Your system is a two wire heating system. What it means is an incoming voltage comes in on the R wire, and when it calls for heating, the thermostat connects the our wire to the W wire and that will initiate the heating cycle. Now if this system is based upon a 24 V AC system, it can be made to work with a nest thermostat. However, you don’t currently have a common wire connected. I noticed in your picture that there is a black wire that’s not connected, and maybe that can be connected to the common in your furnace system, and use as common for the nest.
Ken, AC Cooling Wizard
NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.
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