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Can I pigtail a "C" common wire to power a Smart Thermostat to the 24v wires coming from the outside

JimmyNJ
JimmyNJ Member Posts: 107
edited April 28 in Thermostats and Controls
Hoping someone can help me solve for powering a 24v Common wire to what will be my new smart thermostat. Currently my downstairs thermostat controls a boiler as well as the central air unit for downstairs. The thermostat wire does have an unused blue "C" wire but when I checked in the attic where the thermostat wire gets pigtailed into the wires coming out of the AC handler I can see that the blue wire is NOT actually connected to anything up there so I know it won't power anything. The wire from the outside AC condensing unit has 2 wires (one red and one white) that I believe always carry 24v (but maybe I'm wrong?) and the red is pigtailed into a brown wire that seems to be going into the control board of the AC handler and the white wire is pigtailed into a blue wire that is coming out of a light brown shielding that also has a green wire that is pigtailed into a gray wire coming out of the AC handler and a red wire that is pigtailed into a red wire coming out of the AC handler. The wires are then connected to the wires coming from my indoor thermostat using pigtails. Green is connected to green, red to red, and blue is connected to yellow (the yellow going to the thermostat). The thermostat wire does have two unused wires that are wrapped around the shielding of it, one white and one blue. My questions is, can I connect the blue wire that is currently unused, to the same pigtail that currently has the white wire from the outside AC compressor connected to the blue wire (which later is pigtailed into the yellow wire going to the thermostat)? Alternatively, is buying a 24v transformer adapter to power the thermostat with my current setup of needing the thermostat to manage a separate heating system (boiler) and central air unit? I have included the pictures of my current thermostat and wires as well as the back of the new one. The connections seems pretty straightforward but I believe for the adapter, they want you to connect one of the wires to the "C" and the other to the "RC"...but I need that connection for the AC unit.....is it possible to connect the wire that they want you to connect to the "RC" terminal to the "O/B" terminal instead to complete the circuit? I circled the terminals my current wires would go into.




Comments

  • Jamie Hall
    Jamie Hall Member Posts: 18,941
    Probably not -- although stranger things have happened. It's quite unlikely to be so simple -- and an incorrect connection will likely blow one or the other of the control boards or the thermostat -- or all three.

    First, do not pay much attention to the wire colours. As I have often said, electricity is colourblind.

    In order to power your thermostat, the common -- often, but not always, a green wire -- and the hot feed -- often, but not always, a red wire, must connect to the output leads of the same 24 volt AC transformer. The thermostat switched lead -- often, but not always, white -- must go to the controlled device -- possibly the boiler, possibly a control board or relay or relay block -- and that must then go back to the same terminal of the same 24 volt AC transformer as the common wire. An alternative which is sometimes used is for the common and power wires of the thermostat lead to a 24 VAC transformer, and the power and switched wires of the thermostat connected to the controlled device, powered by a different 24 VAC transformer. This connection is a little tricky, as the two transformers must both float, with the secondaries not connected in any way to a ground.

    In your situation, where you are also controlling a different unit -- the AC -- with the same thermostat, there must be at least one other wire from the thermostat which is switched and controls that device independently, and in that case the hot feed to the thermostat must also be connected in some way to the transformer for that device.

    I doubt very much that the two wires from the outside unit which you mention carry a constant 24 volts AC. It is much more likely that one of them is a hot feed and the other is a switched return (switched by the thermostat when cooling is requested).

    You need to identify exactly which wires are connected to what, and draw a circuit diagram of the complete system; I strongly recommend labels on the wires rather than depending on colour. Once you have done that, it is possible that a connection scheme which will work can be arranged, as at least you seem to have enough independent wires.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,002
    The red and white coming from the outside condenser is 24volt going to the outside condenser to start it. If the white from the outside condenser that is spliced to the blue that goes to the thermostat on the Y terminal that would mean the red wire going to the outside condenser is your common wire.
  • Jamie Hall
    Jamie Hall Member Posts: 18,941
    I wrote a nice long comment on this one... which vanished. Grr...

    Short answer is maybe. But in order to verify what exactly does what, you need to have a clear circuit diagram -- ignoring wire colour, which can be misleading -- showing exactly what is connected to what, including all the power sources (you likely have at least two transformers here). Then you may find that indeed you red wire from the outside unit is constant hot, and the white wire is the switched control signal -- in which case you may find a way to get back to the outside unit's transformer with a common wire and power the thermostat that way.

    Or there may be a way to figure out which wire is the hot from the boiler control system, and get back to that transformer with a common wire.

