Google Nest Thermostat Wiring: C Wire, *OB Star Terminal, Rh/Rc and Heat Pumps

1. Which Nest you have

Google sells three thermostat lines, and their wiring rules differ. This article is about the one Google simply calls the Nest Thermostat: the model released in 2020 with a mirrored glass front, a 2.4 inch color screen and two AAA batteries, according to Google’s Nest thermostat technical specifications.

• Nest Learning Thermostat (3rd and 4th gen): a metal ring you turn, more connectors and a star terminal with different jobs. See our Nest Learning Thermostat (4th gen) wiring page.
• Nest Thermostat E: a frosted white display. Its star terminal works much like the one described here, but check its own guide.

Everything below is specific to Google’s Nest Thermostat unless it says otherwise. Don’t carry these rules over to an ecobee or Honeywell.

2. What each connector does

Google’s spec page lists what the Nest Thermostat can control: one stage of heating (W1) and cooling (Y1) and a second stage of either heating (W2) or cooling (Y2), heat pumps with auxiliary or emergency heat, the fan (G) and power (C, Rh or Rc). Google’s Identify a thermostat wire page explains each label:

The table counts terminals, not wire colors. Google’s compatibility page says to enter the labels on your old thermostat’s connectors, not the wire colors. Google also notes that color alone is not a reliable guide, for example for O/B wires. Our thermostat wire color guide shows the usual colors and where they go wrong.

3. The *OB star terminal

Google’s Nest thermostat star terminal page lists three jobs for it on the Nest Thermostat and Thermostat E:

Three rules from that page matter during install:

1. Only use it when the app says so. The Nest app builds a custom wiring diagram from the wires you enter. Put a wire in the star terminal only if that diagram tells you to.
2. One wire, one job. The terminal holds one wire. You can’t use it for O/B and a second stage at the same time.
3. Tell it the job during setup. The app’s equipment questions set whether the wire is W2, Y2 or O/B. If you don’t know what the wire did, Google suggests checking how your old thermostat was wired.

Because there is one extra slot, a conventional system with both two-stage heat and two-stage cooling (W1, W2, Y1, Y2) has more wires than the Nest Thermostat can use. Run your wires through Google’s compatibility checker at g.co/nest/works or our wiring tool before you buy.

4. When you need a C wire or Power Connector

Google’s common wire article says heating-only, cooling-only, zone-controlled and heat pump systems need a C wire or a power accessory. For the Nest Thermostat on other systems, the app often tells you after a power test during setup whether you should connect a C wire.

Google lists these as signs of a power problem:

• The thermostat battery drains fast or the display won’t turn on.
• Motion sensing is disabled, or it drops off Wi-Fi now and then.
• Heating, cooling or the fan runs constantly or won’t turn on.
• Help codes E1, E297, N260 or N261.

If the old thermostat has no C wire, look for a spare wire folded back in the wall first. If there isn’t one, the Nest Power Connector takes the place of a C wire. Google’s Power Connector help page says:

• It installs at the furnace or air handler control board (or the zone controller on a zoned system), not behind the thermostat.
• It works only with 24 VAC systems. Millivolt and high-voltage systems are not compatible.
• Finish installing the thermostat first. Afterwards, run the power test under Settings > Equipment > Power Connector to confirm it is detected.
• If the control board has no C terminal, Google recommends a professional.

Our no C wire guide compares this with other brands’ adapters, and the C wire terminal page explains what the common wire does. A “No power to Rh” message is a different problem; see Nest E74, E73 and E195 errors.

5. Wiring a Nest Thermostat, step by step

These steps follow Google’s Install your Nest thermostat page and the setup flow in the Google Home app.

1. Turn off power to the heating and cooling system. Google notes some systems have more than one breaker; turn them all off. Change the old thermostat’s setting and wait a few minutes to confirm nothing starts.
2. Remove the old thermostat’s face and take a photo of the wiring and labels.
3. Enter your wires in the app by their terminal labels. Leave out jumper wires. The app tells you whether the setup is compatible and draws a custom diagram.
4. Label each wire with the stickers from the quick start card as you disconnect it. Don’t let the wires fall back into the wall.
5. Mount the Nest base and pull the wires through.
6. Insert each wire. Press the connector button, push the wire in as far as it goes and release. Google says the button should stay down. Put no more than one wire in each connector.
7. Leave spare wires unconnected unless the app tells you otherwise. Google says not to connect wires the old thermostat did not use.
8. Attach the display, restore power and finish setup in the app, including the equipment questions for any wire in *OB.

Not sure a wire is live? Our multimeter guide shows how to check for 24 V.

6. Heat pump wiring and the B wire

On a heat pump, the O/B wire goes in the *OB star terminal. Y1 runs the compressor, G runs the fan and a C wire or Power Connector is required. Google’s spec page lists heat pumps with auxiliary or emergency heat as supported; follow the app’s custom diagram for where the aux heat wire goes on your system. Google strongly recommends professional installation for dual fuel systems (a heat pump with a gas furnace).

Watch for a wire labeled B. Google’s wire identification page warns that in some installations the common wire is disguised as a B wire, and that a common wire in any connector other than C can blow a fuse or damage the thermostat. Before choosing O/B, check where that wire lands on the control board. Some brands use B for the reversing valve; others, including some Rheem and Ruud systems, use B as common. Our Rheem and Ruud B wire guide walks through the check, and the O/B terminal page explains O versus B.

If the system heats in cool mode and cools in heat mode after install, the O or B setting is the likely cause. Our heat pump thermostat wiring guide covers the rest of a heat pump install.

7. Systems it will not work with

Google’s Nest thermostat compatibility page lists these as not compatible:

• High-voltage (line-voltage) systems, such as 120 or 240 V electric baseboard heaters. The spec page rates the Nest Thermostat for 20 to 30 V AC.
• Stranded wires.
• Millivolt systems, often found on gas fireplaces and some wall furnaces.
• Proprietary systems whose thermostat terminals are labeled 1, 2, 3 or A, B, C instead of the standard letters.

Google also notes that some zoned systems and micro-controller based systems need a C wire, and that a gas valve that buzzes or vibrates can usually be fixed by connecting one.

8. Frequently asked questions