Reddit mentions of Venstar ACC0436 2-Wire Kit for all 24VAC Thermostats

https://www.amazon.com/Venstar-ACC0436-2-Wire-24VAC-Thermostats/dp/B00755BZZC

http://www.hvacproblemsolver.com/floor-wall-furnace.asp

Check for voltage on your thermostat wires while heat is off

If you have 24v a nest will work just fine

However

If it's a milivolt system you can either:

Use a UK Nest E (my favorite option): https://store.google.com/gb/product/nest_thermostat_e

Or

Use a Venstar 2-Wire-Kit : Venstar ACC0436 2-Wire Kit for all 24VAC Thermostats https://www.amazon.com/dp/B00755BZZC/ref=cm_sw_r_cp_apa_i_7f20DbBSAYP3J

Running new wires isn't an option right now (old house; giant project for something that's more of a luxury than a necessity for us). But could I use this Venstar transformer so that my ecobee isn't trying to pull power through the furnace wiring? https://www.amazon.com/Venstar-ACC0436-2-Wire-24VAC-Thermostats/dp/B00755BZZC

oh hey, i know a lot about this because i just did it. I had suspected that all i needed was a solid state relay to isolate the 24V of the thermostat from the millivolt 750Ohm two wire system but i wanted someone to blame if i fried it. I bought this dingus https://www.amazon.com/Venstar-ACC0436-2-Wire-24VAC-Thermostats/dp/B00755BZZC and hooked it up to a standard honeywell wifi thermostat that i got for $4 with a utility company rebate and it worked like a charm. Turns out I was right, i cut the hear shrink on this think off and all it was was a PCB with a cheap mechanical relay and a triac.