostat had none, run a spare conductor from the equipment's C (common) terminal.
What it does
On a call for heat the thermostat connects R to W and the equipment starts heating.
What it does
Energized in addition to W when more capacity is needed.
Setup
Set 2 heat stages in the thermostat setup.
What it does
With electric or fan coil equipment, the thermostat also runs the fan when calling for heat.
What it does
On gas or oil furnaces, the furnace runs its own fan while heating; G is for fan-only operation.
Cool only
Air conditioning without heating.
Terminals and components · 5
- R · 24 V
- Common
- Compressor
- Compressor 2
- Fan
How to set it
With a single R wire (it may have been on R, Rh or Rc on the old thermostat), set the switch up and insert that wire into R. With two wires, set it down: the Rc wire goes into Rc and the R or Rh wire into R.
Jumpers
The T5 needs no jumpers: remove any from the old thermostat.
What it does
Brings 24 V AC to the thermostat, which sends it back through W, Y, G… to call for heat, cooling or fan.
How to connect it
A single R wire goes into R with the switch up, even if it was on Rh or Rc on the old thermostat.
What it does
Completes the 24 V circuit and powers the thermostat continuously.
Required
The T5 does not work without a C wire. If the old thermostat had none, run a spare conductor from the equipment's C (common) terminal.
What it does
Energizes the outdoor unit's compressor contactor.
What it does
Energized in addition to Y when more capacity is needed.
Setup
Set 2 cool or compressor stages in the setup (maximum 2).
What it does
On a call for cooling, the thermostat runs the fan with the compressor. It also allows fan-only operation.
Heating and cooling
Conventional system (H = heat stages, C = cool stages), with one or two transformers.
Terminals and components · 6
- R · 24 V
- Rc · cooling 24 V
- Heat
- Heat 2
- Common
- Compressor
- Compressor 2
- Fan
How to set it
With a single R wire (it may have been on R, Rh or Rc on the old thermostat), set the switch up and insert that wire into R. With two wires, set it down: the Rc wire goes into Rc and the R or Rh wire into R.
Jumpers
The T5 needs no jumpers: remove any from the old thermostat.
Two transformers
Typical of a boiler or furnace with a separate air conditioner: each unit has its own transformer.
What it does
Brings 24 V AC to the thermostat, which sends it back through W, Y, G… to call for heat, cooling or fan.
How to connect it
A single R wire goes into R with the switch up, even if it was on Rh or Rc on the old thermostat.
What it does
Brings 24 V AC to the thermostat, which sends it back through W, Y, G… to call for heat, cooling or fan.
How to connect it
With two transformers, R takes the heating unit's one (R or Rh wire on the old thermostat).
How to connect it
Only with two transformers: the old thermostat's Rc wire goes into Rc and the R switch is set down.
What it does
On a call for heat the thermostat connects R to W and the equipment starts heating.
What it does
Energized in addition to W when more capacity is needed.
Setup
Set 2 heat stages in the thermostat setup.
What it does
Completes the 24 V circuit and powers the thermostat continuously.
Required
The T5 does not work without a C wire. If the old thermostat had none, run a spare conductor from the equipment's C (common) terminal.
What it does
Completes the 24 V circuit and powers the thermostat continuously.
Required
The T5 does not work without a C wire. If the old thermostat had none, run a spare conductor from the equipment's C (common) terminal.
Two transformers
The common must come from the cooling transformer.
What it does
Energizes the outdoor unit's compressor contactor.
What it does
Energized in addition to Y when more capacity is needed.
Setup
Set 2 cool or compressor stages in the setup (maximum 2).
What it does
On a call for cooling, the thermostat runs the fan with the compressor. It also allows fan-only operation.
Boiler
Hot water or steam heat, straight to the boiler or through a zone valve.
Terminals and components · 4
- R · 24 V
- Common
- Heat
How to set it
With a single R wire (it may have been on R, Rh or Rc on the old thermostat), set the switch up and insert that wire into R. With two wires, set it down: the Rc wire goes into Rc and the R or Rh wire into R.
Jumpers
The T5 needs no jumpers: remove any from the old thermostat.
What it does
Brings 24 V AC to the thermostat, which sends it back through W, Y, G… to call for heat, cooling or fan.
How to connect it
A single R wire goes into R with the switch up, even if it was on Rh or Rc on the old thermostat.
What it does
Completes the 24 V circuit and powers the thermostat continuously.
Required
The T5 does not work without a C wire. If the old thermostat had none, run a spare conductor from the equipment's C (common) terminal.
What it does
When heat is called, W sends 24 V to the boiler's thermostat input.
Check
Some boilers have thermostat terminals without 24 V AC: then there is no R and C to use and a separate transformer or zone control is needed. Check the boiler manual.
