My nest thermostats are missing a common wire which means they’re not getting enough power to work properly. I actually didn’t realize this until an electrician told me about it (it hadn’t become a problem yet), but now it has and I brought him back in since he indicated he could do the work for $100 which seemed pretty reasonable. It turns out I need to add a new relay and now the price has gone way up so I’m going to try doing it myself.

I’m hoping someone can help me understand why I need a new relay and if this is something I can reasonably do myself. I’ve done some limited electrical work, but this is a bit advanced for my experience. If someone can help me better understand this system I imagine it’s something I could do myself, but perhaps it’s better left to a professional?

The white-coated wire here is a new wire since one of the two zones was a two wire and had to be replaced. There are actually three zones here, but I am only using nest thermostats on two of them at the moment (but it couldn’t hurt to future proof it). The two pre-existing ones are hot-water heat and the one I’m adding is steam heat. The brown-coated wires already have three wires. The green wire is being used as the common wire.

Here is the setup

High level view of the side - the brown and white wires are coming in from above. You can see the existing relay and transformer here.

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transformer and you can also see here there’s a Honeywell box that the white and red wires on the bottom of the relay run to - I’m guessing that’s for the hot water heat?

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That brown wire that runs to the Honeywell part (not sure what that is?) then runs along with the white wire to the oil burner here:

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Some additional images in case there’s anything I missed:

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  • Which model of Nest are you trying to use? Jul 22, 2020 at 0:04
  • The Nest E. Despite indications that it those the two existing wires should be enough I am already having issues.
    – Jordan
    Jul 22, 2020 at 0:09
  • I should probably also note that these are being used for heating only. A/C comes through minisplits.
    – Jordan
    Jul 22, 2020 at 0:14
  • Can you get us a clear photo of the label atop the transformer inside your relay module? That'll tell us how many VA (Volt-Amps) of power the transformer can handle Jul 22, 2020 at 0:16
  • 1
    Ah ok yes that one is 15VA
    – Jordan
    Jul 22, 2020 at 1:42

2 Answers 2


I would try connecting the C-wire to the existing 24V COM terminal on the relay and seeing if it works

Your relay, thankfully, makes its C (Common) connection accessible (not all HVAC relay modules of that nature do). So, given that your Taco's transformer secondary is rated at 15VA (as per the label on the transformer) and the Nest only pulls about 5VA of power, based on this document, the Nest shouldn't have any issue working with your relay once that C wire's hooked up.

  • Is it possible the problem was with the new red and white wires and had nothing to do with the common wires? They are stripped as if he wanted to connect those to the relay.
    – Jordan
    Jul 22, 2020 at 0:33
  • @Jordan -- I doubt the problem would be with the new red wires Jul 22, 2020 at 1:06

You've got 2 transformers here: one mounted to the furnace, and one in the furnace relay box. I don't know why they did that -- maybe an upgrade at some point?

Anyway, it looks like you have 2 thermostats going to one thermostat terminal that controls both hot water zones, and that those are contained in the green box on the left.

It looks like you are adding a new zone, and you'll need a relay to control your steam system valve (or however that thing works). This will require another green box, and that's why the electrician was quoting a higher price.

To power your new nest, it looks like you can either connect the green wire in the white casing to either the common terminal in the green box or to the other green wires on the other transformer. That will get it to power on, but I don't think it will control your steam valve.

  • 1
    This worked as-is but the nests just weren’t getting enough power. The new wire was added simply because the existing wire for the steam heat was only a 2-wire so we had nothing to hook up to common.
    – Jordan
    Jul 22, 2020 at 10:32

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