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My furnace is old and only has 4 wire connections, but I require a C wire. How can I create one based on this diagram?

I do have 5 wires running between the furnace and thermostat.enter image description here

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The way to find a C wire is to look for the transformer, and then find the secondary (low voltage) wire that is not the R wire.

In your case, this is the wire in the diagram going to the right of the part labeled "24 VAC", which is the transformer.

enter image description here

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