Common on Nest Not Starting Up Boiler

[kylegansel]

Hi all, I’ve been reading numerous posts on this website and can’t really find my answer. I have a new boiler system with a two wire transformer. I had a nest that was on the old boiler for about 2 1/2 years and it just recently died and after all my research, I’ve determined that it definitely needs a common. I have a four wire thermostat cable where only the red and white wire is being used. I grabbed one of the other spare wires in the thermostat cable that is not being used and connected that to the common on the transformer and then connected that the common on the nest. I’m not showing any errors on the nest, but the boiler will not fire up.

After numerous steps of troubleshooting, I found that the valve for that zone that has a nest on it does open and close when the thermostat calls for heat, but the boiler will not fire up. After speaking with a few HVAC friends they told me that it should be that simple but I’m curious if anybody knows why the boiler is not firing up when the common is now wired to the nest.
If I remove the common from the nest and the transformer, the boiler then fires up normally but now the nest is not getting powered. Looking for help!!!

[CoolingWizard]

@kylegansel ,

with your new boiler. The real question becomes can any sub zone initiate the boiler to ignite? Most boilers require that a master signal will be sent to the initiate the boiler to turn on and this is sometimes wired in a series with the water valves so that any valve can turn on the boiler. Or you can work in a semi automatic mode where when a valve opens and the water starts flowing through the boiler the boiler senses, a cool water temperature and automatically turns on the boiler and heats up the water tank.  
there’s only one single situation which would cause this whether or not to work when a nest thermostat is attached to it and that is if the transformer is undersized and with the draw of upto  200 mA that the Nest might be pulling, it might not leave enough power to run the boiler.  

AC Cooling Wizard

[kylegansel]

@CoolingWizard For the time being is just using a plug-in transformer, the way to go?  I’ve looked at the nest see adapter, but that looks like a bit more work to just get me through for a while. 

[CoolingWizard]

@kylegansel ,

in order for the system to work properly with any thermostat, the main transformer supplying power must be the same one that’s sending power to the thermostat. If you can give me some pictures and model number of your boiler so I can look up the installation manual I have and see what type it is and what the control system is. 

AC Cooling Wizard 

[kylegansel]

How do I get you photos? Also, would it make sense to wire another 48 V transformer adjacent to the existing transformer on the boiler to power the nest units?