I installed a Nest thermostat last month and the batteries keep dying every 2-3 weeks, which seems way too fast. I checked that the WiFi signal is decent in the hallway where it's mounted. Is this normal or did I get a faulty unit?
The C wire issue is real, but before you go down that path make sure your WiFi isn't actually struggling more than you think - sometimes a thermostat will keep trying to reconnect in the background even if the signal looks okay, which absolutely murders batteries. That said, if it's consistently dying every couple weeks you're probably missing that C wire connection to your HVAC system, which means it's pulling everything from batteries instead of getting trickle power from your heating/cooling setup.
Does your thermostat have a C wire (common wire) connected to your HVAC system? That's the usual culprit - without it, the device runs entirely on batteries and drains them fast. If you don't have a C wire, you'd need an adapter or to run one from your furnace/air handler, which honestly isn't that hard. If you do have a C wire connected and it's still dying that quickly, then yeah, something's wrong with the unit and you should contact Nest support about a replacement.
I ran into this exact same problem with mine - batteries were toast every couple weeks and I was ready to return it. Turned out my HVAC system didn't have a C wire run to the thermostat, so the Nest was basically a battery-powered device 24/7. Once I had an electrician pull a C wire from my furnace, the battery drain stopped being an issue. Before you assume it's faulty, check if your HVAC has that C wire connected - if not, you've got two options: run the wire yourself (or hire someone if it's a pain to access) or grab a power adapter meant for Nest thermostats, though that adds another thing to manage. The WiFi signal being decent doesn't really matter here since the real drain is keeping the display and processor running without proper power from your system.
Don't assume the WiFi signal strength you're seeing on your phone matches what the thermostat experiences - they can be very different, and a device constantly hunting for a stable connection will burn through batteries in weeks. Before investigating the C wire, try moving the thermostat closer to your router for a few days as a test; if the drain noticeably slows, poor connectivity was likely your problem all along. If it doesn't improve, then yes, the missing C wire is almost certainly the issue, and you'll need either to run one from your HVAC system or install a power adapter accessory that Nest makes for exactly this situation.
have you actually swapped in fresh batteries to rule that out, or are you recharging the same ones? the guys above nailed the c wire issue, but here's what they glossed over: even with good wifi and a c wire installed, a thermostat left in a cold garage, basement or exterior wall can drain batteries way faster because the device has to work harder to maintain temperature readings. if your unit's mounted somewhere chilly or drafty, that could be compounding the problem alongside whatever else is going on.
Battery drain that fast usually means your thermostat's pulling power constantly instead of sipping it - and the C wire issue folks mentioned is definitely the most common reason, but here's something worth trying first: pull the battery cover off and check if the batteries are actually making good contact with the terminals. Sometimes they slide in slightly loose or there's a tiny bit of corrosion that kills the connection, forcing the device to work harder. If the contacts look fine and you've confirmed the WiFi isn't constantly dropping, then yeah, you're probably looking at needing that C wire run from your HVAC system to power it properly instead of relying on batteries as the main source.
Log into answer.
