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Trying to swap an old Honeywell thermostat for new with Wi-Fi. Old thermostat doesn’t need C wire, but I have an unused blue wire in the wall. I installed the new thermostat correctly with the C wire, went down to my furnace and securely connected the C wire into the C terminal there (there’s another wire in the terminal as well, I’ve double checked their connections), but still get no power to the thermostat. Furnace is a mid-00s electric Frigidaire, pretty simple setup. I’ve swapped thermostats before but have yet to run into this problem, any help appreciated!

Upstairs (thermostat I have tried to replace): enter image description here

Downstairs (no changes other than connecting C wire to C terminal): enter image description here

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    Can you post the wiring diagram for your furnace please? – ThreePhaseEel May 14 at 11:43
  • Start by verifying you have voltage between C-wire post and hot side at the furnace connection block. Then verify same up at the thermostat location. Are there any other thermostats in the house (in which case there might be multiple 'blue wires' down by the furnace)? – Carl Witthoft May 14 at 14:34
  • I do have another furnace downstairs on a separate circuit breaker with it’s own thermostat, I have not replaced that thermostat yet. I did hook it’s C wire up at furnace, but have not yet done so at the thermostat itself. They appear to be wired separately. I have pictures of the wiring of both furnaces that I will attempt to post. – Chad in Idaho May 15 at 15:12
  • Yes, can you measure 24V between R and C at the upstairs furnace? Also, does the air conditioning upstairs work? Or is that thing connected only to Y and C at the upstairs furnace something else instead of an air conditioner? – ThreePhaseEel May 15 at 15:45
  • I don’t have my multimeter right now but I can probably get another later today. The AC works fine, all of it works. – Chad in Idaho May 15 at 16:00

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