I've looked at just about every post on adding a C-wire to power a WiFi thermostat but remain uncertain on how to proceed. The current installation on a Rheem RGAC-080A is two wire as here:
XFMR - White -> Thermostat - Red -> Valve TH terminal, Valve TR terminal to silver Honeywell box (capacitor?) -> XFMR
(Tried to take a photo but it's too dark to be informative.)
I can run 3-wire from the thermostat to the furnace and reproduce the Red & White connections. But where does the third wire go? Valve TR?