I replaced old thermostat with Honeywell wifi and added C wire from furnace to new thermostat. I left the jumper wire on Rc and R in place and wifi would not come on line. Checked C wire and have 28vdc from C to R. The C wire is grounded to furnace just as it's supposed to be. The old thermostat does not have a C wire or Rc terminal. It is a Trane XT300C thermostat. What am I doing wrong?

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    I'm confused. In your first sentence you said you replaced your old thermostat with a Honeywell. At the end you say it's a Trane. – isherwood Dec 18 '17 at 20:23
  • Side note: HVAC control systems typically use Alternating Current (VAC), not Direct Current (VDC). – Tester101 Dec 19 '17 at 11:48
  • Although the voltage between the R wire and the furnace frame might read 28V, you have to attach the C wire to the neutral lead of the 24VAC side of transformer feeding the furnace control board. – Stavr00 Dec 19 '17 at 14:07

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