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I have an oil furnace that feeds hot water to baseboard in four zones in my house and also to a hot water tank for tap water. There is a 24vac transformer with two output wires that feed power to the electric zone valves. I want to change one thermostat in a bedroom to a Wifi unit that needs a c-wire. I have read that I can connect the c-wire to the transformer (power stealing), but that is not recommended. I have 5 wires running to the old thermostat, but only Rh and W are connected. I think a good option would be to just replace the old transformer with a modern one. Why is it that I cannot find an HVAC transformer that has a dedicated common terminal? To me this makes more sense then adding a second transformer. Any recommendations?

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    Every HVAC transformer has a 'common' terminal. The 'common' terminal is whichever of the 2 terminals is not connected to the R terminal on your thermostat. – brhans Sep 14 '17 at 21:37
  • Are you referring to some sort of transformer/relay combination device when you say "transformer?" – ThreePhaseEel Sep 14 '17 at 22:44
  • No weird situation. The answer below looks correct to me. I have R and W, so I should connect my C from the thermostat to the transformer at the same terminal as W. – johnlo23 Sep 14 '17 at 23:06
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Um, I don't know where you got the term "power stealing". What you describe as that, is exactly the normal way to do that thing, and the right way... unless you are aware of some capacity issue on the transformer (as is sometimes seen in transformer-relay combos).

As far as the R and C terminals, any HVAC transformer has two pins on the 24V side... They are R and C. The one that is not R is C.

  • I realize my error now. Power stealing is when a thermostat taps a small amount of power from the R and W lines to charge its internal battery. I'm amazed with all the write-ups about adding a c-wire, nowhere have I seen someone explain so simply how to wire my situation. Thanks! – johnlo23 Sep 14 '17 at 22:57

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