Switch off your HVAC system at the fuse box. If the HVAC system uses a switch, you should also turn that off. This will prevent your thermostat and HVAC system from getting damaged as you re-insert the thermostat wires.

If the light blinks green and your thermostat is unresponsive for more than 15 minutes, it may have frozen during the update. WARNING: If you attempt to connect parts from different thermostat models, it may damage your thermostat as well as your system. A blinking green light near the top of your Nest thermostat’s display means that it’s updating the software, starting up, or restarting. Typically, this will only last a minute or two. Once your thermostat has finished starting up, it'll turn on so you can use it. If there's noblinking red light, it's possible that you’ve put wires into the wrong thermostat connectors. Make sure you've entered your system’s wires correctly into theapp and follow the wiring diagram it gives you.The Nest Thermostat can be in different colors. If you have a thermostat that looks like one of these, you have a Nest Thermostat. Your thermostat should automatically turn on once it’s connected to your system and your system is powered up. As long as the thermostat base is from the same model, generation, and country/region as the thermostat display, you can swap bases and displays. Note that your Nest thermostat may charge for about an hour, especially if the battery is very low. Now return to your thermostat to see if the problem is resolved. If not, keep reading. Confirm that Thermostat is Compatible with Your Heating System

Large items (delivered by two people, such as appliances and furniture) can be returned using our Home Collection service. Put the other end of the USB cable in a charger and plug it into a power outlet. Avoid using a computer to charge your Nest thermostat. Nest’s sleeker and thinner Learning Thermostat will slash your energy bill and boost your Smart Home credentials. Since 2011, Nest has helped save over 4 billion kWh of energy in homes around the world, and this supremely sophisticated Thermostat can work wonders for you too. This device gets to know you, and adjusts energy usage accordingly, so you only use what you need and get a stylish home accessory to boot. All the features, such as the new Farsight display, will stun you with their bill-busting, hyper-intuitive results. Important: Turn off power to your system at the system switch or fuse box. This helps protect your system and thermostat from possible damage. Proprietary Heating System: If your HVAC system only has 2 wires when you have both heating and cooling systems, your heating system is most likely proprietary.The display connector on the thermostat base is different for all 3 generations of the Nest Learning Thermostat. In addition, the connectors for the system wires are different between thermostat models. If you discover that your heating system is millivolt, proprietary, or high voltage, you might need to replace your Nest thermostat with a compatible alternative. That said, an installation expert can help you rewire your proprietary heating system to make it compatible with the Nest thermostat. Re-insert Your Thermostat’s Wires Appropriately Insert the two new AAA alkaline batteries. Be sure to match the positive "+" and negative "-" ends of the batteries with the markings in the battery compartment. If you suspect that the Nest thermostat isn’t working because you aren’t installing it correctly, consult an installation expert.