Just received a Nest thermostat I would like to install. My system is a McLean boiler steam to radiators. My existing setup has a 2 wire thermostat connected to boiler. From thermostat the R wire goes to boiler Y and the W wire from thermostat goes to G on boiler. I’ve added a C wire and tried numerous combinations on Nest thermostat. I’m receiving power but none of the different combinations I try on Nest seem to work. Do I need to change the Boiler wiring?
The picture I’m going to attach will be a bit confusing since the red/white wires were spliced to 2 black wires. 
Nest Thermostat
nest
Related Solutions
Since you have a R841C (with an integral transformer)
You have your terminals on the relay confused. Connect RH to the HOT terminal from the relay and W1 to to the NEUTRAL terminal from the relay. C on the Nest is left unconnected, and you do not need a separate transformer here.
If this was an R841D (standalone relay)
You have your terminals on the Nest confused here. Connect the hot wire from the transformer to the RH terminal on the Nest, the neutral wire on the transformer to one end of the relay coil (it doesn't care which way it's wired) AND to the C terminal on the Nest, and the other end of the relay coil to the W1 terminal on the Nest.
I fixed it myself. In the manual (page 18) there is the following image.
As shown on the photo in the question:
From left to right: The red "bridge cable" is meant to close the circuit in order for the next two (bus +/-) to work. But the bus connectors only work with with an eBus system. Which is not compatible with the Nest Heatlink.
The two wires that are used to connect the boiler to the heat link (the left brown/blue wires) should be connected to the 4/3 connectors instead. So after removing the red bridge cable and connecting the heat link to 4/3 of the boiler, it worked fine.
Disclaimer for those with the same problem: I'm not a electrician, although I did check the solution with one. Try this at your own risk.
Best Answer
Unfortunately, on non-electronic boilers, labels are all over the place, and the thermostat is reversible -- it doesn't matter which wire coming out of the thermostat is connected to each of the two terminals on it.
The basic principle of how the furnace works with a dumb thermostat is:
It will also work with the following configuration:
So the first challenge is to identify the (R) and (C) terminals on the 24v side of the transformer.
From there, you need to trace the (R) wire and see if it goes to the transformer first or the relay first.
If it goes to to the thermostat first, that's your (Rh) wire, and the other one is your (W) wire (you can check this with a multimeter again -- at the thermostat one wire will have voltage and the other won't). Find the other end of the transformer and attach the (C) wire to it as well as to the Nest's (C) terminal.
If not, you now have your (W) and (C) wires -- find the (R) side of the transformer and attach your wire to it. At the thermostat, plug it into the (Rh) terminal.