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Experiencing low battery notifications and need to know how to charge Nest thermostat? All models of the Nest Learning or E Thermostats come equipped with an integrated rechargeable battery.

From time to time, for a variety of reasons, you may get a low battery message with a blinking red light. In this article, I walk through 10 ways to permanently fix this problem.

Why is my Nest Thermostat Blinking Red?

The flashing red light on your Nest thermostat indicates that its battery charge is critically low but is in the process of recharging.

Nest thermostats rely on power supplied from your HVAC system through the Rh and/or Rc wires, with the C-wire acting as a neutral conductor to complete the circuit.

This consistent power flow allows the thermostat to operate even when the HVAC system is not in use or during a power outage.

The battery is essential for sustaining Nest's advanced features such as backlit displays, Wi-Fi connectivity, and activity sensors. A low battery can result in a “Delayed” message which you can read more about here.

Recharging a significantly depleted battery can take anywhere from several minutes to a couple of hours.

If after waiting for over an hour your thermostat remains inactive, it may be indicative of a different issue requiring further investigation.

How to Fix a Nest Thermostat Blinking Red Light

In this section, we'll dive into nine straightforward steps to help you resolve the flashing red light issue on your Nest Thermostat quickly and efficiently.

Nest Thermostat Pins

1: Double-Check the Nest Thermostat's Connection to the Base

The easiest fix on our list! It’s worth double-checking that your Nest thermostat is correctly connected to the base.

Start by carefully removing the thermostat from the base, then check for any bent or damaged connector pins.

Ensure that the base is securely mounted on the wall and there's a snug fit between the thermostat and base when reattaching.

Proper alignment is crucial for smooth power transmission, so be sure to line up the connectors accurately before pressing the thermostat back into place.

A properly connected Nest thermostat will display its status and allow you to control your home's heating and cooling systems efficiently.

If you continue to experience a low battery or a flashing red light even after confirming the correct connection, continue to the next steps.

2. Charge the Battery Directly (Short-Term Fix)

Quickly charge your Nest thermostat by taking it off the wall and connecting it to a Mini-USB or Micro-USB cable, depending on your device's model.

Charge it for about 1-2 hours to temporarily fix the low battery issue and grant you time to address the underlying problem.

  1. Detach the thermostat display and find the USB port on the back.
  2. Use a USB cable and wall charger to replenish its power.
  3. Be sure to use the correct USB type (mini or micro)
  4. After sufficient charging, the display will light up with a prompt to reconnect it to the thermostat base.
  5. Carefully attach the display and press firmly, listening for a click to ensure it's properly seated.
Circuit Breaker

3. Does your HVAC system have power?

Before troubleshooting your Nest thermostat any further, make sure your HVAC system has power.

If the breaker tripped, the R-wire — responsible for powering the thermostat — won’t receive any electricity.

In this situation, reset the breaker and see if the low battery or blinking red light issue is resolved.

If this was the issue, then you may need to consult with a HVAC professional to diagnose why the breaker tripped.

4. Diagnoise a Low Power Issue

Diagnosing a low power issue in your Nest Thermostat begins with charging it using a USB cable if needed.

After reconnecting the display to the thermostat base, navigate to Settings > Technical Info > Power.

Here, you'll find the values for Voc, Vin, and Iin – these indicate the electrical current sent over your system's wires to the thermostat.

It's essential to verify that these values are within the specific ranges noted below, or you may have a wiring or system issue.

  • For thermostats without a C wire, the values should be as follows: Voc: 29 to 42V, Vin: 29 to 42V when your system is not running and 6 to 7V when your system is running, Iin: 20 to 40 mA.
  • If your thermostat has a C wire, the values should be: Voc: 29 to 42V, Vin: 29 to 42V, Iin: 100 or 200 mA.

Battery Voltage

Additionally, ensure the thermostat's internal rechargeable battery voltage is higher than 3.6V for normal charging.

If the battery falls below 3.6V, the thermostat will disable certain functions to preserve its charge, such as Wi-Fi, software updates, and screen activation.

Nest Thermostat Wiring 1

5. Connect a C-Wire

If you don’t have a C-Wire installed, then connecting one should solve your low battery problem and fix the flashing red light.

Below, I outline 5 ways you can connect a C-Wire.

