Emerson Sensi Touch C wire help needed

Question

I am heating with a fuel oil boiler in the basement and a Unico air conditioning unit in the attic. I some help getting past the c wire issue. Air is using yellow, green and red-c and heat is using red-h and white. No c wire is coming from the boiler but the air conditioning unit is directly above the thermostat. I could pull a c wire if necessary. The problem is getting the Sensi Touch to control both heat and air. Is an isolation transformer a possibility? Emerson also tells me I can use an external transformer to power the Sensi Touch but it will only control heat or air but not both. Picture one is the Sensi Touch. Picture two is the wiring diagram from Unico. Is seems like this could work. Please help. We are pretty far out in the country and finding an A/C service tech who can help has proven to be impossible.

Answer 1

You just need to pull a C-wire from the AC air handler above the thermostat. Most smart thermostats power themselves from Rc and C, and only use Rh to signal heating, so your setup is very "typical" and is what they design for. I have seen a question where someone wanted to pull a C wire from the furnace (power from Rh and C), and that was more complicated and not "supported" by their thermostat, but you should be fine.