Repurpose Green wire to C-Wire

NJstevec

Hi everyone,

I’m a newbie here. I just bought a new house and it has a Weil McLain boiler. The house has multiple zones for the hot water baseboards. There are 6 thermostats that I am switching to Nests. Each thermostat has an unused green wire which I want to repurpose as a C-Wire to power each nest. 

My plan is to go to the boiler side and find all the green wires and then connect them to a power source. I took a picture of all the crazy wiring, My question is where do I connect the other end of the green wire to, that’s on the boiler side? It looks like there are 3 transformers. Do I connect it to them or do I need to get a new transformer? If I need a new transformer, where do I wire that to? 

Thank you very much, any help would be greatly appreciated. 

Jamie Hall

I won't get into why I don't like Nests, and why I think they are unsuitable for hot water heat. You will find that out for yourself.

That said, your best bet is to power all the Nests from their own transformer. The Nest needs a Common connection and a Power connection. The Power connection is Rc which may be jumpered to Rh -- which in your situation should be (but check) the red wire. The first thing to check then is that the three existing transformers are all in phase; may be, but looking closely I see that they may not be, but it's worth checkiing. What you need to make sure of is that the all the red wires going to the thermostats have the same voltage (you will read zero volts from each red to each of the others, and the same voltage (probably 24 but maybe 0) to ground from each wire. If that is the case, then you can install the new transformer and select the terminal on it which also has the same voltage to the existing red wires, then connect that to all the red wires. Then connect your green wires to the other terminal of the new transformer. If two of the red wires do NOT have the same voltage, you'll have to trace the wiring to find out where the wires from that transformer are switched. It doesn't make any difference to the existing thermostats or to the zone valves -- but you can't parallel them that way.

JUGHNE

Jamie, you are saying to get the hot from the existing xfmr and the common from the new xfmr?

It seems like each zone would need a isolation relay if another xfmr was involved?

Hopefully we get the results of the Nest operation this winter.

NJstevec

Thanks Jamie for the detailed response. Can I ask why you do not like nests with this system? Is it a serious enough issue where I should just keep the regular thermostats? 

Thank you for the help

Jamie Hall

JUGHNE said:

Jamie, you are saying to get the hot from the existing xfmr and the common from the new xfmr?

It seems like each zone would need a isolation relay if another xfmr was involved?

Hopefully we get the results of the Nest operation this winter.

I meant to connect the new transformer power to all the Rc or Rh terminals in parallel, and the run the C terininals, in parallel back to the new transformer. The existing transformer power is also connected in parallel to the Rc or Rh terminals, but its common stays inside the control board as it does now.

NJstevec said:

Thanks Jamie for the detailed response. Can I ask why you do not like nests with this system? Is it a serious enough issue where I should just keep the regular thermostats? 

Thank you for the help

Nests are designed to work with forced hot air systems, which can ramp air temperatures up and down very quickly. Thus they can be used to advantage in such systems with large setbacks and with room occupancy sensors and can, in fact, save some energy in the process. For better or worse, hot water and steam systems -- never mind radiant -- cannot ramp up and down quickly. They take time to respond. What can and does happen is that the Nest says ah ha! Someone just walked into this room! And raises the air temperature. The system fires up, and maybe 15 minutes later the room really begins to warm up -- at which point the someone has left the space to do something else and the Nest drops the temperature again. The system shuts off. The person was cold, and still is. The system has burned some fuel to no purpose.

Nests, and similar occupancy thermostats, can be made to work satisfactorily -- though never better than a regular programmable thermostat -- by going into the setup menus and killing all the smart features. If they are then used with reasonable -- a few degrees -- setbacks, they're OK.

But, being cheap, I say to myself why bother with all that when I can go out and get a nice battery powered programmable thermostat -- no C wire! -- which will do the job just as well?