Nest thermostat stops heating early, cycles boiler on and off after 2+ years of normal operation.

[Tnest]

Nest thermostat 3rd gen stops heating in a few mins and doesn’t get to set temperature. Was working fine for 2 years and now on the 3rd winter it says Delayed 3:00 and stops heating. I have a separate 24v transformer for it. Battery reads 3.9x volts, Voc Vin 29-36 range and Lin 200 mA(c).

I contacted Nest support and they did not find the problem and blamed wiring. But why was it working fine for 2 winters?

I have used a multimeter and the 24V transformer is giving 26V, so it’s working fine I assume. 24V transformer connected R wire to Rc at Tstat, C wire to C at Tstat. I bypassed the thermostat and have connected W1 to RH, the 2 wires from my boiler and the boiler has started. It ran for 20 mins without stopping. I tested it again for 30 mins straight and it heats fine without stopping by jumping the boiler w/o nest.

Firmware updated to 6.2-22 on October 26th. I have not changed anything on the boiler or the settings on this thermostat, what is the problem? My mercury tstat worked for 30 years no problems.

[mlane224]

I am having the exact same issue with my thermostat. It too has been working fine for 2+ years it has a C wire and full power. Looking at my usage history for some reason on November 28th it switched to constantly cycling the boiler to get up to temperature, it is clear to see the solid on off that we typically see and then the switch on the 28th to tiny little bars turning on and off before finally getting to temperature.

[Tnest]

Yes it stinks, I actually reinstalled my mercury tstat instead of spending more days to figure it out. Sounds like maybe it is the firmware or something faulty with the nest.