Navigating wiring for boiler - Why do I have two wires on one terminal?

junta

I am trying to understand how my boiler with three zones has been wired so I can see if I can add a C-wire adapter to it. It seems strange that I have two wires twisted together on the W and another two on the R terminal at the thermostat. The red wire that is jumped on the first photo is coming from my C screw on the terminal. I have two questions:

1. What is the purpose of having two terminals on one terminal?
2. Can anyone help me understand if my setup could be adapted with a c-wire adapter to run a smart thermostat? I am trying to avoid using a 24v transformer plug in adapter.

Jamie Hall

Sigh. Someday I will see a neatly done arrangement...

If what I think I'm seeing is what I'm seeing, you have three thermostats with twisted pair wires (red/green and yellow/black). I also see three other cables labelled "from thermostats" but I can't tell where they are landed.

On the thermostat pictured, the yellow/black pair is landed on the W (switched) terminal, and the red/green pair is mounted on the R/RC terminal (power hot).

On the terminal strip, it appears that all three twisted pair red/green wires are landed on a terminal which is also the green lead from the transformer. The three yellow/black pairs are landed on three separate terminals, two of which have heavy yellow wires (one has two heavy yellow wires, the other one). The other terminal with the yellow/black pair doesn't appear to be connected to anything; is this a dead or unused thermostat? The red lead from the transformer appears to be landed on two terminals, one with three heavy yellow wires on it and one with none.

Is it possible that those heavy yellow wires go zone valves? If so, two of the zones are controlled by one thermostat, one zone is controlled by another thermostat, and the third thermostat does nothing at all.

There is also a light gauge red wire on the terminal strip which connects to 3 (I think) heavy read wires, and a light gauge white wire which connects to two or maybe three heavy red wires -- I have no idea where those wires go.

You will have to check your thermostat manuals, but in many cases the B terminal is intended for power common.

To answer your second question, then, it is possible that you could take one of the two wires in the red/green twisted pair and -- having checked it for continuity and lack of shorts -- land it on the B terminal (if that is power common for your thermostat) and then back at the terminal strip land it on an otherwise unused terminal, and connect that terminal to the red wire from the transformer. Without actually being there and checking the manuals -- and the connections of those heavy wires -- I offer no guarantee that this will work.

STEVEusaPA

I saw one like this before where the h/o used phone wire and put two wires on every terminal.
In your case I think someone didn’t know how to gang power and end switches so they ran home runs to/from every zone valve.

Jamie Hall

STEVEusaPA said:

I saw one like this before where the h/o used phone wire and put two wires on every terminal.
In your case I think someone didn’t know how to gang power and end switches so they ran home runs to/from every zone valve.

It does look like 'phone wire, for sure. The only thing I'd be concerned about would be the current carrying capacity of that fine wire.

Steamhead

Jamie Hall said:

STEVEusaPA said:

I saw one like this before where the h/o used phone wire and put two wires on every terminal.
In your case I think someone didn’t know how to gang power and end switches so they ran home runs to/from every zone valve.

It does look like 'phone wire, for sure. The only thing I'd be concerned about would be the current carrying capacity of that fine wire.

You guys nailed it. Phone wire is maybe 26-gauge, so they doubled up the conductors to get the needed current-carrying capacity.

I'd replace that with proper 18-gauge thermostat wire. Use a 3-wire cable so you'll be able to hook up the C wire when ready.

EBEBRATT-Ed

@junta

You would be much better off figuring out how to get a C wire from the new stat even if you have to add another transformer and relay.

All the little adapters and workarounds are just that. Smart thermostats are unreliable without a C wire. We get several posts a week about these stats failing. They may work at the beginning but most rely on "power stealing" to charge the battery when the equipment is off. It doesn't work very well.

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