How to connect it
W goes to the valve motor; the other motor lead returns to the transformer common (C). Once open, the valve usually starts the boiler through its own end switch.
Setup
System type: boiler (hot water or steam).
Heat pump
Air-to-air or geothermal, with or without auxiliary heat. W is not used on heat pumps.
Terminals and components · 6
- R · 24 V
- Common
- Compressor
- Compressor 2
- Fan
- Reversing
- Auxiliary
- Emergency
How to set it
With a single R wire (it may have been on R, Rh or Rc on the old thermostat), set the switch up and insert that wire into R. With two wires, set it down: the Rc wire goes into Rc and the R or Rh wire into R.
Jumpers
The T5 needs no jumpers: remove any from the old thermostat.
What it does
Brings 24 V AC to the thermostat, which sends it back through W, Y, G… to call for heat, cooling or fan.
How to connect it
A single R wire goes into R with the switch up, even if it was on Rh or Rc on the old thermostat.
What it does
Completes the 24 V circuit and powers the thermostat continuously.
Required
The T5 does not work without a C wire. If the old thermostat had none, run a spare conductor from the equipment's C (common) terminal.
What it does
Energizes the compressor contactor. On a heat pump, Y runs in both cooling and heating; the O/B valve sets the mode.
What it does
Energized in addition to Y when more capacity is needed.
Setup
Set 2 cool or compressor stages in the setup (maximum 2).
What it does
On a call for cooling, the thermostat runs the fan with the compressor. It also allows fan-only operation.
Setup
In the setup choose O (valve energized in cooling) or B (energized in heating), as the heat pump maker specifies.
Check
If the unit heats on a call for cooling, the O/B setting is reversed.
What it does
Adds backup heat (for example, electric heat strips) when the heat pump cannot reach the setpoint.
Caution
On a heat pump, nothing connects to W: it can keep the heat running continuously.
What it does
Used in EM Heat mode: heats with the backup source only, for example when the heat pump has failed.
About This Diagram
These diagrams are redrawn from the Honeywell T5 Smart Thermostat manual, and we offer them completely free and in English. We have also categorized it in the Thermostats (HVAC & Water Heating) section on Diagramverse, and it has been rated by 1 user with an average rating of 5.0. If you have any specific questions about the Honeywell T5 Smart Thermostat Wiring Diagram, we suggest checking the Frequently Asked Questions (FAQs) section.
Printable Honeywell T5 Wiring Diagrams
Wiring Instructions
Before you start
You need a small flat-head screwdriver, a Phillips screwdriver, a wire stripper, needle-nose pliers, a pencil, a level and a drill with a 7/32 in bit for the wall anchors. A multimeter that reads AC volts helps you check the 24 V supply. The UWP wall plate comes with anchors and three #8 1-1/2 in screws.
Reuse the existing Honeywell T5 Smart Thermostat cable if it is in good condition. No wire gauge is specified for this thermostat. Standard 18 AWG solid thermostat cable is the usual choice for 24 V AC control wiring, with one conductor per terminal plus one for C. If no C wire reaches the thermostat, pull a new cable or have an HVAC technician add one before you install the T5.
Turn off power at the breaker or switch that feeds the heating and cooling system. Then change the setpoint on the old thermostat: if nothing starts within 5 minutes, the system is off. Remove the old faceplate. Thick black wires with wire nuts, or a thermostat rated 120 V or higher, mean a line voltage system such as electric baseboard heat, and the T5 will not work on it. It also does not work on millivolt systems. Call a licensed HVAC contractor if you find line voltage, wires on L/A, S or U terminals, or a heat pump with V and VR wires. Swapping a thermostat on existing low-voltage wiring rarely needs a permit, but check local rules before running new cable.
Pick the tab that matches your equipment: Heat only for a furnace without air conditioning, Cool only for air conditioning alone, Heating and cooling for a furnace plus central air, Boiler for a boiler or zone valve, and Heat pump when you have an O/B wire. A W2 or Y2 wire means a second stage. Separate R and Rc wires from two transformers match the 1H/1C · 2 transf. variant.
Step-by-step wiring
- Photograph the old wiring, including the letters beside each terminal, and label every wire with its terminal letter, especially if colors are faded or repeated.
- Remove any jumper, such as a short wire or metal staple between R and Rc. The T5 does not use jumpers.
- Disconnect the wires, remove the old wall plate and wrap the wires around a pencil so they cannot fall back into the wall.
- Feed the bundle through the back of the UWP and make sure at least 1/4 in of bare copper is exposed on each conductor.
- Level the plate, mark the holes, drill and insert the anchors flush with the wall. Leave the plate loose for now; wiring is easier off the wall.
- Set the R switch. With one R, Rh or Rc wire, slide it up and insert that wire in R. With two wires, slide it down, put Rc in Rc and R or Rh in R. If the old thermostat had R and RH, the R wire goes to Rc and the RH wire to R.
