Can't figure this 3 wire system out or find anything in postal about it

Household Thermostat Wiring Diagram

Can't figure this 3 wire system out or find anything in postal about it Old thermostat wiring diagram

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Wiring Diagram For Ac Unit Thermostat

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Old Thermostat Wiring Diagram - Collection - Wiring Diagram Sample
Old Thermostat Wiring Diagram - Collection - Wiring Diagram Sample

Wiring Diagram For Ac Unit Thermostat
Wiring Diagram For Ac Unit Thermostat

Can't figure this 3 wire system out or find anything in postal about it
Can't figure this 3 wire system out or find anything in postal about it