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Add-A-Wire - C Wire - 3 Wire Thermostat


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01-27-18, 05:14 PM   #1  
Add-A-Wire - C Wire - 3 Wire Thermostat

I am battling the Nest and losing at every turn. Instead of running a 24V COMMON / C wire from my basement to my master bedroom in the 3rd floor, can I use an "add-a-wire" product such as Venstar (https://venstar.com/thermostats/accessories/add-a-wire/) . I searched a bit and hearing it only works with 4 wire applicatoons. My 2nd floor is a slave and only has 3 wires (POWER - RC/RH; Y - HEAT; W = A/C). I have an outlet right nelow it so there must be something I can do without running lines through the wall from the basement.

 
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01-27-18, 06:03 PM   #2  
Is that thermostat a part of a zoning system ?

You can use a small plug in 24vac wall wart and run it directly to the nest.

That transformer would connect to Rc and C.
Your house wiring would connect to Rh, Y1 and W1.


~ Pete ~

 
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01-27-18, 06:13 PM   #3  
Can you dumb it down a bit for me?

yes master slave and 2nd floor only has the three wires. Not even fan control. First floor has additional wires that I repurposed as common right at the zone controller

 
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01-27-18, 06:36 PM   #4  
You go out and buy a 24vac plug in transformer. Several examples below. Then connect as I've previously written. The slave thermostat port of a zone controller doesn't have enough power to run a wifi stat.

PWS-2420-Supply-Adapter-Transformer-terminals
MG-ELECTRONICS-MGT2420-TRANSFORMER-APPROVED


~ Pete ~

 
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01-27-18, 06:53 PM   #5  
Is there an option that would allow me to wire it directly to a power outlet or a light switch so the wiring is hidden? The outlet is literally right below the thermostat.

 
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01-27-18, 07:59 PM   #6  
Instead of the Venstar you can use Fast-Stat's common maker: https://fast-stat.net/pdf/Common-Mak...ctions-Set.pdf
It can take your R and W and turn into R,W,C.

 
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01-27-18, 10:45 PM   #7  
The slave thermostat port on that zone control board may not supply enough current to run a wifi stat. It's not enough power to support the nest power stealing function.


~ Pete ~

 
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01-28-18, 07:52 AM   #8  
Posted By: "pjmax' The slave thermostat port on that zone control board may not supply enough current to run a wifi stat. It's not enough power to support the nest power stealing function.
Can I test at the zone controller if I have enough power? The other option is the plug you mentioned but I don't want the ugly wire running up the wall. The outlet is directly below it. Will i be able to connect it behind the outlet and run the wire up the wall? If so, without losing the use of that outlet?

 
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01-28-18, 09:31 AM   #9  
Try the nest right at the zone controller.
Connect it to C, Rh, Y1 and W1 and see if it works there.
If it does.... a create a ground adapter will work.

There is no 24v adapter that can go inside the wall.


~ Pete ~

 
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03-23-18, 10:48 AM   #10  
Try the nest right at the zone controller.
Connect it to C, Rh, Y1 and W1 and see if it works there.
If it does.... a create a ground adapter will work.

There is no 24v adapter that can go inside the wall.
Apologies as I thought I responded and just realized the thread wasn't updated. the nest does not power up right at the zone controller. My electrician came over and wired a 24V transformer w/ 2 - 12V wires and ran them down the thermostat and then realized it wasn't going to work as the power would not originate and end at the same source meaning the nest didn't just accept a 'power in' but needed to use the R wire to go back to the zone controller to complete the circuit. I am not knoweldgeable so I cannot comment but it seems odd to me that this is soo difficult given I have 2 12V wires right at the Nest.

 
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03-23-18, 11:27 AM   #11  
When using a transformer with a Nest gen 3 the wires go to Rc and C. Then Rh and W1 go to the furnace/zone controller. So altogether 4 wires go to the thermostat.

 
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03-23-18, 11:34 AM   #12  
Where do the 2 - 12 V power wires go? RC and C? and then the original 3 to the HVAC go HR/W1/Y1? Electrician was convinced that it would cause an issue.

 
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03-23-18, 11:52 AM   #13  
The two wires from the 24vac transformer connect to C and Rc.
The other three connect to Rh, W1 and Y1.

The nest keeps the R connections isolated so they don't interfere with each other.


~ Pete ~

 
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03-23-18, 12:19 PM   #14  
Any concern that this will damage anything? Should I try this? How do I 'test' the 2 - 12V wires to confirm they are indeed 12V?

 
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03-23-18, 02:07 PM   #15  
Do you have a multi-meter? The two wires from the transformer should measure just a bit higher than 24vac.

Sorry - I forgot that you had a Y1. Nest switches Y1 to Rc if present so a separate Rc and Rh won't work. Yours is one of the few situations that bridging might be needed with the Nest.

