I am using Taco 503-4 relay already , not sure if you mean same by
isolation relay. My nest is configured with dual txfr and v wire is
coming from AC air handler.
I m facing same problem and having n72 error. Everything was working
fine till now.ac works fine. But heat is not turning on. Checked
everything..HVAC fuse..drain clog .. everything is fine..can someone
please help? Called Google nest but no help.