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Bought a house built in 1995 with I believe original Ruud Silhouette II gas furnace, central A/C, and a Maple Chase 9600 thermostat powered by two AA batteries. Upgrading to an Ecobee smart thermostat premium.

Original thermostat is using R,W,Y,G wires with an unused blue one available behind it. The wires go to corresponding terminals on the furnace control board, with the blue wire unused. The C terminal on the furnace is wired to a white wire coming from a 2-wire bundle, most likely goes to 24VAC transformer? The other wire in that bundle is red, and is spliced into the Y line with one of those twist-on wire connectors.

My plan is to add the blue wire into the furnace's C terminal (is there any problem putting two wires in one screw terminal, or should I splice it with a twisty wire connector like the Y line?), then connect it to the new thermostat's C terminal.

Could this run in to any problems?

I think the end of this video covers this exact situation.

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  • Please edit to include clear, focused, well lit pictures of the wiring diagram on the furnace and the actual wires on the circuit board, as well as the wiring to the thermostat.
    – FreeMan
    Jan 10 at 13:21

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The two-conductor with white going to C and red splicing and going to Y is the cable that goes out to the contactor (relay) in your air conditioner compressor.

Your plan to use the blue wire, attached along with the white wire on C, is good. Some screw terminals have a square plate below the screw head and will reliably hold one conductor on each side of the screw. If there's any doubt, a twist-on connector ("Wire nut" or other brand, orange size) or a lever connector like Wago is always a safe choice. You'll need at least 4 inches of wire to use as a pigtail between the terminal and the wire connector. A scrap of 18 gauge thermostat wire if there's enough to spare will be great, but really anything 16-20 gauge would reasonably work.

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