I just installed a Emerson Sensi wifi thermostat.It works, and I ran a separate C wire. I had a 24 volt 5 wire Robertshaw programable thermostat that ran my boiler, air handler and AC. It is all single stage. On the old thermostat the W wire went to the Taco boiler control and made a loop back to the common side of the transformer in the 20 year old air handler. RH, RC ( jumper removed), powers the T stat,is on the hot side of the transformer. G goes to the fan control and Y goes to the AC condenser.It did not have a common, which I added to the Sensi with a new wire
When I wired the Sensi up with the same configuration, and the new C wire, it did not energize the heat zone on the Taco controller when the Sensi calls for heat.. I called Emerson and they told me I had to take the return loop from the Taco and attach it to the same hot side of the transformer that the RH and RC are connected to (they are now all wire nutted together). Nobody at Emerson can tell me why the W wire leaves the Sensi on one side of the transformer, and returns to the same side of the transformer. This goes against typical thermostat wiring and my old configuration. It also is not how the G (fan) wire and the Y (AC) are configured. They return to the common side.
Does anyone understand why this new thermostat actually works in this situation. It baffles me. The people at Emerson are very nice, but they are only reading from their manuals. They are clueless about how this stuff works.