Before you even buy a Nest thermostat, you should check to see if it is compatible with your home. To do this, use the Compatibility Checker.
You'll need to remove the cover from your thermostat and see which wires are connected. Simply follow the steps in the compatibility checker. If it is compatible, the guide will provide a wiring diagram that you'll need to use later.
As a safety precaution, go to your breaker box and locate the breaker that controls your heating and cooling systems. Switch the breaker off to cut off power to these systems.
If you're ready to install your Nest, you'll first need to remove the old thermostat. But before you get too far, make sure you label the wires. Your old thermostat should show a letter or combination of letters next to each wire. Use some tape and a pen to label each wire as you remove it.
After removing the old thermostat, there may be damage to the dry wall, screw holes, unpainted areas, etc. Now is the time to repair the dry wall and paint, if necessary.
If you'd rather not mess with patching and painting the wall, you can install the trim plate to cover everything up instead. We'll cover that in a little bit.
If you're going to install the trim plate, you can just slide it over the wires. In the following step, you'll position the base and screw both the base and the trim plate into the wall together.
Run the wires through the center of the base, and position it on the wall. Ensure the base is level using the built in level.
Then screw the base into the wall using the screws provided. These screws are self tapping and do not require dry wall anchors or a pre-drilled hole for installing in dry wall. For harder walls, you should pre-drill the holes using a 3/32 in drill bit.
Using the wiring diagram from step one, connect the wires to the base. Use needle nose pliers to position the wires more easily.
To install a wire, push down on the gray connector button and push the wire in fully. If the wire is connected properly the connector button will remain in place even when you let off.
Push the display onto the base until it snaps in place.
When I installed the Nest in my home, there were a few initial issues. First, the display wouldn't come on because the battery would never charge. As a first step, I charged the Nest via USB. Then with the display installed I tested the A/C unit. The A/C would come on, but only for a second. Also, the battery drained after only a few minutes. I did a little research and realized that I would probably need to use the C wire (or "common" wire).
The C wire is not typically necessary but may be required in some systems. In order to connect the C wire, I had to go into the attic and connect the C wire to my controller (see the image). Note that mine is a dual zone controller. After that I connected the C wire to the Nest base, and plugged the display back in. The display immediately indicated that it was charging, which it didn't do before. After a few minutes the display came on and I was able to configure the Nest.
Blinking red light
There are a few reasons the display might not turn on. The most common reason is that the battery is drained. If you snap the display into the base and see a red light blinking, this indicates that the battery is charging. If this is the case, the display should turn on soon.
Blinking green light
Typically a blinking green light means that the Nest is starting, restarting, or updating. But if the display never comes on, there is likely something else going on. If this is the case, it's possible you need to use the C wire from the previous step.
If you're seeing other issues, let me know in the comments, and I'll try to add more troubleshooting steps. In any case, if you're having issues you should be able to find the solution on the Nest support page.