    As I said in the vanished post, I very much recommend a circuit diagram showing what goes where -- with clear labels on the wires (don't believe the colours!) and then figure it out. Trouble with two different power sources is that unless it is really done correctly, there's a good chance of blowing one or the other of the control boards or the thermostat -- or all three.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JimmyNJ
    JimmyNJ Member Posts: 107
    So the red wire from the condenser that is pigtailed to the brown wire (going into the AC hander in the attic) would be the 'common wire'? any issues if I added another wire to that pigtail and pigtailed the added wire to the unused 'blue' wire going to the thermostat to use it as the common wire with a constant 24v going to the thermostat?
  • Erin Holohan Haskell
    Erin Holohan Haskell Member, Moderator, Administrator Posts: 1,864

    I wrote a nice long comment on this one... which vanished. Grr...

    Sorry @Jamie Hall. There was a duplicate post held in our spam filter with your comment. I've merged the posts.
    President
    HeatingHelp.com
  • Jamie Hall
    Jamie Hall Member Posts: 18,941
    JimmyNJ said:

    So the red wire from the condenser that is pigtailed to the brown wire (going into the AC hander in the attic) would be the 'common wire'? any issues if I added another wire to that pigtail and pigtailed the added wire to the unused 'blue' wire going to the thermostat to use it as the common wire with a constant 24v going to the thermostat?

    No. It is much more likely to be the constant hot from the condenser. There appear to be only two wires going to the condenser. One has to be constant hot, the other has to be the control signal. Now we don't know where the power supply transformer is. Most commonly it would be at the condenser -- in which case you would need a common return going out to the condenser to use that power supply for the thermostat. That wire isn't there. There is a possibility that the power supply for that condesnser constant hot is somewhere inside. In which case, you could use that supply -- if you can find it

    A more general comment: you may be confusing a constant hot wire with a common wire. You need both to power your thermostat, plus the switched wire to carry the signal.

    I suspect that if you add a wire as you suggest, you will blow either the power supply to which they are attached or the thermostat.

    Do NOT depend on voltage measurements to determine which wire is which. In particular, the control may show 24 volts relative to the constant hot wire when the thermostat is not calling. It may also show a random floating voltage, depending on how things are wired, although that's less common. Measurements to ground are also unreliable, as the 24 VAC transformer may -- or may not -- have one side of the secondary grounded.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JimmyNJ
    JimmyNJ Member Posts: 107

    I wrote a nice long comment on this one... which vanished. Grr...

    Sorry @Jamie Hall. There was a duplicate post held in our spam filter with your comment. I've merged the posts.
    My apologies as well @Jamie Hall as I went to edit my original post after you had replied but then somehow it just vanished and I thought I had accidentally deleted it. Sorry about that.
    Erin Holohan Haskell
  • JimmyNJ
    JimmyNJ Member Posts: 107

    JimmyNJ said:

    So the red wire from the condenser that is pigtailed to the brown wire (going into the AC hander in the attic) would be the 'common wire'? any issues if I added another wire to that pigtail and pigtailed the added wire to the unused 'blue' wire going to the thermostat to use it as the common wire with a constant 24v going to the thermostat?

    No. It is much more likely to be the constant hot from the condenser. There appear to be only two wires going to the condenser. One has to be constant hot, the other has to be the control signal. Now we don't know where the power supply transformer is. Most commonly it would be at the condenser -- in which case you would need a common return going out to the condenser to use that power supply for the thermostat. That wire isn't there. There is a possibility that the power supply for that condesnser constant hot is somewhere inside. In which case, you could use that supply -- if you can find it

    A more general comment: you may be confusing a constant hot wire with a common wire. You need both to power your thermostat, plus the switched wire to carry the signal.

    I suspect that if you add a wire as you suggest, you will blow either the power supply to which they are attached or the thermostat.

    Do NOT depend on voltage measurements to determine which wire is which. In particular, the control may show 24 volts relative to the constant hot wire when the thermostat is not calling. It may also show a random floating voltage, depending on how things are wired, although that's less common. Measurements to ground are also unreliable, as the 24 VAC transformer may -- or may not -- have one side of the secondary grounded.
    Thanks Jamie for your time to look at and respond to my post. I thought this was within my DIY capabilities but it seem that maybe I need to call an HVAC company to install the thermostat (which irks me a bit as I am somewhat handy :) Unless there is an option to buy a separate 24v transformer adapter and use that to power the thermostat (although perhaps that option only works for thermostats that control either heat or cool, but not both).
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,002
    Generally, with residential air handlers the transformer is in the air handler, Y from the thermostat and common from the transformer go outside to start the condenser.

