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Help c wire

diego168diego168 Member Posts: 69
Hello everyone

My Home was build in 2013. I have an oil boiler that has a honeywell transformer at72d That leads to 3 honey well zone valves. Behind current thermostats i have free wires not being used so I’m hopping one of those can be used to tap into a 24v source and use it as a c wire for my 2 nest thermostats.
The following picture shows how my boiler is wired. I’m hopping to tap into the secondary transformer white wire that’s connected to all the yellow wires. Would these transformer be overwhelmed by connecting this 2 thermostats to it ? Or even 3 thermostat.
Thank you
«134

Comments

  • HVACNUTHVACNUT Member Posts: 3,350
    If that's a 40 va transformer it should be ok.
    If it's not too confusing, I would rewire so Red is R and White is C on the transformer.
  • diego168diego168 Member Posts: 69
    > @HVACNUT said:
    > If that's a 40 va transformer it should be ok.
    > If it's not too confusing, I would rewire so Red is R and White is C on the transformer.

    Thanks for the reply. Can you explain why I should change red to r on the transformer.
    And you mean just switching does 2 wires.
    I’m trying to understand how things work.
  • diego168diego168 Member Posts: 69
  • ratioratio Member Posts: 2,428
    Red is usually the hot wire, white is usually the common wire when there's only two or three conductors in the cable. Electrically, it doesn't matter, but it could cause confusion later on down the line.
  • diego168diego168 Member Posts: 69
    > @ratio said:
    > Red is usually the hot wire, white is usually the common wire when there's only two or three conductors in the cable. Electrically, it doesn't matter, but it could cause confusion later on down the line.

    But you guys mean only switching red and white at the transformer everything remains the same in my picture scheme? Or do I have to change others.

    At the transformer secondary side c and r, how Are these 2 different or are they both putting out 24v.
    I mean which direction does electricity or current flow
  • diego168diego168 Member Posts: 69
    And if I switch those 2 wires. Connections at the thermostat would still be the same ?
  • Alan (California Radiant) ForbesAlan (California Radiant) Forbes Member Posts: 2,330
    edited June 9
    "R" is hot and "C" is neutral.

    Keep the wiring the same, just add a white wire between the transformer's "C" terminal to the "C" terminal on your Nest stats.

    @HVACNUT and @ratio were just suggesting changing wire colors for convention.

    Often wrong, never in doubt.

    Click here to learn more about this contractor.
  • Jamie HallJamie Hall Member Posts: 12,847
    Conventionally... C is a common return wire. As often as not, yellow. R wires -- commonly red -- may be hot (24 volts to a good ground) and may feed either a heating contact in the thermostat or a cooling command contact in the thermostat (or more than one of each, on multi-stage thermostats). W wires are switched by the thermostat, and will carry power to command something to run.

    On "stupid" thermostats, there are only two wires needed to the thermostat, since all the thermostat does is close a contact when it's too cold (for heat) or too hot (for cooling).
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • HomerJSmithHomerJSmith Member Posts: 923
    edited June 9
    As I recall the Honeywell ZV draws about 7.7 VA. The Nest draws about 4.8 VA. Add them up. Three ZV at 7.7 and 3 Nests at 4.8 = 37.5 VA. I think I would like some extra VA. Perhaps, 50 to 75 VA transformer.

    https://www.supplyhouse.com/Honeywell-AT150A1007-Foot-Mounted-Plate-Mounted-Clamp-mounted-or-Panel-Mounted-120-208-240-Vac-Transformer-w-9-in-Lead-Wires-50VA

    The Nest draws about 200 mA/h or .2 A/h.
  • diego168diego168 Member Posts: 69
    Ive attached a picture of we’re the c and r wire go. I just want to be clear I do it right. The Green circle is where the red wire from the transformer c terminal meet other wires. Black circle is where the white wire from the transformers r terminal meets the yellow wires. So I should connect my c wire to the bundle of red wires ?
    In The diagram I posted On My first post I had my c wire connected to the yellow bundle of wires.
    Just want to make sure I do this right. Thanks everyone
  • HomerJSmithHomerJSmith Member Posts: 923
    edited June 9
    Wire This way. Per your drawing. The connections to the thermostats from the zone valve motors are yellow wires (pictured as green) and the connection from the zone valve motors to the common on the transformer is also yellow.

