Nest thermostat C wire

rob51

Yes. I understand what you are saying. Perhaps I was not being clear. I would connect everything like in this diagram you made:
https://us.v-cdn.net/5021738/uploads/editor/ru/xnlrcozb49iz.png

Instead of installing another transformer, I would use this one. Would that work?

rob51

rob51 said:

Yes. I understand what you are saying. Perhaps I was not being clear. I would connect everything like in this diagram you made: https://us.v-cdn.net/5021738/uploads/editor/ru/xnlrcozb49iz.png Instead of installing another transformer, I would use this one. Would that work?

@EdTheHeaterMan

EdTheHeaterMan

NO.

The transformer you show in the picture is transformer A in this diagram.

In the diagram Transformer A is isolated from the zone control transformers B and C. by way of the isolation relay X X on the Taco control. This it the best way to do it.



If you connect transformer A to the zone thermostat transformer in place of a separate transformer C, then you will be eliminating the isolation circuit created by the Taco Control X X. I guarantee that if you do this, you will be purchasing two new transformers and a new Taco zone relay.

rob51

I understand. I was concerned about the implications of the isolation circuit. I’m glad I asked. Thank you for explaining this to me.

rob51

Also, the 120v input on the relay is taken by a switch that turns the boiler on/off. I’m wondering if I should pull power for the transformer from this switch or elsewhere (I have other options).

EdTheHeaterMan

rob51 said:

Also, the 120v input on the relay is taken by a switch that turns the boiler on/off. I’m wondering if I should pull power for the transformer from this switch or elsewhere (I have other options).

Use the same power source. It is all part of the heating system and it will be easier for service to disconnect one switch to disable everything. Adding that transformer will not overload that circuit. It will also keep the Taco control transformer and the thermostat transformer on the same phase.

rob51

EdTheHeaterMan said:

rob51 said:

Also, the 120v input on the relay is taken by a switch that turns the boiler on/off. I’m wondering if I should pull power for the transformer from this switch or elsewhere (I have other options).

Use the same power source. It is all part of the heating system and it will be easier for service to disconnect one switch to disable everything. Adding that transformer will not overload that circuit. It will also keep the Taco control transformer and the thermostat transformer on the same phase.

Thank you for all your help.

rob51

@EdTheHeaterMan it’s just simple 120 coming into the switch, then going to the 120 input in the relay. I know how to wire this to the transformer, but do you know of a diagram for reference where I can double check my work? Thank you.

EdTheHeaterMan

I don't know of a diagram that has all your components on one page with every wire. I do know that there is a wiring diagram for each component individually. Each of the diagrams have the 120 Line voltage that feeds that diagram indicated by some form of. "L1 H L Common COM N Ground 120 VAC". Some will add the overcurrent protection or a service switch. Some will only have H and Com or L and N. But each one of those diagrams for each individual component are written for electricians that know the electric code and assumes that you already know the answer to the question you just asked. This is the best I can do for you.



If you have any questions on how to wire up something after seeing this, then you are not qualified to DIY this project and you need to call a licensed professional to handle this.

rob51

Ok.  Thank you.

harry22

I have an old system with three Taco 571 zone valves, one circulating pump, no controller, two external 24V transformers wired in parallel (not good form, but they've done the job for 35 years) and all 2 wire thermostats. Ignoring the fact that it will be a PITA, can I just run a common wire to each thermostat location from the transformers and use a nest thermostat without any problems?

Jamie Hall

Well... yes, assuming you get the wire on the correct terminal from the transformers...

But... un... why? What will the Nest do for you that a good programmable battery operated thermostat won't, other than broadcast your doings to Google and whoever else might be interested?

pecmsg

Jamie Hall said:

Well... yes, assuming you get the wire on the correct terminal from the transformers...

But... un... why? What will the Nest do for you that a good programmable battery operated thermostat won't, other than broadcast your doings to Google and whoever else might be interested?

and not work properly!

harry22

1) I hate programmable anything. I change settings manually. 2) Most importantly, control from the smart phone is key- going to the thermostat to mess with the settings is a pain, especially if you change settings often (as is the case for two people with different comfort tastes). I don't care if the Russians, Chinese, Google or the CIA know my thermostat settings . 3) It's Free from the utility, so I might just give it a try- don't know if I'll try it on a heat zone or my central A/C (which has a far more accessible common wire). I won't throw the old thermostat out, that's for sure. Thanks for the reply.

EdTheHeaterMan

harry22 said:

I have an old system with three Taco 571 zone valves, one circulating pump, no controller, two external 24V transformers wired in parallel (not good form, but they've done the job for 35 years) and all 2 wire thermostats. Ignoring the fact that it will be a PITA, can I just run a common wire to each thermostat location from the transformers and use a nest thermostat without any problems?

Yes but you need to be sure the R from the transformer goes to the R on the thermostat, and the W on the thermostat goes to the number 1 terminal on the zone valve. on a 2 wire only thermostat, the R and the W don't matter. When the C wires is in play, IT MATTERS!

Jamie Hall

EdTheHeaterMan said:

harry22 said:

I have an old system with three Taco 571 zone valves, one circulating pump, no controller, two external 24V transformers wired in parallel (not good form, but they've done the job for 35 years) and all 2 wire thermostats. Ignoring the fact that it will be a PITA, can I just run a common wire to each thermostat location from the transformers and use a nest thermostat without any problems?

Yes but you need to be sure the R from the transformer goes to the R on the thermostat, and the W on the thermostat goes to the number 1 terminal on the zone valve. on a 2 wire only thermostat, the R and the W don't matter. When the C wires is in play, IT MATTERS!

Or you will let the magic smoke out of something... or several things...

HomerJSmith

Wiring two transformers in parallel may have a polarity conflict. 50 50 chance. What you have to ask yourself is "Do I feel lucky."

harry22

I didn't wire them, the builder did. I bought the house new in '85. Evidently, the contractor did it right. I suppose if he wired it wrong, he would have found out immediately! Even if wired correctly, there would probably be circulating currents causing unnecessary internal heating if the xfmrs are not exactly identical. I just feel that running a C wire is the most reliable and straightforward solution for an old school guy like me.