5.1 Hardwire the Common Wire

To hardwire a C-wire, connect it to your HVAC system and Nest thermostat. Make sure to turn off the equipment's power during installation. Keep in mind that if no spare wire is available, the process can be more involved. Below are the steps for when there's an unused wire not connected to either the thermostat or HVAC system:

  1. Turn off your HVAC system's power and verify it's off.
  2. Remove the faceplate of your Nest thermostat and photograph the wiring for reference. Label each wire with the corresponding terminal letter.
  3. Find an unused wire to use as a C-Wire, typically colored blue. Connect the wire to the C terminal on the Nest.
  4. Identify the unused wire near your HVAC equipment and connect it to the C/Com terminal on the control board.
  5. Restore the power, reattach the Nest display cover, and test your thermostat. You should see the blue C-wire connected under “Equipment.”

5.2 Use the Nest Power Connector

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The Nest Power Connector provides a stable power source for your Nest thermostat if your HVAC system can't provide consistent power. To install, plug it into the wall and connect the black and white wires to your system's power terminals, and the red wire to the C terminal. The connector will then signal the thermostat to turn the system on or off as needed.

Check out the Nest Power Connector here.


5.3 Use the FAST-STAT Common Maker

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The Fast-STAT Common Maker provides a convenient solution for connecting a C-Wire without pulling additional cables. Using a wireless sender and receiver, it simplifies the process and saves time and money compared to rewiring your thermostat.

Check out the FAST-STAT Common Maker here.


5.4 Install a C-Wire Adapter

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A C-Wire adapter is a DIY solution if your Nest thermostat lacks a C-wire and you don't want to install the Nest Power Connector or Common Maker. These adapters are user-friendly and easy to install, providing power to your thermostat.

Here's a C-Wire Adapter that works with the Nest.


5.5 Reuse your G Wire as a C Wire

Reusing your G-wire as a C-wire is an inexpensive solution, but you'll lose independent fan control. Before starting, verify that your HVAC system is compatible with this option. Remove the G-wire from the G terminal in the thermostat and connect it to the C terminal. Repeat this process at the furnace. Check compatibility with zone panels and connect a jumper cable between the G terminal and Y terminal if necessary.


6. Troubleshoot the C-Wire (if one is installed)

A common cause of wiring issues with Nest thermostats is an improperly connected C-Wire, which may occur during initial installation or due to other factors later on.

  1. To address this issue, first, ensure that power to your system is turned off at the fuse box or system switch – this is crucial for both your safety and preventing damage to the system.
  2. Next, gently remove the Nest thermostat display and inspect the connectors.
  3. Make sure all connectors with wires are fully pressed down.
  4. In some instances, you may need to re-insert the wires to ensure the connector button remains fully pressed down.
  5. If the button isn't down, the Nest thermostat will not detect the presence of the wire in the connector.
  6. To achieve this, you might have to re-strip and straighten the wire ends for proper insertion into the Nest base's connectors.

7. HVAC System Issue

When your Nest thermostat displays a low battery icon or flashes a red light, it could indicate an issue with your HVAC system.

Various problems, such as a clogged air filter, may cause your system to overheat and shut off automatically.

In turn, this prevents the thermostat from drawing power to charge its battery.

You might also notice a Furnace Heads-Up message on the thermostat screen, in your app messages, or in your monthly Home Report email.

To resolve this issue, reach out to a local HVAC professional who can service your system and verify the thermostat wiring.

Nest Reset

8. Factory Reset

A factory reset can help resolve the Nest thermostat's low battery or blinking red light issues.

Before resetting, remember to note down essential information, including system wires, heating type, and Safety Temperature settings.

To initiate a factory reset, access the Quick View menu by pressing the thermostat ring, selecting Settings, and choosing Reset.

There are two options available: restarting your thermostat or resetting specific information.

Restarting your thermostat will not delete any information or settings, similar to rebooting a phone or computer.

On the other hand, resetting specific information entails clearing your temperature schedule, learned Away information, or network settings.

Resetting specific information can be useful if you've moved the thermostat, experienced network issues, or undertaken home renovations.

9. Hire a Nest Pro

If none of the above works, or you aren’t confident in handling wiring you can turn to a local Nest Pro for assistance.

Equipped with the expertise to pinpoint and fix problems with your thermostat and HVAC system, Nest Pros can manage tasks like connecting a C-wire or Nest Power Connector.