  • In your drawing, you show the "C" wire from the transformer connecting to the yellow wire bundle, no? I think that's where you want it.

    It you attach the "C" wire to any of the red wires (end switch), it could blow either the accessory transformer or the transformer in the boiler.
    Often wrong, never in doubt.

    Click here to learn more about this contractor.
  • Jamie HallJamie Hall Member Posts: 12,847
    Um. Well, the colour descriptions in your text don't seem to match the coloured wires on your circuit drawing.

    So...

    The wire from the C terminal of the transformer is your common return. It needs to be connected to the return terminal of each of the zone valves, as your sketch shows. It also needs to be connected the C terminal of any thermostats which need a C connection.

    The other terminal of the transformer is your supply. It needs to be connected to the R terminals of your thermostats -- and nothing else.

    Then each thermostat needs a wire from its W terminal to the input terminal of the corresponding zone valve.

    May I suggest that you place a tag -- masking tape will do -- on each wire in that somewhat scary collection of wires -- indicating what the other end of each wire is connected to. Then start making sure that what should be connected is...

    I also note that there seem to be more wires than are described... they all need to be laballed.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • diego168diego168 Member Posts: 69
    In my picture I didn’t label the c and r terminals on the transformer I think that’s lead to confusion im sorry. I re did my diagram labeled the thermostat plus some more pictures. Thanks everyone for taking the time to help.
  • EdTheHeaterManEdTheHeaterMan Member Posts: 572
    edited June 9
    I have worked at a Supply House as the HVAC Tech support guy in the past. This is always a problem. What color the wire is and what terminal it is attached to always makes the conversation confusing.

    I always told the contractor that asked for my assistance to standardize the wiring so every zone valve is properly connected to each thermostat and to each transformer and each end switch.

    The next thing I tell them is that electric is colorblind and can not read the arrows on the diagram. It goes anywhere it can. If you don't follow the diagram properly, you may let the smoke out of the transformer or control. Once that happens you will need to buy a new transformer or control.

    Zone valve rules:
    Rule #1 Always wire the R terminal on the transformer to the R terminal of the thermostat.
    Rule #2 Always wire the common terminals of the transformer to the common wire of the zone valve. (Honeywell does not use a common wire for end switch and motor or actuator so you just need to wire one of each yellow wires to the C of the transformer)
    Rule #3 Always use the W from the thermostat to power the zone valve motor or actuator (Honeywell: YOU use the other yellow wire from each valve)

    That said. You can now use the C from the transformer to be the C for the thermostat.

    Now your original installer did not follow these rules, so... you may have a problem in communication with other people when describing what's happening. Your original diagram picture is fine and will work. The only problem is that you must be sure that you don't confuse the R and C on the transformer with the White and Red color of the wire.

    If it is not too much to ask, you might want to start the wiring from scratch to be sure that there is no mistake or at least follow @Jamie Hall advise and tag each wire
    HomerJSmith
  • HomerJSmithHomerJSmith Member Posts: 923
    edited June 9
    Rather than look at your wire pictures start from the beginning.

    3 yellow wires from the ZV's are bundled together with the 3 "C" wires coming from the thermostats. That 6 wires add one more wire to the common on the transformer. That's 7 wires. Good.

    Take the other yellow wire from each ZV and connect it to the "W" wire coming from the thermostat that controls THAT zone. Good!

    The 24V wire from the transformer (red?) goes to the R on each thermostat. Easypeasy!

    The other red wires coming from the ZV's are end switches, ignore those.

    If you wanted to switch the red and white wire on the transformer as "ratio" suggested make sure that the connections to the thermostats and ZV's are as I suggested.

    EdTheHeaterMan
  • EdTheHeaterManEdTheHeaterMan Member Posts: 572
    edited June 10
    yea... what he said!
    @HomerJSmith and @Jamie Hall and @ratio and @HVACNUT are all in agreement. If you get this wrong, it's gonna cost ya

    THAT @EdTheHeaterMan guy is off his rocker.

    But all of our diagrams show R to R and C to Yellow and W to the other Yellow. I hope you are getting the idea!
  • Alan (California Radiant) ForbesAlan (California Radiant) Forbes Member Posts: 2,330
    edited June 10
    Better yet, get a zone valve control. The terminals are clearly marked, you don't need to bundle wires together and there are diagnostic lights that come on when a thermostat calls for heat and another light that comes on when the end switch closes which makes for easy diagnostics when the thermostat or zone valve malfunction. Taco and Caleffi are good choices.
    Often wrong, never in doubt.

