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I have an old Honeywell Chronotherm IV thermostat that I am trying to replace with a Honeywell RTF6580WF wifi thermostat. I have a gas furnace and an air conditioner.

Wiring for Chronotherm IV (wires in all terminals but R):

D G B Y W RC R (with wire in RC and jumper from RC to R)

Note that the 'D' terminal above may actually be an 'O' -- it's not clearly marked.

RTF6580WF wiring:

K Y2 W2 G W C Y R Rc (with R-Rc jumper)

I'm not sure what to do with the old D wire (which might actually be a letter 'O' but its very hard to read) and the B wire, and I don't know which one is my C wire on the old thermostat. I know I need a C wire.

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Make a list of all the terminals, and the color of the wire that connects to each.

Rc - Red
R - Jumper
W - White

Next turn the breaker off for the furnace, and/or switch off the serviceman switch for the furnace. Then open the service panel on the furnace. You should be able to locate the control board where these wires connect, which should tell you everything you need to know.

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