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Some background first. 30-ish year old furnace/boiler with a zone system and six zones. This was my parents full-time residence in rural Maine, but after my father passed away in 2022, and my mother moved into an assisted living facility, I installed Nest thermostats to be able to control the temperature remotely (from my home in California) so that the pipes don't freeze, etc. The wiring to the existing Honeywell (mercury) thermostats was only two wires, so I presumed a C-wire was not available, and yet the Nest thermostats functioned normally...until March of this year, when one of them stopped working. I assumed this was due to the missing C-wire situation, and that after a year plus of being underpowered, the Nest gave up the ghost. (I suppose it could be the thermostat just died on it's own...after the warranty expired.)

Thinking I now need to solve the missing C-wire problem, I am left with a few potential options. One, buy those plug in 24V adapters to connect to each thermostat. This is the least desirable option, mostly because I don't want stray wires hanging down the walls. Two, pull new wiring. This is risky, of course, or maybe not possible if any wiring is fastened to a stud. And three, install Google Nest Power Connectors to each zone at the furnace level.

I am posting this from California so I had a neighbor take some photos. There does not appear to be a "proper" zone controller or any real circuit board of any kind. Any advice on how to proceed is appreciated!

Transformers?

Some of the zones

Furnace/boiler

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  • Do you have one thermostat controlling all zones? Or one per zone? Or what? I recently addressed a similar hydronic system and can explain it but will need some details first. Do you know whether it uses zone valves, or zone pumps, or a combination of both?
    – Matthew
    Commented May 5 at 20:02
  • I strongly suspect you had a single thermostat fail, since it's very unlikely that the charge on the battery in the thermostats lasted a whole year. What do the battery voltages look like on the other units? If it's 3.8v or higher things are charging fine and you just need to replace the bad one.
    – KMJ
    Commented May 5 at 20:56
  • @kmj these thermostats can sometimes work on the phantom draw while they're in use. Sometimes you need additional resistors to make them work but if the system does not run often enough they will eventually drain out and require resetting
    – Matthew
    Commented May 5 at 21:05
  • @Matthew There is one thermostat per zone. I've added four Nest thermostats, three of which are functioning as normal. As to the zone valves or pumps, I don't actually know, but I expect it is valves.
    – jsbigs
    Commented May 5 at 22:00
  • @KMJ The other three thermostats are reading 3.81, 3.87 and 3.89 volts, so...normal? The system is always running, at least through the fall, winter and spring, as I have the thermostats set to 54°F when no one is at the house.
    – jsbigs
    Commented May 5 at 22:04

2 Answers 2

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That's a relatively nice installation. Relative to all the much worse ones. Everything in one place, wired in a logical way. It would be nice if all those cables were labelled!

Trace all the cables as far as you can. You can easily follow the ones that go to the zone valves. There will be six going to thermostats, and you need to identify them.

With an unfinished basement ceiling there is a good chance of easily pulling new cable to all the first floor thermostats. Pull new cable wherever you can easily do that. For the remainder, use C-wire adapters. The way this is wired, with a wiring hub done logically right next to the transformers, makes it very easy to do that. Once you identify and label the cables, that is.

You also need to look at the power rating of the two transformers. Add up the power requirements of six zone valves, six Nests, and however many C-wire adapters you need. You may need bigger or more transformers.

If you want to avoid adapters, and if there is an unfinished attic, another approach for upper floors is to run a big (eg 18/8) multiwire cable up to the attic, and fish down to each upstairs thermostat from there. It might not be that hard.

A somewhat cool thing: I think (can't be sure from pic) the two leftmost terminal blocks, jumpered together, are the "C" bus. And lookie: you have a spare block to the left, you can jumper that on top and put the six new C wires in underneath. Almost like it was anticipated. :)

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You have two 24 Volt transformers. One for the furnace and one for the zones.

You also have 3 zones heating set up.

You could borrow a 24 Volt (and C) from one of the transformers to feed the Nest.

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