Smart thermostats and the C wire

What a C wire is

A C wire, or common wire, is the wire that feeds a thermostat's C terminal with 24 V AC power. Wyze describes its thermostat as powered by 24 VAC through the C terminal, and says the C-wire powers the thermostat. Wi-Fi thermostats built this way depend on that wire being present at the wall.[1]

A home without a C wire is common enough that ecobee states it directly: not all homes have the C wire needed to power an ecobee thermostat. Makers answer the gap with a battery, an adapter, or a design that draws power in other ways.[5]

Adapters and extender kits

ecobee's Power Extender Kit works by combining the functions of the Y and G wires into a single wire, which frees a wire for the C connection. It comes in the box with the Smart Thermostat Premium and Enhanced. ecobee does not include it with the Smart Thermostat Essential or the Lite, so it is sold separately for those, and ecobee lists the kit as working with the Essential. The Essential has Wi-Fi 6 at 2.4 GHz, Bluetooth 5.4 and a sub-GHz radio for SmartSensors, so the kit is the route to the C connection on it.[5][7]

Wyze includes a C-wire adapter in the box of its Wyze Thermostat. Honeywell Home lists a free C-wire adapter for the X2S. LG lists an LG C-Wire Power Adapter at $24.95, sold separately, and Schneider Home sells its C-Wire Power Adapter and Trim Plate separately. Both thermostats were announced on 2026-10-13, and LG's adapter price is its announced list price.[1][2][8][9]

Aqara's Thermostat Hub W200 replaces a 24 VAC thermostat and lists power as a C wire or the Aqara C-Wire Adapter.[3][10]

Models that list a C wire as required

• Honeywell Home X2S Smart Thermostat: Honeywell lists a C wire (24 VAC) as required.[2]
• Wyze Thermostat: powered by 24 VAC through the C terminal, with a C-wire adapter in the box. Wyze states it controls low voltage (24 V) heating systems only and does not work with 120 V or 240 V electric baseboard heat.[1]
• Aqara Thermostat Hub W200: 24 VAC through a C wire or the Aqara C-Wire Adapter.[3]
• Honeywell Home T6 Pro Z-Wave Thermostat: three AA batteries or a C wire.[11]

Models designed to work without one

Google designs its Nest thermostats to run in many homes without a C wire. The Nest Learning Thermostat (4th gen) is designed to work whether or not the home has a C wire, and Google says that in limited cases, under 1 percent, a C wire or the Nest Power Connector may be necessary.[4][12]

The Nest Thermostat (2020) works without a C wire on most systems. Google lists heating-only, cooling-only, zone-controlled and some heat pump systems as ones that may need a C wire or a Nest Power Connector, and refers owners to g.co/nest/cwire for details. Google lists it as working with conventional systems that have one stage of heating and cooling and a second stage of either, and with heat pumps that have one stage and either aux or emergency heat.[4][13]

ecobee takes a third approach. Its Power Extender Kit, in the box with the Premium and Enhanced, gives a home without a C wire a path to the C connection.[5]

Thermostats that double as hubs

A thermostat powered from the wall can host radios that battery devices cannot. Aqara's Thermostat Hub W200 replaces a 24 VAC thermostat and is also a Zigbee hub, a Thread border router, a Matter controller and a Matter bridge. Aqara lists it for the contiguous U.S. The tado Wireless Receiver in the 2nd generation starter kit is likewise a Thread border router, and it runs on 230 V AC mains.[3][10][14]

A border router in the thermostat adds to the Thread mesh for other Thread devices. Apple states that Thread accessories need a Thread-enabled home hub such as an Apple TV 4K or HomePod, so a home with a HomePod or Apple TV already has one, and a thermostat hub is an additional border router.[15][16]

HomeKit and Matter by model

Power and platform support are independent. ecobee documents HomeKit setup for its thermostats. Honeywell's X2S combines a required C wire with Matter certification.[2][17][18]

Google lists the Nest Learning Thermostat (4th gen) with Wi-Fi 802.11n at 2.4 and 5 GHz and with power terminals C, RH and RC, alongside heating, cooling, fan and accessory terminals.[12]

Battery-powered and mains-powered receivers

tado's thermostats avoid the question by splitting the hardware. The Wired Smart Thermostat X runs on three AAA batteries, and the 2nd generation has a USB-C rechargeable battery. The Wireless Smart Thermostat X (2nd gen) Starter Kit has a receiver on 230 V AC mains that connects to the boiler or heat pump and acts as a Thread border router, and a sensor with a rechargeable battery.[6][14][19]

tado's wired thermostat connects over Matter over Thread and needs a Bridge X or another Thread border router.[6][20]

Honeywell Home lists the T10+ Pro as hardwired and as the replacement for the ElitePRO S1200, and Honeywell lists the T6 Pro Z-Wave as battery or C-wire powered.[11][21]