    Click here to learn more about this contractor.
    SuperTechHomerJSmith
  • diego168diego168 Member Posts: 69
    Thanks homer and everyone else for taking the time to explain. I understand now.
    As of now I only have 2 nest thermostats. Is it a problem if I change the wiring as described but leave one thermostat as is. Obviously I would skip adding a c wire at old thermostat.
  • EdTheHeaterManEdTheHeaterMan Member Posts: 572
    edited June 10
    NO! do not leave the non-Nest thermostat as it is. The R from the transformer MUST go to R on the thermostat on EVERY zone.

    This is where the problems happen and you let the smoke out of the thermostats or transformers. Every zone must be wired the same. If one thermostat does not need a common, then dont add the common to that thermostat only. BUT ALL THERMOSTATS MUST BE WIRED THE SAME. Rule #1 is R to R
    If the non-Nest thermostat is wired [Transformer C] to [Thermostat R] then you may get the electricity to go the wrong way down the one-way street.
  • diego168diego168 Member Posts: 69
    Thank you ed. Yes I was going to wire them all as per homer and your recommendation. I meant just not adding a c- Wire to one thermostat and I was also wondering if the changes done at the transformer would affect a non WiFi thermostat.
  • HVACNUTHVACNUT Member Posts: 3,350
    > @Alan (California Radiant) Forbes said:
    >
    > @HVACNUT and @ratio were just suggesting changing wire colors for convention.


    That's all I was going for. R power, W common, when there's only 2 wires, and its not for a switch.

    @diego168 , I should have kept my yap shut. Leave it as it is. No harm done. Just connect Common on the transformer to Common on the Nests. Blue is a typical color if you have it.
  • EdTheHeaterManEdTheHeaterMan Member Posts: 572
    edited June 10
    changes done at the transformer will not affect the non-wifi thermostat
  • EdTheHeaterManEdTheHeaterMan Member Posts: 572
    HVACNUT said:

    > @Alan (California Radiant) Forbes said:

    >

    > @HVACNUT and @ratio were just suggesting changing wire colors for convention.





    That's all I was going for. R power, W common, when there's only 2 wires, and its not for a switch.



    @diego168 , I should have kept my yap shut. Leave it as it is. No harm done. Just connect Common on the transformer to Common on the Nests. Blue is a typical color if you have it.

    this is where the communication confusion comes from. @HVACNUT without changing the red and white wire on the transformer, the transformer C is connected to the thermostat R terminals. following your last statement connecting the C on the nest to the C on the transformer would not offer the proper charging circuit. The easiest fix is to follow your suggestion. swap the Red and white wires on the transformer and the wiring matches all of our diagrams.
  • diego168diego168 Member Posts: 69
    > @EdTheHeaterMan said:
    > (Quote)
    > this is where the communication confusion comes from. @HVACNUT without changing the red and white wire on the transformer, the transformer C is connected to the thermostat R terminals. following your last statement connecting the C on the nest to the C on the transformer would not offer the proper charging circuit. The easiest fix is to follow your suggestion. swap the Red and white wires on the transformer and the wiring matches all of our diagrams.

    Yes that’s my plan switch wires at the transformer and then add common wire to the yellow bundle of wires. The existing wire not being use is green and not blue how it should be for a common wire. So I will label it.
    Another questions is will my transformer be able to handle the load of the 3 zone valves plus 3 nest thermostats ?
  • Alan (California Radiant) ForbesAlan (California Radiant) Forbes Member Posts: 2,330
    edited June 10

    As I recall the Honeywell ZV draws about 7.7 VA. The Nest draws about 4.8 VA. Add them up. Three ZV at 7.7 and 3 Nests at 4.8 = 37.5 VA. I think I would like some extra VA. Perhaps, 50 to 75 VA transformer.

    https://www.supplyhouse.com/Honeywell-AT150A1007-Foot-Mounted-Plate-Mounted-Clamp-mounted-or-Panel-Mounted-120-208-240-Vac-Transformer-w-9-in-Lead-Wires-50VA

    The Nest draws about 200 mA/h or .2 A/h.

    If you stick with the 40VA transformer, it's going to get pretty hot when all three zones are calling for heat.

