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I under stand that it can be done using a transformer and a relay along with a WiFi thermostat. I want to keep the present Milli volt thermostat functioning in case I loose electrical power and I realize that I need to put the WiFi thermostat on a Parnell line to the Milli volt thermostat. However, I'm not confident of the wiring for that. Is it possible for someone to draw a wiring diaragram for this setup?

  • It's pretty simple, just wire your relay in parallel to the millivolt stat and keep the millivolt stat set much lower, so that either will complete the circuit. The potential problem that you will run into is that a few additional ohms of resistance means a lot in a millivolt circut which can prevent thus from working as expected. – Tyson Dec 19 '16 at 1:59
  • Exactly. Just hang the new relay in parallel with the original thermostat, so if either one closes, the furnace will fire. Your logic is entirely correct; the old thermostat is a backup to keep your house from freezing. – Harper Dec 19 '16 at 4:25

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