Honeywell RTH8500 User Manual page 19

Installing your thermostat
Alternate wiring key (conventional system)
Remove metal jumper connecting R and RC only if you must connect both
1
R and RC.
If your old thermostat had both R and RH wires, remove metal jumper.
2
Connect the R wire to the RC terminal, and the RH wire to the R terminal.
3 Do not use K terminal. For future use.
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