Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Nest thermostat C wire

Options
Hi,

I have 3 Nest Thermostats that require a common wire.  I have a taco SR 504 switching relay that the thermostat wires (currently 18/2 cable) go into.  It is a 4 switch relay.  One is dedicated to domestic hot water and the rest go to circulator pumps for forced hot water (baseboard).  My questions are:

1.) it appears there is only one place to hook up the common wire.  Is this correct?

2.) can I hook up multiple common wires from each zone to this one common wire connection in the relay box?

3.) of the alternatives available: nest power connector/ running new 3 or 5 cable/ plugging a transformer into a wall and running to the thermostat/ a solution I don’t currently know about, which is considered the best/better solution or are there complications associated with using a particular method?  

Thanks everyone.  This community is great. 

-Rob
«1

Comments

  • SteveSan
    SteveSan Member Posts: 235
    edited June 2023
    Options
    Hi Rob, does your Taco control have a metal or plastic cover? If you have 3 Nest t-stats then your going to have to add an external transformer. I'll attach two different wiring diagrams based on the metal or plastic cover.

    If you have any questions, please give Taco Technical Services a call during normal business hours Mon-Fri 8am-5pm EST 401-942-8000
  • pecmsg
    pecmsg Member Posts: 4,883
    Options
    Anyone’s T-Stat is better than the NEST!
  • HomerJSmith
    HomerJSmith Member Posts: 2,468
    Options
    If you have only two wires at the thermostat, a red and white, you are going to need a third wire or some way to charge the Nest battery. Look closely at the wire cable and see if there is a third wire wrapped around or cut short in the cable.

    Report back and a pic of the Taco panel would answer a lot of questions.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,806
    edited June 2023
    Options
    You will need a three wire thermostat low voltage wire to power the wifi thermostat .. I can not give you a recommendation on any after market gismo to bypass the common needed .Maybe some one else can.. Three wire you can pick up a common in any control....

    There was an error rendering this rich post.

    ronbugg
  • rob51
    rob51 Member Posts: 67
    Options
    @SteveSan. The cover is metal.  Thank you for the diagrams.  It looks like there is a common on the panel that’s labeled “24vac common”.  Can I use that instead of a transformer?  I’ll give taco a call too tomorrow.  Thanks.
    HVACNUT
  • rob51
    rob51 Member Posts: 67
    Options
    @HomerJSmith. I looked very closely at the cable.  It’s definitely 18/2 cable, without a doubt.  Thank you for this thought though. 
  • rob51
    rob51 Member Posts: 67
    Options
    @HomerJSmith.  I’m out right now, so I don’t have a picture of the relay, but it is an SR504 taco 4 switch relay.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,092
    edited June 2023
    Options
    pecmsg said:

    Anyone’s T-Stat is better than the NEST!

    Pay no attention to that funny man behind the curtain



    @rob2816 ... if you were using only one Nest thermostat then you may be able to use the COM on the Taco control. Since you want to use three thermostats, that transformer that powers the Taco Relay is not powerful enough to supply 4 relays (that are on the Taco Control) and 3 thermostats. There may be a time that you are drawing too much power from that tiny transformer. If that happens, you will see all the factory installed smoke leak out of the transformer and you will need to buy new parts.


    So this is what Taco SteveSan is trying to tell you. DO NOT use the COM on the taco control. Use the C on a separate transformer. (like this one https://www.supplyhouse.com/Packard-PF4031VM-Foot-Mounted-120-208-240V-Primary-24V-Secondary-40VA-Transformer ) this diagram will be more like what you need to do.



    With the additional transformer to power the thermostat via the R on the transformer to R on the thermostat and the C on the transformer to C on the thermostat.


    The R on the Taco Control will also use that same R on the TACO to the R on the thermostat to operate the relay by way of W on the thermostat to W on the Taco relay for the appropriate zone. You do not need the R on the other two thermostats to go to R on the zone relay for each zone. That is because all the R terminals are connected internally inside the control. And the R on each thermostat is also connected to each R on the other two thermostats. I have illustrated that with the dashed Red line


    If you have any other questions @SteveSan will be able to help or you can always post here on The Wall



    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    SteveSan
  • SteveSan
    SteveSan Member Posts: 235
    Options
    Thanks Ed, very well said. He has the "older" style metal cover so when wiring a three wire t-stat he must wire the R to the right T and W to the left T. If only two wire t-stat then it doesn't matter which T he lands the R & W.