    If you don't like taking chances, get a larger transformer.

    I like this one because it has a circuit breaker that trips if you wire it wrong and is very reasonably priced:

    https://www.supplyhouse.com/Functional-Devices-TR100VA002-Transformer-w-Circuit-Breaker-Foot-Dual-Threaded-Hub-Mount-100VA-120-to-24-Vac
    Often wrong, never in doubt.

    Click here to learn more about this contractor.
  • EdTheHeaterManEdTheHeaterMan Member Posts: 572
    edited June 10


    I believe so. but to be absolutely sure you don't have a problem, you can include a 3 amp inline fuse. connect the fuse to the R terminal and the red wire to the other end of the fuse. automotive fuses are easy to come by. 3 amp will blow before the 40 VA transformer fails. If that happens then you can get a 50 VA transformer and use a 5 amp fuse.
  • diego168diego168 Member Posts: 69
    Ok thank you ed and Alan.
    This forum has been the most helpful forum I’ve encounter ever.
  • diego168diego168 Member Posts: 69
    > @Alan (California Radiant) Forbes said:
    > (Quote)
    > If you stick with the 40VA transformer, it's going to get pretty hot when all three zones are calling for heat.
    >
    > If you don't like taking chances, get a larger transformer.
    >
    > I like this one because it has a circuit breaker that trips if you wire it wrong and is very reasonably priced:
    >
    > https://www.supplyhouse.com/Functional-Devices-TR100VA002-Transformer-w-Circuit-Breaker-Foot-Dual-Threaded-Hub-Mount-100VA-120-to-24-Vac

    Alan I was looking at the diagram of the transformer you quoted and it tells you which side is high voltage “120” and which side is secondary. But on the secondary side I couldn’t tell which is R and C. It just looks like 2 wires coming out.

    which other transformers Do you guys recommend.
  • Jamie HallJamie Hall Member Posts: 12,847
    The side labelled "120" is -- clearly -- the side which hooks to your house wiring. The other side is low voltage -- 24 Volts AC, nominal. Some transformers have one of the low voltage terminals also connected to the transformer case. Some -- most -- do not. You can use your multimeter to find out very quickly. If one side is connected to the transformer case, that's the C terminal and the other is R. Otherwise, it doesn't matter.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
    ratio
  • T Otherwise, it doesn't matter.

    Thanks for bringing this up, Jamie.

    I purchased this transformer earlier this year because I was installing a Safgard 1100 low-water cutoff and I kept blowing the transformer. There were other devices in series with the Safgard, so I didn't know where the short was. The circuit breaker let me search with abandon. Finally discovered that I was connecting the "hot" lead to the Safgard neutral which was grounded.

    Both 24 volt leads were yellow and I remember testing them to find which lead was "hot" by meter to ground; I didn't get 24 volts to either of them. I was only able to verify polarity once I switched the wires without the circuit breaker tripping.

    Can anyone explain how to properly test polarity when it's not obvious?

    Often wrong, never in doubt.

    Click here to learn more about this contractor.
  • HomerJSmithHomerJSmith Member Posts: 923
    edited June 10
    These transformers are all good. The reason that I chose my suggested trans was that it had screw connection on the face like your old trans.

    The other suggestions have lead wires that connect to the Thermo's and ZV's with wire nuts and you have to bring the 24 V leads out of the 4X4 junction mounting box.
    diego168
  • diego168diego168 Member Posts: 69
    > @HomerJSmith said:
    > These transformers are all good. The reason that I chose my suggested trans was that it had screw connection on the face like your old trans.
    >
    > The other suggestions have lead wires that connect to the Thermo's and ZV's with wire nuts and you have to bring the 24 V leads out of the 4X4 junction mounting box.

