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Five ways to get power to a smart thermostat with no C wire
Updated
If the checker says you need a C wire, you have options short of opening walls. Which one works depends on the brand you buy and how many wires you already have. Only one pick on this site, the ecobee Premium, lists its adapter as included.
| Fix | Works with | Limits |
|---|---|---|
| Use a spare wire in the bundle | Any brand | Both ends must be connected: C at the thermostat and C on the equipment board. |
| ecobee Power Extender Kit (PEK) | Premium (included), Essential (sold separately) | ecobee says you need at least 4 wires. It combines the jobs of G and Y to free a wire for C. Not for heat only or two transformer installs. |
| Nest Power Connector | Nest Thermostat, Nest Learning (4th gen) and older Nests | Sold separately. Google says if your system has no C terminal, you need a pro. |
| Use the G (fan) wire as C | Sensi names this option | You give up separate fan control from the thermostat. |
| Third party add a wire kit or a plug in 24 VAC transformer | Sensi names both. The Amazon listing points to a separate power adapter kit | Google warns that some third party C wire adapters can cause problems or damage a Nest, even if they claim to be Nest compatible. |
| Run a new wire | Any brand | A pro job in most homes. Google notes the cost depends on how hard it is to route wire through your walls. |
How the ecobee kit works
The PEK is a small module that mounts at the furnace or air handler, not at the wall. ecobee describes it as a device that attaches to the system control board. It lets four wires do the work of five. That is why ecobee sets a floor of 4 wires, and why a two wire heat only system is out. For those, ecobee says you need a new wire from the C terminal on the equipment, or third party parts.
How the Nest Power Connector fits in
Google says the Power Connector gives a Nest a stable power source when the system cannot deliver enough. Google also says you should finish installing the thermostat first, then add the connector at the equipment with power off. If the Google Home app tells you during setup that you need one, you may have to pause setup until you have it.
Which picks leave you shopping for a second part
In our heat pump example with no C wire, the checker puts ecobee Smart Thermostat Essential, Google Nest Thermostat, Sensi Smart Thermostat ST55 and Amazon Smart Thermostat in the "needs a C wire or an adapter sold separately" group. The ecobee Premium is the only one that covers it from the box.
Safety first. Turn off power at the breaker or the system switch before you pull a thermostat off the wall, then change the set temperature by 5 degrees and wait 5 minutes to confirm nothing starts. If you see thick wires, wire nuts, L1 or L2, or a 120 V or 240 V label, stop. That is line voltage. Put the cover back and call a licensed electrician.
Tick your own wires in the wire checker, or compare all seven picks on the roundup page.