Specializing in Nest device installations, Google's partners Handy and OnTech provide online scheduling, evening and weekend availability, and a one-year workmanship warranty.

10. Get a replacement

Should your Nest Thermostat have a fully drained battery and remain under warranty, reach out to Google for a replacement, as the battery is a non-serviceable component.

FAQs

Here are some common questions and answers related to the Nest Thermostat battery.

What happens if the battery is showing as low?

When your Nest thermostat displays a “Battery Low” message and a flashing red light, it means the battery is running out of charge, potentially affecting its performance.

As a result, your thermostat could lose the ability to activate your HVAC system for heating or cooling, create odd noises due to rapid on-off cycles, or display delayed heating or cooling messages.

Additionally, the Wi-Fi connection may become unstable or your thermostat's screen might become unresponsive.

To temporarily resolve this issue, immediately charge your Nest thermostat using a USB.

However, you will need to investigate the underlying problem, which could involve the lack of a C-wire or issues with the voltage coming from the Rh or Rc wire on your HVAC unit. See my 10 troubleshooting steps above.

What’s the charge time for the Nest thermostat?

A Nest thermostat typically takes around 30 minutes to fully recharge. That being said, if the battery is entirely depleted, this process may extend up to 2 hours.

During this time, the thermostat will gradually regain its functionality, ensuring efficient temperature control and energy management within your smart home.

nest thermostat low battery

What kind of battery does the Nest Thermostat have?

The Nest Thermostat houses two types of batteries, depending on the model.

Newer models like the Nest Thermostat E and later generations of the Nest Learning Thermostat use an internal, non-replaceable rechargeable battery.

This battery charges when the furnace is on and through a common wire.

In contrast, older Nest Thermostat models rely on two replaceable 1.5V AAA alkaline batteries that need replacement as they deplete, ensuring optimal performance and seamless operation.

Is it possible to replace a Nest Thermostat battery

In instances where your long-used Nest thermostat's battery seems to require replacement, it's important to note that the battery is not designed for user servicing as it is integrated within the device.

If experiencing low battery or power issues, first attempt the troubleshooting steps detailed above before exploring battery replacement options.

If your internal battery is undeniably dead and unable to hold a charge, assess your warranty status; if still covered, contact your provider for a replacement.

For devices with an expired warranty, replacing the internal battery or seeking local repair services are viable options, but be aware that these actions will void any remaining warranty.

Check out these replacement batteries and this IFIXIT guide here on how to proceed with this.

What is the Battery Life of a Nest Thermostat?

The battery life of a Nest thermostat varies based on its power source and usage.

Typically, its internal battery can last several hours to a few days without charging from the HVAC system.

A steady power supply from the HVAC system can extend the battery life to 5-10 years without problems.

However, if the battery is under stress, it may fail as early as 2 years.

How to Charge Nest Thermostat: Sources

https://support.google.com/googlenest/answer/9373007?hl=en

https://support.google.com/googlenest/answer/9241211#power&zippy=%2Cpower%2Cbase%2Cdisplay%2Cequipment%2Csensors%2Cnetwork

https://support.google.com/googlenest/answer/9233319?hl=en#zippy=%2Cblinking-red-light-on-your-thermostat%2Cblinking-green-light-on-your-thermostat%2Cno-light-on-your-thermostat

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3 Comments

  • Allen Miller
    Posted December 1, 2022 at 4:32 am

    Do you know about how much time i would need to schedule the fan to run per day to keep the battery charged if I don’t have the C wire and don’t do the Green wire changeover?
    Thanks.

  • Andrew
    Posted November 27, 2021 at 4:48 pm

    Thanks for this. VERY helpful as I bought a used Nest thermostat, installed and verified correctly hot wired to the furnace, am getting the right charge and Lin my numbers are good. However, when The furnace runs for a while (catching up more than say 5 degrees in warming (heat so far, haven’t tested cool) it seems like it *drains* the battery rather than charges it. Hmmm.

    Anyways your post gives me solutions. Thanks!

    Also note that your link to IFixIt for the battery replacement has changed. It’s this now:

    https://www.ifixit.com/Guide/Nest+Thermostat+E+battery+replacement/133827

    • Daniel Walsh
      Posted November 29, 2021 at 9:18 am

      Thanks! I’ve updated the link 🙂

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