Nest Thermostat Wiring: Every Wire and Where It Goes

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

Nest Thermostat Wiring: Every Wire and Where It Goes

Short answer

Each wire goes to the Nest connector with the same letter, with three rules Google is firm about. Leave any jumper wires out, because a Learning Thermostat joins Rh and Rc inside itself. Put only one wire in each connector. And a common wire in any connector other than C can blow a fuse, which matters on systems where B is the common.

Wiring a Nest thermostat is mostly matching letters, and the Nest app will draw a wiring diagram for your own system as you go. The problems come from three things the old thermostat did that the Nest does differently.

Everything on this page comes from Google’s own Nest help pages, mainly Identify a thermostat wire, and Learn about thermostat jumper wires. Before buying, Google’s compatibility checker will tell you whether your system works with a Nest at all.

Before you start

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

Power off completely. Google’s words: your system uses high voltage, which can be dangerous, so turn the power off to the system completely. That means the breaker for the furnace or air handler, and the outdoor unit too.

Gas, electric or heat pump? It decides which connectors you use. A heat pump uses O/B, and often AUX or W2 for backup heat.

Photograph and label everything first. Google warns that if two or more wires are the same colour, you will not be able to tell them apart later.

Three rules Google is firm about. Leave jumper wires out. Google says not to connect jumper wires to a Nest thermostat. The Nest Learning Thermostat has an internal jumper between Rh and Rc and uses it by itself if your system needs it.

One wire per connector. Google says the connectors are built for one wire each, and putting more than one in can damage your system and the thermostat.

A common wire only goes in C. Google warns that a common wire in any other connector can blow a fuse on your system and damage the thermostat. On some older systems the common is labelled B, not C, which is how people get caught.

What each wire does

The labels you will actually meet, grouped the way Google explains them. Most houses use only a handful.

Label	What Google says it does
R, Rh, Rc	Power. Rh for heating and Rc for cooling when both exist
C	The common wire, which helps power the thermostat
W or W1	Controls the heating
W2	Second stage heat, or auxiliary heat on a heat pump
Y or Y1	Controls the cooling in most systems
Y2	Second stage cooling, or second stage compressor
G or G1	Controls the fan
O/B	The heat pump changeover valve
AUX	Auxiliary heat on a heat pump
E	Turns emergency heat on or off
S, L	Not needed, the Nest uses its screen instead

The R wires and the jumper

A short wire between two terminals on the old thermostat is a jumper, and it stays out. Two separate R wires from the system are not jumpers, and each goes to its own connector.

Check 1Free, five minutes

Sort out R before anything else

A short wire looping between Rh and Rc on the old thermostat is a jumper. Loosen the screws, take it off, and do not put it on the Nest.

One R wire from the wall. Google says an R wire can go into either Rc or Rh on a Learning Thermostat, which joins them inside when needed. The Nest Thermostat E has a single R connector.

Two R wires from the wall, one marked Rh and one Rc. Google says these are not jumper wires: you have separate heating and cooling systems. Rh goes in Rh and Rc goes in Rc.

Wires that were never connected. Google says most systems do not use every wire in the bundle. If a wire was not connected to your old thermostat and has no label, it typically is not connected to your system either, so leave it tucked away.

Heat pumps and the extra connectors

Check 2

Wire the heat pump connectors

O/B to O/B. Google calls it the changeover valve for heat pump systems. If your old thermostat had separate O and B wires, Google says that system may need professional help.

Backup heat. A wire marked AUX, or W2 on a heat pump, is auxiliary heat. E is emergency heat.

Then check the setting in the app. A heat pump that heats when asked to cool is often just the O or B setting, not a fault: how O and B reverse a heat pump.

Humidifiers, dehumidifiers and multi speed fans. Wires marked HUM, DH or DHUM, and G2 or G3, are in Google’s list of wires that need professional help to connect.

Wires that mean it will not work

Check 3

Stop if you find any of these

Labels Google lists as incompatible. Y3, the numbers 1 2 3 4, the letters A B C D, V or VR, and Water or H2O. Google says the numbers and letters point at proprietary systems and Y3 at three stages of cooling, though its spec sheet says the Learning Thermostat (4th gen) takes Y3 on its star connector.

Thick wires or wire nuts at the thermostat. That is mains voltage, found on baseboard heaters and some electric heat. Nest thermostats are low voltage and are not for these.

Two wires the same colour with no labels. Google’s advice is to get a professional rather than guess.

No C wire. Many Nest models can run without one, but some systems will not power them properly, and the symptoms look like faults: what a Nest without enough power does.

Questions people ask

Do I need to keep the jumper wire for my Nest?

No. Google says not to connect jumper wires to a Nest thermostat. The Learning Thermostat has an internal jumper between Rh and Rc.

My wire is labelled B. Where does it go?

It depends on the system. On a heat pump it can be the changeover valve, and on some older systems B is the common. Google warns that a common wire in any connector other than C can blow a fuse, so find out which yours is before connecting it.

Where does the star connector come in?

Google says the star connector can do one of several extra jobs, such as a humidifier, dehumidifier or emergency heat, one at a time. The app asks which when you set it up.

Can I put two wires in one Nest connector?

No. Google says the connectors take one wire each and more than one can damage your system and the thermostat.

The app says a wire is not detected.

Push the wire in until the button stays down, with a straight clean end. That is the first thing Google’s wiring checks ask for.

It works with a different thermostat but not with the Nest.

Power is the usual reason, a system with no C wire that cannot keep the Nest charged: what a C wire does and how to add one.

Sources: Google Nest Help: Identify a thermostat wire, Learn about thermostat jumper wires, Troubleshoot Nest thermostat help codes, and Nest thermostat technical specifications