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5 Ways to Fix Nest Thermostat Low Battery Problems

The Nest range of wireless controls and monitors have become very popular. They provide remote and automatic control for a range of devices, including heating and air conditioning systems.

The Nest Thermostat is battery powered but is recharged by the base unit that it attaches to. This removes the need for users to recharge the battery manually.

But sometimes things go wrong, and the displays a Nest thermostat low battery warning.

This brief article looks at the causes of the error and how to remedy the fault.

Why Does My Nest Say Low Battery?

There are several reasons why your Nest Thermometer will display a low battery warning. But in the end, they all lead back to a single cause. The base unit is not recharging your thermostat battery.

The Nest thermostat takes a small charging current via your HVAC unit to charge the battery. In some circumstances, this may not be enough to keep the battery fully charged.

What to Do When Nest Thermostat Says Low Battery?

First, we need to establish why the battery is no charging. If it is an old unit, then there is a possibility that the rechargeable battery is failing.

If you have the original Nest Thermostat, then the batteries are easily replaced:

  • Remove the thermostat from the base unit.
  • Replace the two batteries in the back of the thermostat.
  • Only use quality AAA alkaline batteries.
  • Refit the thermostat to the base unit.

The Nest Thermostat E or the Nest Learning Thermostats are sealed units, and the battery is not designed for user replacement. Replacing the battery on these units is not a simple process, and it’s beyond the scope of this article.

You can test for a faulty battery by manually recharging it as described below and refitting it to the base unit.

If the low battery warning has gone and remains off for several hours, then the battery is fine. The sealed unit battery usually is good for at least five years.

So assuming the battery is good, we need to discover why the base unit is not recharging it.

Where Does Nest Thermostat Get Power?

When the Nest Thermostat is plugged into the base unit, it receives power via its rear connector.

In a standard setup, the Nest is not connected directly to a power source for battery charging. Instead, it takes a small charge from the HVAC control connections.

In some cases, the charge coming from this source isn’t enough to keep the battery fully charged, and this causes the battery-low warning.

Problems and Solutions

#1: Charging the Nest Thermostat by using the charging port on the rear of the Nest

There are several reasons why you may need to charge your Nest thermostat manually, for example:

  • It has been in storage for some time
  • Your HVAC has been switched off for a long time

If the battery voltage drops below about 3.6 volts, your Nest unit may not switch on or recharge from the base unit. In this case, we need to give the battery a boost.

It’s straightforward to recharge the battery by using the charging point on the rear. You will need a phone charger and micro or mini USB cable, depending on your model.

  • Pull the thermostat away from the base unit.
  • Plug the phone charger into the wall socket.
  • Connect the cable to the charger and the micro-USB port on the rear of the thermostat.
  • Once the red light on the top of the unit stops blinking, your unit is charged. This should take between 30 minutes and two hours.

If you have an older unit without a USB port, you will need to recharge the batteries in a dedicated battery charger unit.

#2: Install a C wire/ common wire

If you have established that your battery is good, we need to determine why it is not charging correctly and fix it.

Your thermostat typically gets enough power from the heating and cooling control circuits. If that’s not the case, we can install a C wire, also known as a Common wire.

There may already be a spare cable that you can utilize for the connection. If not, then you will need to run a cable from the furnace to your Nest thermostat.

  • For safety, switch off the power to your furnace.
  • Remove the Nest thermostat from the base unit.
  • Find the C connector, and if there is a wire connected, make a note of the color.
  • Now, look at the connections on the furnace.
  • Make sure the same color wire connected to the Nest C connecter is connected to the C terminal on your furnace.
  • If there is no cable connected, then a new wire will need to be run in between the thermostat and the furnace.
  • With a new cable installed, connect to the C connector on the Nest and the C terminal on the furnace.
  • Now refit the Nest to the base unit.
  • Switch on the furnace power and make sure it is working correctly.

#3: Good solution when you don’t have a C wire

If you look at the Nest base unit and find no common wire connected, there may be an easy solution.

Older HVAC systems, around 20 years or more, often require a fan control cable. Modern systems don’t need it, and we can take advantage of this.

We can utilize the fan cable to connect to the common wire connector and provide a charging circuit for your battery.

In most cases, the C connector wire should be blue. However, it doesn’t matter in reality. The colors are just for identification, and all the cables between the Nest and your furnace are solid copper.

  • For safety, switch off the power to your furnace.
  • Remove your Nest thermostat from the base unit.
  • Find the G connector.
  • The G wire controls the fan and is not needed in modern systems.
  • Switch the cable from the G connector to the C connector.
  • Now go to the furnace and switch the cable from the G terminal to the C terminal.
  • Refit the Nest thermostat
  • Switch on the furnace power
  • Make sure everything is working correctly.

This video shows you how to switch the Nest cable from the G to the C connector in more detail.

#4: Use a common wire transformer

If you remove the Nest from the base unit and find no C wire and don’t want to install another cable, there is another option.

The fundamental problem is that the thermostat does not have a reliable power supply. We can provide a constant power supply to the thermostat by using a simple 24Vac transformer.

The procedure is straightforward:

  • Switch off the power to your furnace for safety
  • Remove the Nest thermostat from the base unit.
  • Feed the transformer cables behind the base unit and out through the center hole
  • If there is a cable connected to the Rh connector, disconnect it and tie it back safely.
  • Connect one of the transformer cables to the Rh connector
  • Connect the other transformer cable to the C connector
  • Plug the transformer into a mains outlet and switch on
  • Refit the thermostat and test
  • The thermostat indicator light should start flashing as it goes through its startup process.

Watch this video for more information on connecting a transformer to your Nest thermostat.

Once you have confirmed this is all working, you will probably want to tidy up the transformer cables. You can either run them behind the wall or cover them with an electrical conduit.

#5: Settings – reset – restart

There was a software issue with some Nest thermostats that caused the battery to stop charging and the unit to stop responding eventually.

Nest has been working on a solution, but there is a temporary fix if your thermostat exhibits this problem. You need to restart your Nest thermostat, as follows manually:

  • If your thermostat battery is flat, remove it from the base unit, recharge it, and refit it. See the procedure described earlier.
  • On the Nest thermostat, bring up the main menu by pressing the ring.
  • Select “Settings” by turning the ring and selecting.
  • Select “Reset” by turning the ring and selecting.
  • You can then select the “Restart” function.
  • Here you have a choice:
    • Either select restart, which will restart your Nest without losing all your data.
    • Or, select factory reset, which deletes everything and reboots your thermostat.

Watch this short video on how to reset your Nest thermostat for more information.

FAQs

How long does the Nest thermostat battery last without power?

Without power, your Nest thermostat battery will last between one and two hours.

How many volts should my Nest thermostat have?

To charge the battery, your Nest thermostat requires a reliable 24-volt power supply. This can usually be achieved through the standard connections to your HVAC system.

If not, there some workarounds that we have mentioned above.

How do I know if the Nest thermostat is charging?

When your thermostat is connected to a power source, you’ll see a blinking red light. This indicates that the battery is charging, and it will stop blinking once the battery is fully charged.

Can Nest thermostat work without C wire?

According to Nest, yes, it can, but this will depend on your HVAC system and the current setup.

If you don’t have a C wire, and your Nest battery keeps going flat, then you’ll need one of our solutions above.

Conclusion

A low battery on your Nest thermostat will cause the system to stop working, but there are solutions.

First, you need to establish the cause of the low battery then choose a resolution. Most of the fixes are simple enough for the home DIYer to accomplish, but if in doubt, call a professional.

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