Nest Gen 3 Learning Not Holding Charge – Need Advice
TL;DR:
My Nest Gen 3 Learning thermostat isn’t taking a charge from USB. It worked fine for a year after a previous USB charge but is now draining quickly. There’s no C-wire available behind the baseplate, just four bundled wires from the furnace (Carrier 58STS/STX).
Biggest concern: Will the power from the furnace wiring keep the thermostat functioning even if the smart features fail? Or should my landlord replace it ASAP since single-digit temperatures are coming?
Full Story & Troubleshooting So Far:
We rent, and I don’t know the thermostat’s exact age.
The Nest worked fine until yesterday’s internet outage.
Afterward, it showed low battery and lost WiFi.
Plugged it into USB at midnight, removed it at 7 AM → Heat turned on, WiFi connected.
Later in the day, WiFi lost again due to low battery.
What I Noticed:
Green flashing light when plugged into USB, but unclear if it’s actually charging.
When forced off and on, it only turns back on when placed back on the baseplate.
Once back on the wall, furnace kicks in immediately but still shows a low battery warning.
Questions:
Is the battery dead? Can it be replaced, or do I need a new unit? Will furnace power keep it working, even if the smart features fail? Are there any troubleshooting steps left, or is it time for a replacement?
Any advice would be appreciated before I have to freeze this week.
@Muhammad
If your furnace doesn’t support a common wire, you can add a 24v transformer and connect it to the C and Rc terminals to provide constant power. Might need extra wiring if there aren’t unused wires available, but it works well. I did this at my parents’ place since their boiler had no common wire.
As the Nest ages, the battery loses its ability to hold a charge. If you have a common wire, the battery is just a backup and doesn’t matter much. But without a common wire, the battery has to hold power between heating cycles, and if it’s weak, you’ll get shutdowns.
There are simple kits that add a C wire if you don’t have one. I installed one on my two Gen 3 units, and they’re still running fine six years later.
@Willie
Since I’m renting, I let my landlord know and suggested the kit. If it were my own place, I’d do it myself, but I doubt he’d want me messing with the system.
I’m an electrician. Check the wiring at the furnace to make sure the wires are connected to the correct terminals. I had an issue like this, and it turned out the wiring wasn’t the same at the furnace.
Also, Nest thermostats charge very slowly if they don’t have a common wire. The trickle charge from the furnace isn’t much. You can speed it up a bit by charging it with USB before putting it back on the wall.
@Kenny
When we moved in, we had to charge it via USB because the battery was too low to connect to WiFi. It worked fine after that but now won’t hold a charge. It charged overnight for about 7 hours, but once back on the wall, it quickly showed low battery again.
I let my landlord know. He might replace it, but we’ll see what happens.
@Muhammad
If it belongs to the landlord, I wouldn’t mess with it. If the thermostat stops working completely, that’s on him to fix since heat is a major need.
That said, batteries in these don’t fail that fast, so it could just need a proper power source rather than a replacement.
@Kenny
His response was that he has three others in his rentals and never had this issue. He’ll check it when he’s back in town in June. If it dies completely, he’ll replace it.
Since it’s still working (barely), I guess I’ll just keep an eye on it.
@Muhammad
If he’s making you wait months for a fix, check your lease and local laws. If the battery fully drains, the heat won’t work at all, and then he’ll have to act fast.
That’s a long time to ignore a problem that’s only going to get worse.
I had a similar issue with my mom’s house. Her Nest worked fine at first, but after a couple of years, one of them wouldn’t hold a charge. The other two were fine. Ended up replacing the battery, and it worked again.
Nest thermostats are only designed to last about ten years. The battery isn’t meant to be replaced, and once it dies completely, the unit stops working. Mine started having issues at seven years, so I ran a C wire, which helped, but I’m switching to an Ecobee soon since it doesn’t rely on a battery at all.
Since you’re renting, let your landlord know. If the battery fully dies, the thermostat won’t work even with a C wire.
@Dallas122
I told my landlord. He said he’ll check it when he’s back in town, or if it fully dies, he’ll replace it. I’ll just keep an eye on it for now.
I had the same issue. My HVAC tech told me the G wire only controls the fan, so if you don’t need manual fan control, you can repurpose it as a C wire.
Here’s how: Turn off power to the furnace, move the G wire from the furnace terminal to the common terminal, then move the G wire on the thermostat to C. Turn the power back on. It takes about 15 minutes.
After doing this, my Nest stopped losing power and charged properly. If you don’t feel confident doing it, an HVAC tech can do it for about $85.
Is the battery dead? Can it be replaced, or do I need a new unit?
Will furnace power keep it working, even if the smart features fail?
Are there any troubleshooting steps left, or is it time for a replacement?