    If you find the wire coming off "Y" on the thermostat (color doesn't matter) the {other) wire in the cable that goes out to the condensing unit will be your common wire.
  • Jamie Hall
    Jamie Hall Member Posts: 18,941
    edited April 28
    Actually there are several approaches to a separate power supply for these things. I think that even Nest has a wall wart arrangement which can be used. The trick is to determine which of the "R" connections is used by the thermostat as its constant 24 volt supply for charging -- Usually Rc, but sometimes Rh if there are two -- and connect the outboard 24 volt supply to that and to the "common" (usually C) terminal. Nothing else gets connected to C, but the leads from the heating and cooling controls get connected to Rc or Rh as usual and the switched lines for heating and cooling (almost always W for heating, for instance; cooling, particularly multi-stage, may be various letters) to the controlled terminals.

    There is a history behind all this nonsense -- no surprise there! -- and it goes surprisingly far back. There are some very ingenious wiring tricks which can be used, but -- for your purposes the main thing is to get power to the little widget, and the wall wart type adapters will do that. The secret to their being reliable and not blowing things up is that their low voltage output wires aren't connected to anything else -- or ground -- so they can be connected safely between those two power supply terminals, R and C, in the thermostat, so long as only ONE of those two (R or C) is connected to something else.

    I might add, perhaps a little cynically, that you may well have just as good a chance of getting things right or wrong as many HVAC techs. Nothing against them, but control wiring often is not something they are really comfortable with, particularly mixed setups like yours.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JimmyNJ
    JimmyNJ Member Posts: 107

    Generally, with residential air handlers the transformer is in the air handler, Y from the thermostat and common from the transformer go outside to start the condenser.

    If you find the wire coming off "Y" on the thermostat (color doesn't matter) the {other) wire in the cable that goes out to the condensing unit will be your common wire.

    I added more pics in the original post above. The "Y" wire from the thermostat is pigtailed into a "blue" wire that is later pigtailed into a "white" wire that goes directly out to the AC condenser outside As far as I can tell, the 'white' wire from the condenser basically gets connected via 2 separate pigtails directly to the "Y" wire going to the thermostat and by-pass the AC handler transformer altogether. So I don't think I could hook something up there as I think I need the "C" wire to actually be hooked up to the control board within the AC handler to have constant power (?). It looks like just the 'red' wire that goes out to the condenser unit is pigtailed into a thicker brown wire coming out of the AC handler so presumably goes to the control board within the handler.
  • JimmyNJ
    JimmyNJ Member Posts: 107
    edited April 28

    Actually there are several approaches to a separate power supply for these things. I think that even Nest has a wall wart arrangement which can be used. The trick is to determine which of the "R" connections is used by the thermostat as its constant 24 volt supply for charging -- Usually Rc, but sometimes Rh if there are two -- and connect the outboard 24 volt supply to that and to the "common" (usually C) terminal. Nothing else gets connected to C, but the leads from the heating and cooling controls get connected to Rc or Rh as usual and the switched lines for heating and cooling (almost always W for heating, for instance; cooling, particularly multi-stage, may be various letters) to the controlled terminals.

    There is a history behind all this nonsense -- no surprise there! -- and it goes surprisingly far back. There are some very ingenious wiring tricks which can be used, but -- for your purposes the main thing is to get power to the little widget, and the wall wart type adapters will do that. The secret to their being reliable and not blowing things up is that their low voltage output wires aren't connected to anything else -- or ground -- so they can be connected safely between those two power supply terminals, R and C, in the thermostat, so long as only ONE of those two (R or C) is connected to something else.

    I might add, perhaps a little cynically, that you may well have just as good a chance of getting things right or wrong as many HVAC techs. Nothing against them, but control wiring often is not something they are really comfortable with, particularly mixed setups like yours.

    @Jamie Hall so are you saying (and i added more pictures of "future thermostat terminal options" and my "current setup") that I can buy a constant 24v supply to make the new thermostat work (and not mess around with the current wiring structure up at the AC handler). My understanding is that the 24v adapter wants to be connected to the "C" and then the "R/RH" or "RC" - but since my current set up is already using those terminals and I was planning on connecting them the same way for the "new" thermostat (current "R" connection to the new "RH" terminal and then my current "RC" to the "RC" it seems like I am unable to actually use a 24v adapter as both the RH and RC terminals will be used.....or can I put one of the 24v wires into the same terminal as "RC" (so have two wires connected to one terminal) and the other going to the "C"? Or are you saying that "yes, you can connect a 24v adapter BUT ONLY IF the current thermostat only uses "R/RH" or "RC" -- if both are being used (like in my case as the boiler is connected to "R" and the AC handler connected to "RC", the whole adapter idea goes out the window? Sorry, all this is just a bit confusing to me. I've attached the wiring diagram for the Sensi smart thermostat (not sure if helpful or not). I also have attached the schematic for the wiring for the AC handler and it seems like the "common wire" from the outside compressor is hooked up to the "yellow" T-stat wire directly per my prior post and the "hot" red wire is connected to the "brown" wire which ostensibly goes to the "C" connector on the AC handler control board. Very unclear to me how I can get a "C" wire going here as I don't think I can just tie into the red/brown wire pigtail and simply connect the T-stat unused 'blue' wire to them and voila have 24v going to T-stat.



