The thermostat is no longer managing the fan.
The thermostat is no longer managing the fan.
I reached out into the unknown for HVAC enthusiasts before purchasing another Nest to keep testing. When I moved into my new apartment, I brought my Nest Learning Thermostat (third generation) with me and set it up. From what I remember, all features operated normally during initial tests, including fan control. Last night I realized the fan wasn’t working anymore (actually, I’d been suspecting it for some time because it wasn’t running as much as expected, especially since I really wanted it that evening). After restarting the thermostat, rewiring the end of the wiring, checking the furnace controller, confirming power and performing a factory reset with no difference, I found the issue. The fan does activate when I connect the G wire to the R wire (so it’s likely not a problem on the furnace side), and it also runs when I switch the furnace into second stage heating—since it’s a central hot water system, I’m not too worried about the fan not working, as hot water can’t melt anything there. The thermostat recognizes the G wire and lets me control the fan (it isn’t set to “off”). Using a short jumper wire, I measured roughly 2.2VAC from the fan output to the G wire. This seems close to the expected range of 24–27VAC the system should operate at? I’m probably missing something simple. I might be handy with electronics, but home wiring can feel confusing. All signs point to a failed Nest, which is good news since it’s now past the three-month warranty period.
Honestly, I’m not sure if you can push them into working. They seem to refresh themselves automatically. I just placed a new one, scheduled for tomorrow, so I’ll check then. It looks like the wiring backplate—with its internal circuits—might be the issue.
Just checked in quickly: got a fresh NLTv3 version installed and it’s running smoothly. Tried using the old NLT on the new baseplate but it still doesn’t connect properly, so the issue seems to be inside the thermostat module itself. Then yesterday the new unit stopped working the same way as the previous one. I replaced the original thermostat and am planning to give up until I figure out what’s causing the problem. It looks like something in the AC relay is likely the culprit—possibly drawing too much current and damaging the solid-state relay in the NLT, or maybe it’s not stopping the fly-back from the coil deenergizing.