Honeywell Thermostat Wiring: Terminals, Slider and Setup

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the maker’s manual

Honeywell Thermostat Wiring: Terminals, Slider and Setup

Short answer

On a Honeywell T6 Pro the wires go to the letters they are marked with, and two things catch people out. The R slider tab is a built in jumper that must match one R wire or two, and on a heat pump Honeywell says never to use the W terminal. Then the installer setup has to be told whether the changeover valve is O or B.

Honeywell thermostats are wired by letter, like every other thermostat, but the popular Pro models have two details of their own that decide whether the system works after the swap.

Everything on this page comes from Honeywell’s own installation instructions for the T6 Pro, document 33-00181EFS, now published under the Resideo name. Other Honeywell models use the same letters, and their own instructions have the final word on anything specific to them.

Before you start

What to carry: a small screwdriver, a phone for a photo, and a pen. Thirty minutes.

Power off at the breaker for the furnace or air handler. Do it before any wire comes off. A thermostat wire touching the wrong terminal while live can blow the fuse on the furnace board.

Gas, electric or heat pump? It matters here more than anywhere. A gas or oil furnace with air conditioning is wired one way, and a heat pump is wired another, with its own setting in the menu.

Photograph the old wiring before a single wire comes off. Then label each wire with the letter it came from.

Three mistakes that stop a new thermostat working. The R slider tab in the wrong position. It is a built in jumper between R and Rc. It has to match whether you have one R wire or two.

A wire on W on a heat pump. Honeywell’s heat pump diagrams say it plainly: do not use the W terminal for heat pump applications.

The O or B setting left wrong. A heat pump that heats when you ask for cooling, and cools when you ask for heat, is usually this one menu setting, not a fault: how O and B reverse a heat pump.

What each terminal is for

The terminals on the T6 Pro wall plate, as Honeywell names them. Most systems only use five or six of them.

Terminal	What Honeywell says it is
R	24 volt power from the heating transformer
Rc	24 volt power from the cooling transformer
C	24 volt common, from the cooling transformer on a two transformer system
W	Heat, first stage
W2	Heat, second stage
Y	Compressor contactor, first stage
Y2	Compressor contactor, second stage
G	Fan
O/B	Heat pump changeover valve
AUX	Auxiliary heat on a heat pump
E	Emergency heat on a heat pump
L/A	Heat pump fault input, needs a C wire
S	Wired indoor or outdoor sensor, some models only
K, U	Marked unused on the T6 Pro

The R slider tab

One R wire: slider up. A wire on R and another on Rc: slider down. It replaces the little jumper wire older thermostats used.

Check 1Free, one minute

Set the slider to match your R wires

Only one R wire. On R, Rc or RH at the old thermostat. Honeywell says to set the slider to the up position, marked for one wire. That joins R and Rc inside the wall plate.

Two R wires, one on R and one on Rc. Your system has a separate heating and cooling transformer. Set the slider down, marked for two wires, so the two supplies stay apart.

If the old thermostat had a short wire joining R and Rc, that was the jumper. The slider does its job now, so leave that wire out.

The C wire. Honeywell says C is optional if batteries are fitted, but recommended. Without it the thermostat runs on its batteries: what to do when there is no C wire.

Furnace with air conditioning, or a heat pump

Check 2

Wire it the way Honeywell's diagram shows

Furnace and one stage of cooling. R, Y to the compressor, G to the fan, W to the heat, and C if you have one. Slider up for one transformer.

Two stage furnace and air conditioning. Add W2 and Y2, then in Installer Setup set the system type to 2 Heat/2 Cool conventional, as Honeywell’s note says.

A heat pump. R, Y to the compressor, G to the fan, O/B to the changeover valve, and C. Nothing on W. On a heat pump with backup heat, the backup wire goes to AUX, and E is emergency heat.

Then tell the thermostat which valve you have. Honeywell says to set the changeover valve in Installer Setup to O for cool changeover or B for heat changeover. On the T6 Pro that is setting 218. Most heat pumps use O, and some brands use B, which the heat pump’s own label or manual will say.

Hot water heat only. Honeywell’s diagrams use R, W and C, and set the heat system type to radiant with no cooling stages.

Power on and test

Check 3Free, ten minutes

Test every mode before you put the tools away

Snap the thermostat onto the wall plate and turn the breaker back on.

Heat. Set it five degrees above room temperature. The furnace or heat pump should start.

Cool. Set it five degrees below. The outdoor unit should start after its delay of up to five minutes.

Fan. Honeywell’s fan modes are Auto, On, and Circ, which runs the fan about a third of the time.

Nothing happens at all. Check the slider, then the breaker, then the small fuse on the furnace board, which a wire touching the wrong place can blow: how to prove a thermostat is really calling.

Put the heat setting back to where you want it once you have heard each part start.

Questions people ask

My old thermostat had a jumper between R and Rc. What do I do with it?

Leave it out. The slider tab on the Honeywell wall plate does the same job, set to the one wire position.

Where do I put the W wire on a heat pump?

On a Honeywell T6 Pro, not on W. Honeywell says not to use W for heat pump applications. A backup heat wire goes to AUX.

What gauge thermostat wire do I need?

Honeywell says 18 to 22 gauge thermostat wire, and shielded cable is not required.

The heat pump heats when I set it to cool.

The O or B setting is the wrong way round. Change it in Installer Setup and test again.

I have a wire that was on K on my old thermostat.

K is used by some wire saver setups that carry two signals on one wire. On the T6 Pro, Honeywell marks K as unused, so check the instructions for your old setup before connecting anything.

Do I need a C wire for a Honeywell thermostat?

For the T6 Pro, Honeywell says C is optional if batteries are fitted but recommended. Wi-Fi thermostats usually need one, so check the box before buying.

Sources: Honeywell Home T6 Pro installation instructions 33-00181EFS (Resideo)