Glossary
Equipment
C-wire (common wire)
The 'common' wire in thermostat wiring that provides continuous 24-volt power. Smart thermostats need constant power for their screens and Wi-Fi, which older thermostat wiring often didn't include.
Why it matters for your home
The missing C-wire is the number-one surprise in DIY smart-thermostat installs. Workarounds like power-stealing adapters exist but can cause odd behavior on some systems. If your wiring bundle has an unused wire, a technician can often repurpose it in minutes, worth checking before you buy the thermostat, not after.
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