I had a difficult time getting a Nest to work my steam heating system, and google failed me, so I engineered through it. The nest says that it works with 95% of all systems out of the box, but when you have that 5%, it's rather vague. The Nest, and most other thermostats, are in the class of "power stealing thermostats", which basically means they use a RC circuit to leach a bit of power off the lines from the heater. This means that the power is acquired from two lines from the heater. At a minimum, you need two lines to turn on your heater, which are the R and the W. In my case, the Nest did not work; however, I guessed this might happen so I ordered a 120v to 24v transformer in advance.
I have a
Beckett 7505B burner controller that does not have a common line. It has just two lugs, Tw and Tr, which are the R and W wire terminals. If you short these two wires together, the system will turn on. The issue was that the Nest need more power. The solution is to add a 24VAC transformer and connect them to the Rc and C lines. The "C" line is the common from the transformer, which is most likely a black line. The "Rc" line is the power from my non-existent air conditioner. In this way, the Nest is powered by Rc and C wires while the control is passed via the Rh and W1 wires. The final schematic follows and worked very well.
