Nest Learning Thermostat (third gen) with no C wire and no constant power

[cbroverman]

Hi, Nest claims that there is no C wire needed for the Learning Thermostat. I only have 2 wires and an old boiler with T/T terminals only. My questions are: (1) Without constant power how long will the battery charge last on the thermostat? (2) How long does it take to recharge the thermostat? (3) Others seem to say a C wire is needed to keep it powered. If that is true (a bit misleading that Nest says it is not needed for two wire systems), can i use an external transformer plugged into an outlet below? If so, where do I plug in the two wires that come with the transformer? Thank you.

[Patrick_Caezza]

@cbroverman 

Nest says the C wire isn't normally required, it also says it works with 95% of systems (3rd Gen) and 85% (4th Gen.

Boilers are a completely different beast since most use a switch closer to turn them on and off.

The TT terminals on your boiler are just for a switch closer which rarely works with smart thermostats like the Nest.

Sometimes you can get it to work using the Nest Power Connector.

Give this wiring a try.