No C wire? Which smart thermostats still work
Oct 23, 2026 · 2 sourcesYou take the old thermostat off the wall and count the wires. If there's no C (common) wire, the one that gives a smart thermostat steady power, you might think you're stuck. In most homes you're not. Here's how Google and ecobee handle it, when you still need a C wire, and what you get once it's on the wall.
Nest Learning Thermostat: Power Sharing
Google says the Nest Learning Thermostat (4th gen) is designed to work in most homes without a C wire, using what it calls Power Sharing, and that it works with 85% of systems.
The exceptions matter: heating-only, cooling-only, zoned and heat pump systems may need a Nest Power Connector or a C wire.
Sources: [1] Google
ecobee: a Power Extender Kit in the box
ecobee's Smart Thermostat Premium runs on a C wire, but ecobee includes a Power Extender Kit that lets it install in most homes without one.
Sources: [2] ecobee
Check before you buy
Both makers say "most homes", not all. Before you order, take a photo of the wires behind your current thermostat and check it against the maker's compatibility information. That's quicker than a return.
Sources: [1] Google, [2] ecobee
What you get once it's installed
Both come with a room sensor, so the thermostat can heat or cool for the room you're in instead of the hallway it hangs in. Nest includes one Nest Temperature Sensor (more cost $39.99 each); ecobee's SmartSensor also detects which rooms are occupied.
On schedules they differ: Nest learns your routine and builds the schedule itself, while ecobee's Schedule Assistant suggests changes for you to accept. Prices: Nest is $279.99 and ecobee Premium $259.99.
Sources: [1] Google, [2] ecobee
Keep going
- Can a smart thermostat work without a C-wire?
- Can a smart thermostat balance temperature room by room?
- Can a smart thermostat set its own schedule?
Status pages: Google Nest Learning Thermostat (4th gen) · ecobee Smart Thermostat Premium
How we know (2 sources)
Google Store page for the Nest Learning Thermostat (4th gen), read Oct 23, 2026: $279.99, includes one Nest Temperature Sensor (2nd gen) for room-by-room comfort (extra sensors $39.99). Automatically creates a schedule by learning your routine and preferences. Designed to work in most homes without a C wire (Power Sharing); some systems, such as heating-only, cooling-only, zoned and heat pump systems, need a Nest Power Connector or C wire. Works with 85% of systems.
Read the sourceProduct page for the ecobee Smart Thermostat Premium, read Oct 23, 2026: $259.99; ecobee claims savings of up to 26% on energy bills. Includes a SmartSensor that detects which rooms are occupied and adjusts the temperature, and lowers heating or cooling when the home is empty. eco+ uses occupancy and schedules; Schedule Assistant recommends schedule changes based on your routine. Runs on a C wire, with an included Power Extender Kit for most homes without one. Works with Apple Home; Siri or Alexa built in. Works with over 95% of HVAC systems.
Read the source