To replace the backup battery in a Honeywell VisionPro 8000 thermostat, gently pull the main display faceplate straight away from the wall mounting subbase. Locate the coin-cell battery compartment (typically a slide-out tray on the side or a holder on the back of the display unit). Remove the old lithium coin-cell battery and insert a fresh replacement cell with the positive side facing outward. Snap the thermostat display firmly back onto the wallplate. Note that units powered by a continuous 24V C-wire use the battery strictly for backup memory retention.
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