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I have 3 wires coming out from the wall at my new house (red, white, blue). It is a water boiler heat only system. The tstat was already removed unfortunately.

I thought the blue wire was the C wire, so I hooked it up to a nest but it didn't work. It said there was no power.

I checked the voltage as following:

  • red and white: 24V
  • red and blue: 0V
  • white and blue: 24V

I put R and W together and the heat is on.

So I think the blue wire is not a C wire? because it looks like it is hot. So what is this blue wire for? or I think the red and white are somehow swapped... so white is hot and red is neutral? At the relays, does it matter which wire goes to the terminals? It is a heat only boiler system, no cooling.

These are the wires from the relays. Zone 3 is the zone. I don't even see any blue wire.

enter image description here

  • 1
    Can you post a photo of the wiring at the zone valve or circulator pump? – ThreePhaseEel Dec 31 '18 at 2:32
  • 1
    Hello, and welcome to Home Improvement. Looks like you've created two accounts. You should use the Help/Contact page to ask SE to merge them. – Daniel Griscom Dec 31 '18 at 20:29

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