Need help on identifying and setting up c wire

[Samir A]

Hello all,

I’m new to this forum and would like somebody to really help me. I had two Navien NCB 240 boilers installed three years ago, One boiler for my tenant upstairs and one boiler for the first floor and basement and I’ve attached the picture of a switching relay and my nest thermostat. It looks like the thermostat wires are all not being used and the switching relay looks like nothing is connected to common and 24V. Every time I try to connect my nest thermostat to Internet it says low battery so I figure it’s not getting power from the switching relay.

Can someone help me see if I can set up or connect the wires not being used for the thermostats to the common or 24V If it’s possible or would I need a venstar add a wire. Never messed with any electronics before so step-by-step would be really great and appreciated!

 

[Jadnashua]

It looks like the two terminal strip in the upper right is labeled 24 V and common. IF that's correct, take an unused wire from the cable going to your thermostat, connect one end there to the common, and then hook the other end up to the thermostat's common (C) input.

With the shadows, it's hard to read all of the labels.

 

[Sponsor]

 

[Samir A]

Sorry about the shadows in the pics, basement is kind of dark lol. One screw on the top right says common and the other is 24V, there are 3 thermostats in the house total, can I connect 3 unused wires from the 3 different thermostat wires coming in and wire them together to one of the screws in the top right? And if I so, which one, the common or the 24V screw? Also, can I do it without shutting down the boilers or switching relay?

 

[Fitter30]

Looked at the install.manual yes you can use the common. Might be easier to use a wire nut and a piece of thermostat wire then theres only one wire under the terminal.

 

[Samir A]

You guys are awesome!! Thank you so much for helping me! I thread both thermostat blue wires with a piece of thermostat wire to a wire nut and connected it to the common screw, worked like a charm!

 

[Jadnashua]

The 24vac transformer will have a VA rating. The total of the devices attached should not exceed the VA rating of the source. So, there is a limit on how many devices you can power from the source. You could be fine, but don't know for sure.

 

[Fitter30]

The 24vac transformer will have a VA rating. The total of the devices attached should not exceed the VA rating of the source. So, there is a limit on how many devices you can power from the source. You could be fine, but don't know for sure.

30 va transformer 1.25 amps nest stat pull 40ma x 3= .12 amps

 

[Samir A]

30 va transformer 1.25 amps nest stat pull 40ma x 3= .12 amps

So I should be good for 3 nest thermostats?

 

[Jadnashua]

The smart thermostats are a load if their battery is charged by the supply, but also the things they control are a load as well. The output of the thermostat pulls in a relay or some other device in the furnace or a/c unit...those don't normally take a whole lot of power, but again, without knowing, can't say. The spec sheet or installation manual will often list their load.

In reality, you probably won't have an issue. I brought it up in case it could be, and it is a consideration. A dumb thermostat or one that uses batteries isn't a load on 24vac...it just opens and closes switches that cause the other things to operate.

 

[DIYorBust]

I did have an issue witht his once. A contracter removed the thermostat and allowed the c wire to short to neutral. There is no overcurrent protection on the 24vac circuit and it fried the transformer. I was happy the thermostat was connected to the relay and not the boiler. The transformer can be replaced, but I couldn't find one that fit in the relay case and ended up replacing the whole relay unit, which was annoying.

 

[Stuff]

If you add a C wire then Nest will likely start reporting that it is using 200 ma. I haven't put a meter on one and measured it though.

 

[WorthFlorida]

Here is a diagram I modified a few years ago to show how the C wire fits in the scheme of things. Wifi thermostats need continuous power and the WiFi take much more power for the wifi and the led screen, therefore batteries would not last very long. Older digital programmable thermostats used batteries to power the electronics and it was because most homes did not have more than four wires. With a hydronic heat system most old homes had only two wires.

 

[DIYorBust]

There are a few workarounds if you don't have a c wire. But that doesn't seem like the issue here since he has access to the relay.

 

[Jadnashua]

Depending somewhat on how often the thermostat actually is calling for heat or cooling, some can charge themselves by parasitically drawing power from that running through it to activate the HVAC system, but that may not be enough to keep it's internal battery charged which is why it's easiest if you can actually tap the common of the transformer.

A basic, simple thermostat doesn't need power. Many have replaceable batteries, so don't need the C wire. Nest, with its WiFi connection and display, work best when they can be powered constantly, and that takes both sides of the transformer.

 

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