Run a 24v from a Tri-Volt Transform to a Nest 2nd Gen Thermostat

I'm trying to run a 24 volt wire up to my thermostat. In the basement almost directly below the thermostat there's a junction box w/ a tri-volt transformer connected to it already. I've uploaded a picture of it and here's a link to something very similar.

I'm assuming that I connect the two 12v secondaries and I can run a single wire up to the transformer. I figured I'd ask before trying anything.

I have 2 wires coming from the furnace a W and Rh. I'm assuming that I will not need the Rh after I wire the new 24v. Thoughts?

 

Welcome to the forums.

You still need to connect the W and Rh to the nest. These are the control wires that turn the heat on.

You will need to run two wires from that transformer. Use the outer two terminals that are labeled 24v. You will connect those two wires to Rc and C. No need to observe polarity.

Be sure there is no jumper between Rh and Rc.

 

I did this exact thing with this exact same equipment tonight, on the advice of PJmax. It was incredibly easy and worked perfectly (I'm guessing you were having the same "pulsing" problem I was having with the Nest).

 

I ran the wire and connected it to the transformer, but I think the transformer is blown. I'm not getting a reading across them. Should I buy another tri-volt transformer or something different?

 

I decided to hook up the nest and try and see what happens. Going through the setup, and it detects the W, Rc and C wires, but not the Rh wire. I measured this across the Rh and W and i was getting around 13.5v.

nestor87 did you receive the same error message?

Edit: here's the link to n22
https://nest.com/support/#troublesho...m-chooser=true

 

Pretty hard to help you..... you didn't leave much information.

Why can you get C from the furnace ?
With connecting Rh and W to stat..... you should get 24v across them.

 

Sorry,
I have a boiler (not sure if relevant) and when I measure across the transformer's Rh & W I get about 13.5v. The house has a junction box directly below my thermostat with a tri-volt stepdown transformer off it (see pic below)

According to your advice, I ran 2 new wires to the tri-volt 24v load. The wires go to Rc & C.

The nest is powered & charging but I'm getting an error (n22) that it's not receiving power from the Rh. I unplugged W & Rh, measured across them, got 13.5v.

Going through nest's error troubling shooting wizard, it said to ignore this error if the furnace turns off/on. I verified that the furnace wasn't running, cranked the heat, and the furnace turned on. It turned off when I turned down the temp too.

I was just wondering if nestor87 also received the same error.

I just looked at the schematics for the boiler and the transformer should be 24 volts.
I could run another wire down to the boiler and connect to the C. But then what would I do w/ the 2 wires coming from the extra tr-volt transformer.

 

If you are just running your boiler off that stat then get your C connection from that.
Use one of the wires that is currently going to that tri volt transformer.

You use W, Rh and C directly from the boiler.

What is the Rh and W connected to..... a zone valve.... an aquastat ?