I have a house/furnace built in the 1980s. We have only red and white wires connected to thermostat. Thankfully they ran a third wire from the taco valves up to all three thermostats. Is it as easy as connecting the third wire to the c wire connect on nest and to the common port (second) on the taco valve?
Theoretically, you should be able to connect the C terminal of the thermostat to terminal 2 on the valve, the W terminal to terminal 1, and the R terminal to the 24Vac transformer.
Figure C shows the wiring that I am referring to.
Thanks for the helpful reply Patrick_Caezza.
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