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Can I connectenter image description here C-wire to the empty terminal(red arrow)

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No, no, no!

That terminal is on the wrong side of the transformer! It's a primary tap used when the furnace is hooked up to a circuit or service that is "two-thirds" of a three-phase system, and has nothing to do with your quest for the C wire connection point.

Instead, you want to look at the wires that connect at the top of the transformer (helpfully labeled "24VAC" with the upward pointing arrows) in your picture. One of them will measure 0VAC or thereabouts to the R (red) wire going off to the thermostat (that's where the R wire comes from); the other will measure 24VAC or so to the R wire, so this second wire will be where you want to tap your C wire connection at.

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  • Your second paragraph is extremely confusing. You make it sound like if the red wire from the transformer is the one going to the thermostat that you hook the C wire there.....when in fact that would be incorrect. – Michael Karas Jun 29 '18 at 12:13

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