Smart home · fix it
Smart thermostat wiring? Here's what to do first
Photograph the old wiring, match the letters, and most swaps take under an hour. The one thing to check first is whether your system has a C wire.
DIY with caution
Doable for a careful homeowner — read the safety note first and stop if anything doesn't match what you see.
Intermediate About 60 min
Kill the HVAC breaker before touching wires. If you find high-voltage wiring or no low-voltage letters, stop and call an HVAC pro.
Video: Google Nest (YouTube)
What to do, in order
- Step 1: Turn off the furnace/air handler at its breaker — not just at the thermostat.
- Step 2: Pull the old thermostat face off and photograph the wires with their letters clearly visible.
- Step 3: Label each wire, then release them one at a time.
- Step 4: Mount the new base, feed the wires through, and match each one to the same letter.
- Step 5: Restore power, then finish setup in the app and run a short heat and cool test.
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This page also helps with
- replace old thermostat
- c wire
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