I have an older thermostat I'd like to replace with the more modern Nest thermostat. The online Nest compatibility checker tells me that I have a compatible device. However, I'm not sure how it could work since my existing thermostat runs on batteries. How would the Nest draw it's power? The older thermostat (pictured) has R, W, Y wires and a B which is not connected to anything. The other end of the wire (pictured) mixes colors with two smaller wires (the heater and the AC I'm assuming?). Is this a DIY job or something that should be handled by professionals?

Thanks in advance!

thermostat connections

  • Can you post a wiring diagram for your furnace + some zoomed out shots of the wiring there? Dec 17, 2018 at 3:57

1 Answer 1


The Nest runs on power supplied by your furnace. Typically the red wire is +24v. It doesn't need battery backup partly because its scheduling and other configuration data are stored online. It may also have some non-volatile memory for basic data.

In some cases, the common (C) wire isn't present. In your case, you have an extra conductor that should serve that purpose if properly connected. Here's more on that.

No need to call in the pros. If you find that something doesn't work, contact Nest. If all else fails, hook the old thermostat back up until you figure out the issue.

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