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I am installing a Honeywell Wi-Fi thermostat. I do not have a C wire, but there is an unused blue wire in the thermostat wiring, and also in the furnace wiring.

The videos I've watched say that I can use the unused blue wire as the C wire. That's no problem for the thermostat, but when I look at the furnace, there is already a wire in the C port. Can I put the unused blue wire in the C port, while still having the existing wire there (so, having both wires in the C port)?

I've included pictures of the thermostat and furnace.

Thermostat wiring

Furnace wiring

  • LOL I would certainly hope the C terminal is in use at the furnace! If it wasn't used anywhere, there'd be no reason to have it! Seriously, "C" is the current return for the relay or solenoid. It's like the neutral wire, if a thermostat was a switch loop... and then you put in a "smart device" and it needs a neutral. In fact it's exactly like that. – Harper Jan 5 '17 at 0:30
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    @Harper If the home did not have A/C, the C terminal on the thermostat terminal strip may not be in use. The red wire on the Y terminal, and the white wire on the C terminal surely go to the contactor in the condensing unit outside. – Tester101 Jan 5 '17 at 16:00
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Yes. It's not exactly code compliant (as far as NEC goes), but it is an accepted practice in the HVAC industry. Notice that there are two wires on the Y terminal.

I believe this question has been asked a few times before, so may get closed as a duplicate.

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Can you double-stack two wires onto that terminal? Yes if the terminal is listed for that application, which the maker could say.

Amateur reverse-engineering the design, it certainly looks like "yes", it looks like the lower plate has reliefs cut along its edges and the upper plate has downbent teeth on all 4 corners, to bite into a wire laid into that relief. As done on the Y terminal, almost.

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