Using Extra Wire at Thermostat for C Wire

kevinbrandon

Hello,

We are replacing our old thermostats with Nest E's. The wire that we have pulled to both of our 2 thermostats is 3-wire. Only two used at the thermostats. The third wire is used in the zone valve wiring.

We have a Laars boiler with Taco zone valves. We have an indirect hot water tank.

There is a red/green wire coming from the load on the transformer.

I've attached some photos to see if anyone would be able to answer based on what is seen.

My question is whether or not I can simply connect the green wire to the common on the thermostats.

Any and all help is greatly appreciated.

EBEBRATT-Ed

Using the one transformer to power two Nests, two zone valves and the boiler may be a bit much.

Let's see what others have to say

109A_5

Maybe this thread will help you.
https://forum.heatinghelp.com/discussion/188644/why-is-thermostat-zone-valve-wiring-not-done-neater

Connecting the green wires to the "C" terminal at the NEST units and connecting the coiled up green wires at the green rows in the picture to the white wires at the white arrow may get the wiring closer to what you need (assuming the annotations for the cables are correct).

HVACNUT

The pics are a little tough to make out but the White wire that's connected to C on the transformer is what needs to get connected to C at the thermostats. 
If it's a 40 va transformer it should be ok bit wiring in an inline 3 amp fuse at the R side of the transformer would be wise.

HomerJSmith

This information maybe inaccurate. See Ed's post below about an isolation relay.

Whether you can do it is how many wires do you have in the cable at the thermostats, 3 or more. It appears so. Those Taco 570 zone valves draw a lot of current. One 40VA transformer can accommodate 3 Taco ZV's. I surmise that your boiler transformer is a 40 VA transformer. So, the boiler control has a 40VA transformer in it at the boiler (T) connections.

Three functions need to be considered.
1). The Zone Valve need to turn on the boiler.
2). The thermostat need to turn on the ZV.
3). The thermostat need to be charged to function.



This diagram is probably your set up.

1). You have the white wire coming from the (T) connection in the boiler control to the #2 screw on both Taco ZV's same wire. You have a green wire from the other (T) connection on the boiler control to the #3 screw on both Taco ZV's same wire. That is #one circuit. When the thermostat turns on the ZV, the ZV turns on the boiler.

2). For this you need an auxiliary transformer. The one that you show on your last pic would do.

You connect a red wire from one of the screws on the auxiliary transformer to the Rh connection on the base of the Nest thermostat. You have 2 thermostats so the red wire from the transformer would go to both thermostats. The white wire in the thermostat cable would go from the W1 connection on the Nest base to each controlling ZV #1 screw. Two thermostats--two ZV's. A (white wire) would go from the #2 screw on the ZV's to the auxiliary transformer other connection. That is circuit #two.

3). Now the charging circuit. The green wire goes from the Nest's (C) connection on the base to the screw on the auxiliary transformer where the white wire is connected. That's circuit # three.

Any complaints about what I have written are handled by my office in Budapest.


That's how it must be wired, the color of the wires are up to you. My assumption is that the Laars has a control transformer.

pecmsg

kevinbrandon said:

Hello, We are replacing our old thermostats with Nest E's.

D O N T !

EdTheHeaterMan

I just responded to someone with the same query
You need a isolation relay!

Here is my post

EdTheHeaterMan said:

The Taco 571 power head is not compatible with the Nest thermostat. This is because the valve actuator is not always powered. As the wax in the hydraulic piston heats up and expands three things happen in this order.
1. The valve opens to allow flow thru the valve... as the wax increases in volume there is a cam that (see #2)
2. The cam actuates the end switch contacts in order top operate the boiler or circulator relay... As the wax increases in volume and pushes the piston further that same cam (see #3)
3. The normally closed Heater Switch contacts open to stop the wax expansion and over heat causing a piston leak failure ... the wax cools and decreases in volume closing allowing the piston to move towards closing. the cam allows the heat switch to close and the wax heats up. this causes the cam to open the heat switch that opens the circuit. and on and on and on.

This opening of the heat element circuit is interpreted by the Nest as the open circuit or NO PPOWER fault and the thermostat goes thru a lockout and reboot procedure. Since this is normal for the valve actuator and may happen 6 or more times a minute, and not normal for the Nest, you will experience a no heat condition. The fix for Taco 3 terminal zone valves is to use an isolation relay like this one. by DiversiTech part number D90-290Q. it is also made by White Rogers and Supco. It is a pretty standard HVAC relay.

