Trying to get Nest to work with a weil mclain cga-4-spdn boiler

Jimbo60666

For my HVAC I have 2 separate /thermostats and system transformers and I am trying to consolidate them onto a Nest. AC was straightforward and Nest reported as such for the Heat side too. My old heating thermostat was connected to Red/White wires from the Weil-McLain Gold CGa Boiler, via the Red and Green terminals on the TRANSFORMER120/24 V.A.C as listed in the attached diagram. Oddly enough the Nest worked like a charm for 4 days and then the battery drained and threw the N72 error, "no power from the Rh line". I'm trying to hook up a Common line to provide power. Any ideas on a spot for power?
http://homeappliance.manualsonline.com/manuals/mfg/weilmclain/weilmclain_gold_cga_gasfired_water_boiler.html?p=34

HVACNUT

You'll need a common from somewhere connected to the C terminal on the thermostat. Is there an extra unused wire?
Connect it to C at the boiler terminal.
If not, is there an extra wire from the A/C unit? You can connect to C on the stat and common on the A/C board or transformer.
Common on low volt is typically a blue or brown wire.