Old thermostat has an unused blue wire, should I use it for Nest 3rd

Blue Wire Not Connected To Thermostat

Can't figure this 3 wire system out or find anything in postal about it Thermostat wiring diagram mean letters wire 2wire only where ifixit thermostats which goes

Thermostat unused wires Thermostat wiring wire heat color diagram pump air nest guide common goodman colors honeywell code labels heater aruf handler wires Old thermostat has an unused blue wire, should i use it for nest 3rd

What All Those Letters Mean on Your Thermostat’s Wiring - iFixit

What all those letters mean on your thermostat’s wiring
What All Those Letters Mean on Your Thermostat’s Wiring - iFixit
What All Those Letters Mean on Your Thermostat’s Wiring - iFixit

Old thermostat has an unused blue wire, should I use it for Nest 3rd
Old thermostat has an unused blue wire, should I use it for Nest 3rd

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