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.
Emerson Sensi Touch C wire help needed
thermostat-c-wire
Related Solutions
Connect the blue wire from the thermostat cable as shown above, then connect the other end to the C terminal of the thermostat.
This is my attempt to "demystify" the whole "C" wire thing for non-technical folks. This explanation intentionally ignores lots of details that are not relevant to a basic understanding of the concept.
An old-timey thermostat is mechanical in nature. You can think of it very much like 3 separate mechanical on/off switches. (the "switches" in this case are mechanically actuated by temperature using springs and tilting glass bulbs full of mercury and other old-timey mechanical mechanisms)
The circuit is actually VERY simple. The old-timey thermostat switches on and off the "juice" which comes in on the RED wire...separately on and off to each of the other 3 wires (often green for the fan, white for the heat, and yellow for the AC).
That's all the old-timey thermostat does. Hook or not hook the red wire to each of the other three as needed.
Modern thermostats control your HVAC system with the same connections. They do the same thing...hook the red wire up to the proper green/white/yellow to turn various parts of you HVAC on and off as needed.
Since the old-timey thermostats were mechanical in nature, they didn't need any juice of their own to operate. If you think of a modern thermostat as an old-timey thermostat plus a smart phone stuck to the wall right next to it...you need something to power the smartphone. (some "kind of modern" thermostats have batteries in them for this purpose...to power the "smarts")
The red wire brings juice into the thermostat...but there is no "ground wire". Our "smartphone stuck on the wall next to the old-timey thermostat" needs both a "hot wire" and a "ground" (or "common"). (again, this is simplified so as to not get lost in irrelevant details and confuse the non-technical readers this was written for)
The "C Wire" (often blue) is this additional "ground wire" needed in order to complete a circuit for the electronics (which old-timey thermostats didn't have). The "other end" of the C wire is hooked to the other side of the transformer that supplies juice to the R or red wire. It is the other wire needed by the electronics in the newer thermostats in order to power up their "smarts".
Hope this helps a few people understand what the C wire is about.
Best Answer
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.