    I think I like the idea of having the face plate with wire nuts like the one I have just makes it easier for me. Would it be overkill to get the 75 rather than the 50.
  • diego168diego168 Member Posts: 69
    > @diego168 said:
    > > @HomerJSmith said:
    > > These transformers are all good. The reason that I chose my suggested trans was that it had screw connection on the face like your old trans.
    > >
    > > The other suggestions have lead wires that connect to the Thermo's and ZV's with wire nuts and you have to bring the 24 V leads out of the 4X4 junction mounting box.
    >
    > I think I like the idea of having the face plate with wire nuts like the one I have just makes it easier for me. Would it be overkill to get the 75 rather than the 50.
  • Jamie HallJamie Hall Member Posts: 12,847
    I'd ohm it out, @Alan (California Radiant) Forbes . If one side is grounded to the case, that side should read 0 ohms -- or certainly not more than a few hundredths (make sure your probes are really well placed!). The other side will read more, but quite likely not more than a few ohms -- it's just a wire coil, after all. But I would have thought that a voltage reading would have shown it, if the transformer was sitting on an insulator, but energized (mind how you fry yourself there). There should be 24 volts , more or less, between the two terminals, and nothing or just odd stray voltage (which could be almost anything, if your multimeter has high impedance) to the case if neither terminal is grounded to the case. If one is grounded, that terminal should read zero, and the other one about 24 volts.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
    Alan (California Radiant) Forbes
  • diego168diego168 Member Posts: 69
    I made This graph because I want to understand the flow in this circuit. Starting at thermostat.
    Thermostat receives 120v converts to 24v. Out of the R terminal electricity flows to thermostat and when the circuit closes at thermostat electricity goes across to w down to zone valve and valve opens. Back to transformer the C terminal also puts out 24v??? So does that mean that the valve needs 48v to open? Or
    If valve only needs 24v and transformer C terminal is always putting 24v why is the valve not always open? I’m
    Assuming R and C are always hot and putting out 24v.
    Or when electricity flows down W down to the valve it closes a circuit at the valve which causes it to open. So it uses 48v. Where does that energy go from there.
  • ratioratio Member Posts: 2,428
    Not quite right. The C terminal on the thermostat is to provide a return path to the transformer to allow the thermostat to have power to operate regardless of whether it's calling for heat or not. While not entirely accurate, you can think of power as coming out of the R terminal on the transformer, & going back on the C. It always starts at R, & always ends at C. If there isn't a path, somewhere, from R to C, no current will flow.

    Basically, the electronics of the thermostat are connected to R & C & receive power from those wires. When the thermostat wants to turn on the heat, it connects R to W, which sends power to the zone valve, which has it's own return connection to C. (Once the zone valve opens, the switch in it makes & that is used to start the boiler, but that is separate from the part we're looking at, so it can be ignored for the time being.)

    STEVEusaPA
  • Jamie HallJamie Hall Member Posts: 12,847
    The thermostat does not convert 120 to 24 internally. It is connected to the R terminal of the transformer through it's R terminal. If heat is demanded, a switch closes in the thermostat effectively connecting R to W. The circuit continues on the to zone valve motor, effectively connecting 24 volts to one terminal of the zone valve motor, the other terminal of which is connected to the transformer C. End result: a complete circuit from transformer R to the thermostat R, through the closed switch in the thermostat to thermostat W, from there to one terminal of the control valve motor, through the control valve motor, powering it, and back to C.

    Now in thermostats which themselves require more or less constant power to operate their internal circuits, that won't work. The only available return path is through W and the zone valve motor. This is where the confusion can set in: thermostats don't require that much power to operate, although they do require some, so the thermostat electronics power supply -- which has nothing to do with that closed switch -- is connected between the two terminals of that switch (R and W). It then has a sort of circuit back to the transformer C through W and the zone valve motor, but usually there isn't enough current to operate the zone valve. Usually. This is referred to as "power stealing" and, in my not very humble opinion, is a perfectly horrible kludge. Instead, the thermostat kindly is provided with a C terminal -- which is there only to power the electronics -- which is connected back to the transformer. With that arrangement, the thermostat electronics always have power from R and C.

    Bottom line: to power the thermostat electronics, you need to feed the thermostat R from the transformer R, and return the thermostat C to the transformer C. W is used to complete a circuit from transformer R to thermostat R, through the thermostat's heat demand switch or relay, to the zone valve through the thermostat's W and thence back to C.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • diego168diego168 Member Posts: 69
    Thanks for explaining!

    I wired everything as discussed and the 2 nest E thermostat work fine. As for the nest learn there is an issue when I connect the AC. On the previous thermostat the ac had a G, Y and an Rc wire.
    When I connect the RC wire to the rc terminal the nest gives error e79 saying No power detected to the c terminal. If I unplug the rc wire it works. Or if I plug the rc and unplug the c wire it works. I’m
    Attaching pics of wiring.
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