parasitics

I have a Danfloss tp9000 programmer and TS2 room sensor which is controlling the Ideal Heat 15 system boiler. I'm trying to replace it with a smart thermostat by Nest, replicating this set up https://d3vlabs.com/2016/12/09/install-nest-tp9000-ts2-danfoss-3rd-gen-uk/ There are 3 lives, 3...