Honeywell RTH8500D Owner's Manual page 56

4. Align the screw blocks with the pins on the back of the
thermostat.
TERMINAL
SCREW
BLOCK
5. Push the thermostat straight onto the wallplate until it snaps
into place.
69-1725-1
Replace Batteries (Cont)
WALLPLATE
56
PINS ON
BACK OF
THERMOSTAT
M22213