Nest thermostat not charging – Causes and how to fix

Your Nest thermostat needs a constant power supply to maintain the Wi-Fi connection and backlit display to keep managing your home temperatures and saving you on costs. It has a rechargeable battery made of lithium-ion drained by powering these processes and recharges itself using your HVAC wiring system.

In case you see a blinking red on your thermostat’s near-top display then the battery charge is low, but worries not because it is charging. Your thermostat’s screen may go off and the Wi-Fi disconnected because the battery has been drained and the thermostat conserves the available power and uses it to control your system. What if it’s not charging?

 When your Nest thermostat is not charging, it’s becauseits internal battery is not getting a constant power supply from the HVAC system. This can be resolved by removing the base and connecting a Common wire (C-wire).

Some systems contain a C-wire or common wire and others don’t. If it has a C-wire, the Nest thermostat uses it to charge itself from the C-wire’s current while the one without relies on your heating and cooling system’s wires to recharge itself.

Summary of Causes and fix

What is causing my thermostat not to charge?

The C-wire ensures constant power flow to your thermostat even when your HVAC equipment is off unlike the rest of the wires in the system used for heating or cooling (labeled Rc and Rh respectively). Power flows from the R (red) wire though not continuously. For energy to flow continuously, it needs a common wire for the circuit to be complete.

Mostly, a Nest thermostat can work just fine without a C-wire since its getting power supply from your systems heating and cooling wires and use as little as 1kWh power a month. But in some cases, a common wire is needed. When installing the mobile app, it will let you if a C-wire is needed.

In a system lacking a C-wire, the Nest thermostat is still able to recharge itself using your HVAC’s heating and cooling system’s wires. This works fine if you are frequently running your heating and cooling. If it isn’t running often, then the Nest thermostat will stop any attempts to recharge itself and your furnace goes off. However, since it’s low on battery, the thermostat attempts to recharge again leading to a furnace on and off.

First, just to be sure it’s the power system and not another problem such as connecting to your Wi-Fi, detach your thermostat and charge it using a micro-USB. Power it on and see if it works. The thermostat comes with a USB port which you can use to quickly charge the thermostat’s battery.

How to connect a Common wire

To avoid any accidents as you’re dealing with an electrical appliance, go to the circuit breaker and turn it off for safety purposes

Checking for the Common Wire

Check to see if your system already has a Common wire. It’s usually located in the wall at the back of your thermostat and can be of any color and not always labeled.

Remove your thermostat from the wall and check the wires that connect to it and look for a wire labeled ‘C’ – this is the Common wire. If you can’t see it, don’t give up just yet. You can check for it in the wall as it might have been stuffed in there during installation since it was not needed at the time. Check for unused wires.

You can also check for it inside your AC/furnace. If you happen to see it, then possibly the other end is inside the wall behind your thermostat. Before dealing with any electrical stuff, it’s always a good idea to switch off the power at the main switch.

Once switched off, remove the cover to expose the wiring. Then check for wires coming from under a line of screws labeled R, C, W, W2, G, and Y/Y2. This can be easy or hard depending on your furnace’s model. If you see a wire from the ‘C’ screw, then proceed, if not then you need a new wiring installation.

Connecting the Common wire

Make sure the wires are straight, if not straighten and trim it leaving 10 millimeters exposed.

Insert the Common wire into the connector button labeled C by pushing it all the way in and ensure that the button stays down.

Connect the Nest display by making sure the log is on top and push until you hear a click sound. Go back to the circuit breaker and turn it back on.

Your Nest thermostat should now start up and you should be able to continue enjoying your room temperature. If these steps don’t solve your problem, you should reach out to the Nest support team for further help or get an expert to fix it for you.

How do I know if my thermostat needs a C-wire?

If you notice the following issues with your Nest thermostat, then it may be time to connect a Common wire to your thermostat.

• Reduced battery life
• Your thermostat keeps disconnecting from Wi-Fi occasionally
• Your system suddenly turns on/off
• Your system produces strange sounds such as clicking or stuttering
• Heating /cooling is always on and doesn’t turn off
• A Delayed or Starts in message displaying on your thermostat’s screen
• The system fan turns on and off frequently in a short period

FAQs

My Nest thermostat works fine; do I still need a C-wire?

You don’t run the risk of your Nest thermostat experiencing power issues unexpectedly when you need it the most. It’s better to have a C-wire connected to prevent such issues from occurring and inconveniencing you when least expected.

How long will my Nest thermostat last without power?

Your thermostat will last for one to two hours and will go off to conserve the remaining power to control your system.