I am trying to install an Ecobee 3 thermostat to replace the upstairs one I have now but the 3rd wire isn't connected to anything on either end. I added the basement thermostat just to complete the wiring but I won't be changing that one. Can I connect the 3rd wire to the C terminal on the external transformer (top left)?

Wiring Transformer


Yes, the terminal labeled C on the transformer is where you want to connect the C wire to the thermostat. However, you'll want to make sure that the transformer is large enough to also supply power to the thermostat. If the thermostat draws too much power, you could end up burning out your transformer. It might also be a good idea to put an appropriately sized fuse in line with the R wire.

  • I added a picture of the transformer and the Ecobee 3 says: Power Consumption less than 3.5 VA. Is there anything else I can supply that would help show if the transformer could handle it? – Ian Nov 16 '17 at 15:57
  • @Ian Does the label on the zone valves have a current (ampere) or VA (Volt-Ampere) rating listed? Assuming ~7 VA per valve + 3.5 VA for the thermostat (7 * 3 + 3.5 = 24.5 VA), the 40 VA transformer should be fine. – Tester101 Nov 16 '17 at 18:44
  • I am assuming all 3 are the same but I'll have to verify that. I know one of them says 24V 0.9A. I also read that a class 2 transformer can run at 200% based off NEMA Class II Transformers Standard DC 20-92. So if I have 3 24V 0.9A that would be 21.6VA x 3 = 64.8VA + 3.5 = 68.3 VA with all 3 valves on. If the Transformer can run at 200% then it should be able to handle 80VA correct? – Ian Nov 16 '17 at 20:57
  • @Ian wasn't able to find the NEMA standard online, so I'm not sure what it says. – Tester101 Nov 16 '17 at 21:13
  • Someone else had mentioned it in another forum, but I believe this is it: nema.org/standards/pages/… – Ian Nov 16 '17 at 21:28

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