  • JimmyNJ
    JimmyNJ Member Posts: 107
    edited April 28
    Actually - I am starting to think that I can just pigtail the unused 'C' wire going to the thermostat into the same junction where the "red hot" wire is pigtailed to the brown wire going to the handler transformer since that should be connected to the 'C' terminal there. Since the white wire from the outside compressor is connected directly to the "Y" (call for cooling) wire -- wouldn't having the unused blue connected to the red/brown junction work? This video seems to show how to connect it and the wiring seem to mimic mine - although he doesn't show what happened when he put the power back on...maybe something got fried so would be more comfortable if the smart folks here chimed in to say that "yes that would work" or "no it may not" :)

    https://youtube.com/watch?v=ocvF2_ZfD-M
  • Jamie Hall
    Jamie Hall Member Posts: 18,941
    Sorry I wasn't clear. Most thermostats seem to use either Rh or Rc (if both are present) for charging. Usually Rc. In some setups, Rh and Rc will be jumpered but ONLY if they both are using the same power supply. C is common, and is connected ONLY to whichever power supply is used for charging.

    So.

    Yes, you can connect one side of the outboard power supply to Rh or Rc (together with the control power supply wire already there) and the other side to C, but ONLY if nothing else is connected to C and the secondaries of the outboard power supply are not grounded. Using the power supplies for the two units you would use the upper diagram you posted (the one with one transformer on one side and the other one the other). Note that in this diagram that the C terminal is connected ONLY to the cooling transformer so charging power is being provided by that transformer (the other loads connected to that transformer aren't a problem). Note that the Rc Rh jumper is clipped or removed in that installation
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JimmyNJ
    JimmyNJ Member Posts: 107

    Sorry I wasn't clear. Most thermostats seem to use either Rh or Rc (if both are present) for charging. Usually Rc. In some setups, Rh and Rc will be jumpered but ONLY if they both are using the same power supply. C is common, and is connected ONLY to whichever power supply is used for charging.

    So.

    Yes, you can connect one side of the outboard power supply to Rh or Rc (together with the control power supply wire already there) and the other side to C, but ONLY if nothing else is connected to C and the secondaries of the outboard power supply are not grounded. Using the power supplies for the two units you would use the upper diagram you posted (the one with one transformer on one side and the other one the other). Note that in this diagram that the C terminal is connected ONLY to the cooling transformer so charging power is being provided by that transformer (the other loads connected to that transformer aren't a problem). Note that the Rc Rh jumper is clipped or removed in that installation

    Thanks for clarifying Jamie - So in my case if I used the 24v adapter, I would plug one wire into the "C" and the other into the "RC" terminal along with thethe Red wire from my AC handler currently goes into it and I should be good to go? I am not sure what you mean by "secondaries" of the outboard power being not grounded. This is the adapter I was looking at: https://smile.amazon.com/Transformer-Thermostat-Competible-Versions-Honeywell/dp/B07DJ7RHS5/?_encoding=UTF8&pd_rd_w=fGlmx&pf_rd_p=bbb6bbd8-d236-47cb-b42f-734cb0cacc1f&pf_rd_r=QX4VZ2MBC0K3S3QHJ7JW&pd_rd_r=025a01d7-1834-4a3c-9289-c3daf531e5ff&pd_rd_wg=ij7ts&ref_=pd_gw_ci_mcx_mi

    would that work (are the 'secondaries' not grounded?).
  • Jamie Hall
    Jamie Hall Member Posts: 18,941
    That should work. Those are specifically built for just this purpose -- and the secondaries (the fine output wires) are, quite deliberately, isolated for just that reason.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JimmyNJ
    JimmyNJ Member Posts: 107
    To close the loop on this.  I ended up pigtailing the unused common wire from the thermostat to the common wire going to the outside AC compressor and it worked like a charm.  AC works and so does the boiler. Thanks for the input and suggestions!  
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,002
    @JimmyNJ

    Yes, there is no need for the outdoor condenser wires to terminate on the control board. Some ynits have a Y terminal in the Air handler, but if they do it does nothing.....its just a "dummy" terminal where you can connect the y from the outside condenser to the y from the stat. You can connect to the common anywhere in the circuit.

    Glad it worked out