Here is how I would wire it:

This diagram was made by the person asking the question about their thermostat and the taco zone valve. I just added the part where to isolation relay needed to be added in Blue

EdTheHeaterMan

You might want to look at the full discussion here https://forum.heatinghelp.com/discussion/comment/1706043#Comment_1706043
There are diagrams that show the internal workings of your 3 wire Taco zone valve actuator. You can see the cam that opens and closes the end switch and the heat switch i talk about in my comment above. This person had 4 zone valves and needed 2 transformers. Since you have only 2 zone valves, you need only one transformer. That transformer will have enough power to operate the Nest, the Isolation relay and two zone valves.

If you need more information or a better explanation, you can message me!

Alan (California Radiant) Forbes

Kevin: Are we going in the right direction? If not, there is an easier ways to do this.

If you get a Taco ZVC403 zone valve control, it will allow you to use the third wire to the thermostat as the "C" wire.

HomerJSmith

You hit a home run, Ed. I knew that the 571 intermittently broke the heater circuit, but I didn't know that was a problem with the Nest. I wonder about the Ecobee or Honeywell.

This is the relay that I would use. One for each thermostat.
https://www.supplyhouse.com/Supco-90290-SUPCO-24v-N-o-Spst-Relay
This is how I would wire it in.

W1 connection at the Nest base to the coil of the isolation relay tab. A white wire from the other isolation relay coil tab to the #2 screw connection on the Taco ZV.

Connect a red wire to the transformer red wire that goes to the Nest Rh connection and connect it to the isolation relay point tab. Connect a red wire from the other isolation relay point tab to the Taco ZV #1 screw connection.

This is the way I would wire in an isolation relay.

Steamhead

@kevinbrandon , given all the factors here, I'll second @Alan (California Radiant) Forbes 's suggestion. This way all the relay logic is in the one unit, and there's far less chance of crossed wires causing a short.

Zman

Stop the madness!
Just install a zone valve controller and save all the brain damage
https://www.supplyhouse.com/Taco-ZVC403-4-3-Zone-Valve-Control-Module-with-Priority

HomerJSmith

Yes, Zman, "stop the madness". But alas, the whole world has gone completely 'Mad Hatter'. It is called "Mass Formation Psychosis".

It's been said that men go mad in crowds and regain their sanity one by one. When I was a kid in WW2, I didn't see the kind of madness that I see today and that was in the mist of a world war. Some one said, "I'm as mad as hell and I'm not going to take it anymore." A different kind of mad which people are waking up to. hmmm

EdTheHeaterMan

@Alan (California Radiant) Forbes & @Zman ...As far as the Zone Valve Relay is concerned, I have tried that with no success. The 571 actuator is still breaking the circuit to the Thermostat and the error code still shows up. I would still recommend the ZVC controls for ease of installation and trouble shooting, however that does not eliminate the problem. Of course the old ZVC controls with the metal cover and the new ZVC controls with the plastic cover may have addressed this problem but I can not speak to that. I have no experience with that.

Zman

EdTheHeaterMan said:

@Alan (California Radiant) Forbes & @Zman ...As far as the Zone Valve Relay is concerned, I have tried that with no success. The 571 actuator is still breaking the circuit to the Thermostat and the error code still shows up. I would still recommend the ZVC controls for ease of installation and trouble shooting, however that does not eliminate the problem. Of course the old ZVC controls with the metal cover and the new ZVC controls with the plastic cover may have addressed this problem but I can not speak to that. I have no experience with that.

You would still need the "C" wire to the t-stat landed on the terminal in the zone controller. The zone controller would clean up the wiring, make troubleshooting a breeze, and isolate the boiler end switch.

EdTheHeaterMan

Zman said:

You would still need the "C" wire to the t-stat landed on the terminal in the zone controller. The zone controller would clean up the wiring, make troubleshooting a breeze, and isolate the boiler end switch.

Agree the C wire would be useful on most Nest applications. 571 zone valve actuator still breaks the circuit to the W with the older ZVC (metal cover) and requires the isolation on each actuator that uses this Nest that I have on my home, and those that I have installed on customers with Taco ZVC and 571 equipped systems.
The newer models may be different but from the questions posted herein I think the Isolation Relay is the best way to match any Nest with any problem controls. I believe it is a great workaround for any of the other "Smart" thermostat brands as @HomerJSmith mentioned