Trying to decode what the previous HVAC Tech did is like trying to determine why the previous Network Analyst setup a port that way.
I got excited when I took down the ancient thermostat to install my device, finding 18/5 wiring!
It was also capped to another 18/5 outside the furnace! What my luck! A C wire!?
Oh, they clipped off about 4” of the C wire and only ran 4 to the board. Why? Who the fuck knows. Fixed with a 4” 18AWG jumper wire.
Apparently I do not only CAT 6A/6/5e/5/3… but HVAC as well. Also… electricians… up your Low Voltage game please, not everything belongs on a network. Kthxbye.