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I want to replace a Honeywell Q674L with a Honeywell RTH6580WF for my heat pump. Which wire is the common or C wire?

Old thermostat is a Honeywell Q674L 1207 and the heat pump is an older Trane.

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The old mercury Q674's didn't require a common terminal, so if there's not an extra wire in the bundle you'll have to fish one up... Inside the old Trane, there should be either a terminal strip with the appropriate terminal designations or possibly a bundle of color coded wires. If they are wires, originally they may have had individual tags on them, but if there are none, you're going to have to refer to the wiring diagram for the correct color for transformer common. Or... find the secondary of the transformer to determine the correct one, then extend it up to the new digital stat.

Posted on Apr 17, 2015

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