    Like Ed said: So this is what Taco SteveSan is trying to tell you. DO NOT use the COM on the taco control. Use the C on a separate transformer. (like this one https://www.supplyhouse.com/Packard-PF4031VM-Foot-Mounted-120-208-240V-Primary-24V-Secondary-40VA-Transformer )
  • rob51
    rob51 Member Posts: 67
    Options
    @EdTheHeaterMan @SteveSan thank you for explaining this to me.
    SteveSan
  • SteveSan
    SteveSan Member Posts: 235
    Options
    You're welcome Rob, please give us a call if you have any questions. Always happy to help our customers!
  • rob51
    rob51 Member Posts: 67
    edited June 2023
    Options
    @SteveSan Talked to Taco today. Steve was very professional and patient with me to explain the SR504 relay. I’ve called Taco in the past too and have only had good experiences. Great customer service!
    SteveSanErin Holohan HaskellEdTheHeaterManJUGHNE
  • rob51
    rob51 Member Posts: 67
    Options
    @SteveSan in your diagram, it appears the red wire from the transformer is connected to the red wire that goes to the zone connection at the relay box. Where do these wires connect? At the thermostat is what I think, but it’s not clear from the diagram. Thank you.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,092
    edited June 2023
    Options
    rob2816 said:

    In your diagram, it appears the red wire from the transformer is connected to the red wire that goes to the zone connection at the relay box. Where do these wires connect? At the thermostat is what I think, but it’s not clear from the diagram. Thank you.

    So the diagram can be confusing for those that are not used to reading diagrams. Here is the identical diagram with the brown covering over the three conductors. All the connections can be made in the boiler room/basement. Just because the wires look like they go from the C on the thermostat to the next C on the next thermostat and ultimately to the C on the aftermarket transformer, as you can see in the original diagram, does not mean that you need to connect it exactly the same way. The wires can join in different locations. With electricity if you connect terminal 1 to terminal 6 and connect terminal 6 to terminal 8 then terminal 1 is also connected to terminal 8 without actually pitting a wire from 1 to 8

    The R wire on the aux transformer and the R on each thermostat can connect to any R on the Taco control board. Since Steve said you have the old metal cover control, the T terminal on the left is R the T terminal on the right is W. I have made this for you so you can get it right.


    You can use the end of the wire in the basement as if it is the terminal on the thermostat in the room(s).

    Make Sense?

    PS. both the original and the modified diagrams are in the file below. I find them easier to zoom in on, to look at details close up, on attached files

    EDIT: I think I found a picture of your Taco SR 504 (old style) where the thermostat terminals are marked T T. Check the third file attached for the updated drawing.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • rob51
    rob51 Member Posts: 67
    Options
    @EdTheHeaterMan.  Your diagram is very easy to read and makes sense.  Thank you so much for taking the time to create it. If I’m understanding it correctly, all 4 commons are wire nutted together in the basement.  Then the red wires are wire nutted with a pigtail coming off to zone 2 (though I think it’s irrelevant which zone it’s attached).  The whites are attached to their respective zones. When you draw the dashed lines, is that the pre-existing wiring in the relay box system?  Thank you again.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,092
    edited June 2023
    Options
    You have a perfect understanding of the connections!

    Yes The dashed lines are the internal connections on the printed circuit. (on the under side where you can't see them.). I did that so you can easily see that connecting one of the R wires is the same as connecting all of the R wires.

    As I may have explained, and I believe you understand, if you make a connection on a given control system from a #1 terminal to a #6 terminal, then connect the #6 terminal to the #8 terminal, you have also connected the #1 terminal to the #8 terminal without actually putting the wire from #1 to #8.

    I have one other thought. When ever I install a transformer, I will use this setup.
    This helps to keep the factory installed smoke inside the transformer in case you make a mistake. A 3 amp fuse is good for a 40VA transformer

    The control has a 15 VA transformer onboard so those fuses are rated for less amperage.


    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    Redbaran
  • rob51
    rob51 Member Posts: 67
    Options
    @EdTheHeaterMan. I will use fuse you described.  Thanks again!
  • rob51
    rob51 Member Posts: 67
    Options
    @EdTheHeaterMan do you have a link for the 3amp fuse? I’m assuming it will work with the 40VA transformer linked earlier in this thread.  Thank you.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,092
    edited June 2023
    Options
    Fuse: Pep Boys Auto parts store. Spade connectors: O'Reilly's Auto parts store. 14 gauge strand wire: NAPA Auto parts store. I hate to make you run around to all these different places. Or you could try Amazon.com

    I hope you know that this response is not intended as a real answer to your question.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • SteveSan
    SteveSan Member Posts: 235
    Options
    The R wire on the aux transformer and the R on each thermostat can connect to any R on the Taco control board. Since Steve said you have the old metal cover control, the T terminal on the left is R the T terminal on the right is W. I have made this for you so you can get it right.

