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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.

  • Can you please provide a better description of how all the wires are connected to the thermostat, possibly including a diagram? Do you have two separate Rh and Rc wires, or just a single R wire? Is the C wire from the boiler, or the air handler? – Tester101 Nov 3 '14 at 11:19
  • I have separate Rh and RC wires going to the thermostat. I have removed the jumper between them at the thermostat. The C wire is from the common side of the air handler coming out of the fan control, that originates from the common side of the air handler transformer. The G wire goes to the fan control in the air handler, It returns to the common wire nut. – David Nov 3 '14 at 22:31
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    Without knowing more about the entire system, it's difficult to provide definitive answers. Typically Rh would come from the boiler transformer, while Rc comes from the air handler transformer. From your description, it sounds like both Rh and Rc come from the air handler. When you have separate heating and cooling systems, the only thing that should be in common is the thermostat. Rh and W should run between the boiler and the thermostat. While Rc, G, and Y should connect the air handler to the thermostat. – Tester101 Nov 4 '14 at 0:21
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    Y and C are also used to connect the air handler to the condensing unit. – Tester101 Nov 4 '14 at 0:23
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    Than you very much. I rewired the RH and W to the boiler controller and that worked. When a previous plumber hooked a steam humidifier to the air handler, he switched the wiring around.I have removed that EWE autoflo, it only worked properly for a few years. Again, thank you !!!!! – David Nov 8 '14 at 23:02
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The boiler probably has its own transformer. Removing the RH-RC jumper allows the thermostat to close the switch that the boiler needs closed; not powered.

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    Thank you for your reply.Yes, the Taco boiler control has it's own transformer. However, why did the previous thermostat work with the return wire of the Taco going back to the common in the air handler, and this Sensi does not? Is there a malfunction in the Sensi? – David Nov 3 '14 at 0:10
  • Your setup was ad-hocked: "When a previous plumber hooked a steam humidifier to the air handler, he switched the wiring around." – Mazura Apr 28 '16 at 3:21

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