One other solution that Nest recommends as a work around is to put a 220 ohm, 5 watt resistor between Y and C at the zone controller end. This gives Nest something to steal power from.

 
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03-23-18, 04:24 PM   #16  
You may be right about bridging the R's together. That means you would connect your old Rh wire and the new R from the transformer together with a short piece of wire. Cap with a wirenut and connect the short piece to Rc.

You can try the way we mentioned earlier first. It won't hurt the nest. If it's incorrect you will get an error message with a number.


~ Pete ~

 
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03-23-18, 07:30 PM   #17  
Do you have a multi-meter? The two wires from the transformer should measure just a bit higher than 24vac.

Sorry - I forgot that you had a Y1. Nest switches Y1 to Rc if present so a separate Rc and Rh won't work. Yours is one of the few situations that bridging might be needed with the Nest.

One other solution that Nest recommends as a work around is to put a 220 ohm, 5 watt resistor between Y and C at the zone controller end. This gives Nest something to steal power from.
'
You may be right about bridging the R's together. That means you would connect your old Rh wire and the new R from the transformer together with a short piece of wire. Cap with a wirenut and connect the short piece to Rc.

You can try the way we mentioned earlier first. It won't hurt the nest. If it's incorrect you will get an error message with a number.
I have a multi meter. Do I just use the probe on both 12V wires at the same time and look for 24V?

Yes, Y/W/R for AC/Heat/Power - No fan control for zone 2. Blah.

Should I just try the 2 12V into the RC and C and existing HVAC wiring RH/W1/Y1 to see if it works? If not, I am bridging the existing HVAC power and the new R (is that both 12V - 2 total) together for a total of 3 wires and then into the RH? So, no C at all? Do I just use a simple wirenut with a new wire connecting all 4 together and then the new short wire into the Nest RH? Did I get that right?

 
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03-23-18, 07:45 PM   #18  
You can check those two wires but you will be measuring near 24vac..... not 12vdc. A thermostat and furnace run on 24vac.

Try the way we mentioned first. Like i said.... if the nest doesn't like it... it will show a fault code.

In the even you need to combine the R wires..... you will take one of the 24vac wires and the old Rh wire and combine them. You will need to add a short piece of wire to that splice to connect to Rc. The nest won't allow two wires in its terminals. The other 24vac wire connect to C.


~ Pete ~

 
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03-23-18, 07:50 PM   #19  
Do I just connect probes against both wires off the 24v transformer?


Understanding the bridging now. Does it matter if I use RC OR RH?

 
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03-23-18, 07:56 PM   #20  
I believe Rc since there is a wire in Y1.

Yes.. touch the wires with the probes. Try not to let the wires touch. Don't want to burn out the new transformer.


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03-23-18, 08:20 PM   #21  
sagosto63: I have an outlet right below it so there must be something I can do without running lines through the wall from the basement.

Here is link to that something for $7.69

https://www.amazon.com/SMAKN-Switchi...H3NF2B19GEYCDR

Just cut off adapter plug and attach one wire to C . The other to Rh and if you have air conditioning to Rc.

 
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03-25-18, 10:48 AM   #22  
Thermostat basics

sagosto63: My electrician came over and wired a 24V transformer w/ 2 - 12V wires and ran them down the thermostat and then realized it wasn't going to work as the power would not originate and end at the same source meaning the nest didn't just accept a 'power in' but needed to use the R wire to go back to the zone controller to complete the circuit.

The “electrician” is WRONG and does not understand these controls!

In typical residential systems thermostats do not originate power to 24 volt controls. To the system they only provide a contact, switch closure that temporarily connects two wires that activates it. Thermostat contacts complete a circuit, not power it.

Modern electronic thermostats require continuous 24 VAC power for internal circuits which is typically supplied by the C wire. Thermostats convert that 24 VAC on C and Rh wires to a low voltage DC to power thermostat electronics. Batteries in some thermostats provide DC power it when there is a outage.

 
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04-14-18, 08:07 PM   #23  
My apologies as I thought I updated this thread? From the 24V transformer, I connected the C and the RC on the Nest and it powered up immediately! I then connected RH/Y/W and I now have heat. Life is good. Then, today, it was 85F in New Jersey so I tried cool and the fan turned on 1st floor but no cool. I forgot that I needed to change the jumper (R1/B) at the zone controller (This was necessary as my system requires O/B connections until I upgrade the zone controller). I changed the jumper to R1/O and the primary (1st floor) cooled the downstairs but NOTHING upstairs? I e-mailed EWC and was very lucky to hear back immediately from their chief engineer who went over my entire setup and told my that my setup will not work because the 2nd floor nest is being powered by external source and the the circuit isn't being closed? I am probably butchering that but I believe it's due to the Nest not being powered by the HVAC that the zone controller isn't reading the signal right. The only reason why heat works is because RC is connected on the Nest to the zone controller. Again, I am probably missing something but it sounds like all that money/time spent was wasted at this point. He did mention a possible fix but it's possible that I could blow a transformer or a 'fire' was a SMALL possibility. With 2 kids and an infant, that scares the hell out of me. I am not comfortable with this at this point so I am not sure what to do at this point.