    @EdTheHeaterMan , please see the wiring diagram for our older, metal cover controls. SR504 and SR506 when adding in the common you need to make sure the Red is wired to the right T and White is wired to the Left T. Doesn't matter where you land the R-W if only a two wire t-stat. Hope this helps.

    I'm going to attach for the older SR502-SR503 & ZVC controls.
    EdTheHeaterMan
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,092
    edited June 2023
    Options
    The R wire on the aux transformer and the R on each thermostat can connect to any R on the Taco control board. Since Steve said you have the old metal cover control, the T terminal on the left is R the T terminal on the right is W. I have made this for you so you can get it right.

    Steve, I got the wires backwards? Sorry. I will fix the diagrams for @rob2816. I believe this is this correct?



    It is nice to know that Taco Engineering Department is consistent with alternating the R and W side of the control with each new generation. This keeps us on our toes, and also sells a lot of transformers over the long range. That is why I started using this set up on every transformer I install.

    LOL

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    SteveSan
  • SteveSan
    SteveSan Member Posts: 235
    Options
    That is correct Ed
    EdTheHeaterMan
  • HomerJSmith
    HomerJSmith Member Posts: 2,468
    edited June 2023
    Options
    The transformer on the Taco SR has to be in phase with the aux transformer, I would think. That is why I prefer a isolation relay. You still have to deal with the missing third wire. I one were to use an adapter on a two wire connection to the Nest, a isolation relay would be the only way to go. That would be one relay for each nest. The cumulative VA of the transformer would be all the relays + Nests + any adapters.
    You are obviously zoning with circulators. I don't have time to draw this up now. Maybe later.
    I like the idea of a fuse wire. It could save a transformer if things go awry.
  • rob51
    rob51 Member Posts: 67
    edited June 2023
    Options
    @EdTheHeaterMan yes, I realized the terminals were backwards.  I would be paying attention to the R and W connections at the terminal, so I doubt I’d get it backwards, but thank you for clarifying.  I appreciate it.  Also, thank you for the info on the relay.  
    @SteveSan. Thank you For clarifying.
    @HomerJSmith. Thank you for the info on the transformer.
    SteveSan
  • rob51
    rob51 Member Posts: 67
    Options
    Any thoughts on if I should put this stuff in an electrical box to protect it and maybe run the wiring through that metal flexible tubing? I guess I’m thinking how I should mount the whole thing.  
  • pecmsg
    pecmsg Member Posts: 4,883
    Options
    Easier to use anyone else’s stat!

    just saying!
  • rob51
    rob51 Member Posts: 67
    Options
    @EdTheHeaterMan. what’s this new blue thing you drew in the new diagram?  It looks like it’s connected to zone 4.   Again, thank you for revising your diagrams and for all the help.  I really appreciate it.  
  • rob51
    rob51 Member Posts: 67
    Options
    @pecmsg. How are they easier?  I would think most wifi enabled thermostats would require significant power to run, thus requiring a C wire.  Perhaps I am wrong and you can enlighten me.
  • Jamie Hall
    Jamie Hall Member Posts: 23,436
    Options
    It's in the setup of the thermostat. You are correct that all wi-fi thermostats should have a C wire or other independent power source to work more or less reliably.

    There's really more to it -- many of them will work reliably without a C wire, or with the resistor trick -- but only if the cycle on times are short (a few minutes). This is characteristic of forced hot air -- which is what they were designed for. Both steam heat and hot water heat -- with longer cycles -- are pretty rare out in la-la land, which is where the folks who design and build them live.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,092
    edited June 2023
    Options
    rob2816 said:

    @EdTheHeaterMan. what’s this new blue thing you drew in the new diagram?  It looks like it’s connected to zone 4.   Again, thank you for revising your diagrams and for all the help.  I really appreciate it.  

    Blue thing is DHW

    I read this comment in your original post
    I have 3 Nest Thermostats that require a common wire. I have a taco SR 504 switching relay that the thermostat wires (currently 18/2 cable) go into. It is a 4 switch relay. One is dedicated to domestic hot water and the rest go to circulator pumps for forced hot water (baseboard). My questions are:


    I made the Domestic Hot Water (Blue tank) the circulator zone to the Indirect Hot Water thermostat. That tank would have a thermostat for water temperature in the tank. We like to call them Aquastats. Do you heat the water with something else?

