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

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