- Connect the common to C. On conventional systems an old C1, X or B wire is the common.
- Connect the remaining wires to matching letters: W and W2 for heat, Y and Y2 for cooling or compressor, G for the fan, plus O/B, AUX and E on heat pumps. Old W1 or H labels go to W, Y1 or M to Y, and F to G. On a heat pump with separate O and B wires, or with Y1, W1 and W2 wires, stop and call a qualified contractor.
- Press each terminal tab, insert the wire into the inner hole, one wire per terminal, and release. Tug each wire gently to confirm it holds.
- Push excess wire into the wall, mount the UWP with all three screws and close its door. Compare every terminal with your photo, then snap the thermostat firmly onto the plate.
Testing and start-up
Restore power at the breaker or switch. The display should come on; if it stays blank, the C circuit is not complete. Pull the thermostat off the plate and measure AC volts between R and C: you should read about 24 V AC.
Go through the installer setup and set the heating system type, the heating equipment type, the number of cool or compressor stages and heat stages, and, on heat pumps, whether O/B energizes in cool (O) or in heat (B). Wrong settings cause as many start-up complaints as wrong wires.
Test each mode: Heat with the setpoint above room temperature, Cool with it below, then fan On. Allow up to 5 minutes for a response; Heat On or Cool On flashing is compressor protection, not a fault. On a heat pump, confirm warm air in Heat, cool air in Cool, and that Em Heat runs the backup heat without the compressor.
Safety notes
- The thermostat circuit is 24 V AC, but the furnace, air handler and outdoor unit carry 120 V or 240 V. Keep equipment power off while you work and leave equipment panels to qualified technicians.
- Never let a bare R wire touch C: the short can blow a low-voltage fuse on the control board, if fitted, or damage the transformer.
- Never connect the T5 to 120–240 V line voltage heaters or millivolt systems.
- On two-transformer systems the R switch must be down so the transformers stay separate.
- If the old thermostat has a sealed mercury tube, do not trash it; ask your local waste authority about recycling.
Troubleshooting
| Symptom | Probable Cause | What to Check |
|---|---|---|
| The screen stays blank after I turned the power back on | No C wire, C wire loose at the thermostat or equipment, or equipment power still off | Remove the thermostat and measure AC volts between R and C on the plate: expect about 24 V AC. If you read 0 V, check the breaker, the equipment power switch, the furnace door and the C connection at the control board. |
| The thermostat turns on but the furnace or AC never starts | Equipment service switch off, furnace door not closed, or compressor delay running | Make sure the equipment switch is on and the furnace door is closed securely. Set Heat above or Cool below room temperature and wait 5 minutes before you decide it has failed. |
| The heat runs nonstop as soon as power is restored | A leftover jumper or bare copper touching between R and W | Turn off power, take the thermostat off and look for staples, short jumper wires or stray strands across terminals. Trim each wire so only about 1/4 in is bare and sits fully inside the terminal. |
| My heat pump blows cold air in Heat and warm air in Cool | Reversing valve setting does not match the equipment, or O/B wire on the wrong terminal | Confirm the O/B wire is on O/B, then change the O/B setting in the installer setup from O (energize in cool) to B (energize in heat), or the reverse. |
| I cannot select Cool mode | Cool or compressor stages set to 0 | In the installer setup, set cool stages to 1 or 2 and make sure the Y wire is landed on Y. |
| Heat works but the AC does nothing on my system with two transformers | R switch left up, or the Rc wire missing or in the wrong terminal | Turn off power, set the R switch down, put the cooling transformer wire in Rc and the heating wire in R. Use the 1H/1C · 2 transf. variant. |
| The fan does not come on when the heat is running | Heating equipment type set wrong, or G wire not connected | Set the heating equipment type to match your furnace or fan coil, and check the G wire at both the thermostat and the equipment. |
| The thermostat went dead and the system stopped right after I wired it | R shorted to C, blowing the control board fuse or overloading the transformer | With power off, check that no R strand touches C. Inspect the low-voltage fuse on the control board, if fitted, and replace it with the same rating. Measure 24 V AC between R and C again. |
| The second stage never comes on | W2 or Y2 not connected, or stages set to 1 | Confirm the wire sits in W2 or Y2 and set heat stages or compressor stages to 2 in the installer setup. |
| Em Heat or auxiliary heat does nothing on my heat pump | AUX or E wire missing, or backup heat stages set to 0 | Check that the heat strip wire is in AUX or E, and set backup heat stages to 1. |
| The system works only sometimes or cuts out randomly | A wire not fully seated in the push terminal or not stripped enough | Press each terminal tab, reinsert the wire into the inner hole with 1/4 in of bare copper, release and give it a gentle tug. |