 
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04-14-18, 09:06 PM   #24  
I respond to so many threads I can't always keep them straight. I re-read the thread but not quite sure where we stand.

The primary nest is working but the upstairs one is not.
Did it work for heat ?
If so.... then cool should also work.

What wiring do you have on the upstairs nest.


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04-14-18, 09:22 PM   #25  
To power nest #1, I replaced O at zone controller with C to provide common wire to Nest #1. I used a jumper (paper clip) between zone controller's R1 and B due to my zone controllers O/B requirement. I had heat both zones independently. I connected zone 2 nest and had no wire to multipurpose as common so used 24V transformer to RC/C as recommended and it powered. I then connected RH/Y/W to complete the wiring. I now had 2 Nests w/ power and heat on both floors independently. I replaced jumper from R1 to B and went R1 to O to lock in the cool mode and only ac on primary nest (1st floor).

EWC is saying that is due to the C/RH wiring on the nest 2 and how it is not a closed circuit so zone controller isn't reading the the signal. I am probably butchering this.

 
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04-15-18, 07:41 AM   #26  
Any ideas? Wife wants to scrap the entire setup which is a ton of loss and a hit to the ego.

 
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04-15-18, 09:54 PM   #27  
When Nest (or any other thermostat with both Rc and Rh) calls for cooling it connects Rc and Y. With Rc going to the local transformer instead of the zone controller your 2nd floor Nest won't properly send a cooling request.

Did you ever get a Venstar or similar? That should solve the problem.

EWC's fix is probably paralleling the transformer. Some do this successfully but it is best to first get a multimeter to get the local transformer "in phase" with the voltage/transformer at the zone controller.

Lastly you could add a relay at the upstairs Nest but would be beyond what most would want to try.

 
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04-16-18, 07:48 AM   #28  
When Nest (or any other thermostat with both Rc and Rh) calls for cooling it connects Rc and Y. With Rc going to the local transformer instead of the zone controller your 2nd floor Nest won't properly send a cooling request.
This sounds exactly how ECW explained it to me. I asked if I could power the nest with C and RH instead of RC and they said no. Wouldn't that make COOL work and not HEAT (inverse)?

Did you ever get a Venstar or similar? That should solve the problem.
No because I had power and HEAT was working perfectly. This caught me off guard. To be honest, I am hesitant to make any changes due to lack of knowledge. I am debating the Emerson Sensi which doesn't require C but 2 batteries. It's a way to cheat out but fear is not preventing me from going further.

EWC's fix is probably paralleling the transformer. Some do this successfully but it is best to first get a multimeter to get the local transformer "in phase" with the voltage/transformer at the zone controller.

Lastly you could add a relay at the upstairs Nest but would be beyond what most would want to try.
I am not sure the specifics but the word 'phase' sounds familiar. He said he wasn't 100% sure but had to let me know the SMALL risk of blowing a transformer and/or fire. He emphasized small risk but it is still a risk. For a non-handy guy like me, that's hard to swallow.

 
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04-16-18, 09:13 AM   #29  
EWC responded to my inquiry:

• Connect both 24v (common wires) together from existing transformer in the basement to new transformer in the attic.
o This would be accomplished at the Nest thermostat.
o Multiple Voltage readings needed at various steps to be make sure nothing blows.
 Voltage readings will occur at the Nest and the ST controller.
o Theoretically allows the 24v hot (R) to be recognized by the ST controller or any new controller you decide upon.
• If voltage readings indicate a short circuit will occur, then we stop. Otherwise this:
o Carries with it the possibility of blowing one or both transformers (and/or a thermostat) if the primary voltages serving each transformer are out of phase.
 ie. Existing transformer 120v = L1 & Neutral / New transformer 120v = L2 & Neutral.

 
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04-16-18, 09:20 AM   #30  
If you have both Rh and Rc connections then Nest always draws power from Rc. So stuck there.

First confirm the transformer installed by the electrician is similar to what was recommended above.
Get a cheap meter from Harbor Freight or maybe Walmart. Take one transformer lead and connect it to the R coming from the zone controller. Then measure the AC voltage between the other lead from transformer and the Y from the zone controller. You want a voltage close to zero. If 50vac then swap the two leads coming from transformer. After doing that put a pigtail on the combined transformer/R and connect to either Rc or Rh (doesn't matter which as Nest will connect the two internally). Put the other transformer wire on C and you should be good to go.

 
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04-16-18, 10:07 AM   #31  
This is way over my head if I am being honest.

 
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