    If the control you have is the one that I selected... AND you wanted to provide Priority for the DHW tank zone, you would need to use zone 4 for the DHW. Since you are using a 2 wire thermostat (aquastat) for that zone, you do not need to follow the R and W as you do when you are using a thermostat that requires a C terminal.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,092
    Options
    pecmsg said:

    Easier to use anyone else’s stat!

    just saying!

    This diagram works for "anyone else’s stat" that requires a C terminal connection. SO your statement is not exactly accurate.

    As I said earlier @rob2816

    Pay no attention to the man behind the curtain

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • rob51
    rob51 Member Posts: 67
    Options
    @Jamie Hall thank you for the explanation
  • rob51
    rob51 Member Posts: 67
    Options
    @EdTheHeaterMan ah, yes. DHW. Makes sense. Yes, this is how I heat my hot water. I have an aqua stat as you say on the indirect fire tank. I believe I have it connected to zone 1, so in this configuration, I can’t prioritize DHW. Perhaps I could change this though I’ve never had a problem with not getting enough HW.
    EdTheHeaterMan
  • rob51
    rob51 Member Posts: 67
    Options
    @EdTheHeaterMan also, the fuse is between the transformer and the thermostats, correct? Not from the 120v line to the transformer. It doesn’t make sense to me for it to be between 120v line to the thermostat because that’s too much current. It will blow as soon as you power everything up. Just making sure.
  • pecmsg
    pecmsg Member Posts: 4,883
    edited June 2023
    Options
    rob2816 said:

    @pecmsg. How are they easier?  I would think most wifi enabled thermostats would require significant power to run, thus requiring a C wire.  Perhaps I am wrong and you can enlighten me.

    NEST uses power stealing to charge their battery. That drives computer boards crazy causing all kinds of issues.
    Honeywell, Ecobee, and most others just use AA batteries!
    rob51
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,092
    Options
    rob2816 said:

    @EdTheHeaterMan also, the fuse is between the transformer and the thermostats, correct? Not from the 120v line to the transformer. It doesn’t make sense to me for it to be between 120v line to the thermostat because that’s too much current. It will blow as soon as you power everything up. Just making sure.


    The low voltage fuse is designed for the low voltage current. You would need a different fuse or circuit breaker for the 120V side. The fuse pictured is designed to blow before there is an overcurrent event that would melt the windings on the transformer. The amount of current needed to melt a transformer winding is far less than the 120 v current fuse you would place on the 120V side. That is why I would place the 3 amp automotive fuse on the R terminal of the transformer then connect all other R terminals from all other components (thermostats or controls) to the other side of the fuse. This is known as placing the fuse in series with the transformer circuit.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • rob51
    rob51 Member Posts: 67
    Options
    @EdTheHeaterMan. This all makes sense.  Thank you!
  • rob51
    rob51 Member Posts: 67
    Options
    I was about to install a new transformer when I realized I already have a second one? There is a switch on the other side of the box. The switch connects to the 120 line in the relay box. Also, there is 18/2 cable that runs to the isolated end switch on the relay. It appears to run from “R” and “G”. Can I hook up to the R and C terminals here? Also, since the isolated end switch is connected to R and G, does this have any implications as to where I connect? Thank you as always.

  • rob51
    rob51 Member Posts: 67
    Options
    As a side note, I tested between “R” and “C” with my multimeter.  I’m getting 26 volts.  
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,092
    edited July 2023
    Options
    rob51 said:

    I was about to install a new transformer when I realized I already have a second one? There is a switch on the other side of the box. The switch connects to the 120 line in the relay box. Also, there is 18/2 cable that runs to the isolated end switch on the relay. It appears to run from “R” and “G”. Can I hook up to the R and C terminals here? Also, since the isolated end switch is connected to R and G, does this have any implications as to where I connect? Thank you as always.

    Probably not a good idea. The R from the thermostat and the C from the thermostat must come from the same transformer. I believe that the R from any of those thermostats do not get their power form the transformer in the photo. Connecting the thermostat R from one transformer and the thermostat C to a different thermostat would be like connecting a flash light bulb to the Pos (+) end of a battery and the Neg (-) end of a different battery without connecting the batteries together.
    Try this to see if the light glows when you turn on the switch.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?