Have an old school mercury thermostat using only 2-wires controlling a
zone valve (water).There are two un-used wires in the 4-wire bundle. Can
the 24-V power the batter ybackup in the NEST?
I found that the original install used a 4-wire bundle. There is 24V on
the Honeywell water valve in the basement.It looks like the current
2-wire has doubled up at T-stat and valve. I think I can break out a "C"
wire
