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How to add a 24v common wire to a 240v furnace for 120v nest thermostat

I just recently installed a nest thermostat not knowing that the furnace was 240v. The thermostat is being powered by only one leg of the 240 transformer inside the furnace. I need to add a common wire and do not know how. There are four wires going to the thermostat. Red, white, green, and yellow (which is not connected inside the furnace as I do not have a/c). Where would I get the 24v common wire to power the nest thermostat?

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 20,416
    What kind of furnace is this? Electric? Gas or oil fired? Hot air, hot water, steam?

    And what kind of thermostat is presently controlling it?

    For you to be able to power the Nest -- or use it to control the furnace -- there has to be a 24 volt (in this case, 240 to 24 volt) transformer associated with the furnace -- but there may not be, if the furnace is electric.

    So a LOt more information on what you are trying to control is needed here!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • David_S
    David_S Member Posts: 12
    @Jamie Hall it is an electric furnace forced air. Previous thermostat was a 24v max digital Honeywell. I installed the nest and it controls the furnace but shows an error and in order to connect to the app and internet, it requires a common wire. The old thermostat did not have a common wire. So there is 24v power going to the thermostat but now I need a 24v common and not sure how to accomplish that
  • David_S
    David_S Member Posts: 12

  • David_S
    David_S Member Posts: 12

  • David_S
    David_S Member Posts: 12
    It’s kind of hard to get a good picture but this is what I am seeing. I’m not too familiar with any of this but the red wire is going to the thermostat. The black wire has multiple wire and the one going out of the furnace is not being used
  • Jamie Hall
    Jamie Hall Member Posts: 20,416
    The black wire (shown as going to terminal Y on the thermostat) should be your common. You can verify that if you have a good voltmeter. The voltmeter should show 24 volts or thereabouts between black and red anytime the furnace is powered on. It should show 24 volts between black and white only when the thermostat is calling for heat.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • David_S
    David_S Member Posts: 12
    I don’t have a black wire going to the thermostat. The only wire going to the thermostat are red, white, green, and yellow. The yellow wire is not connected to anything on the furnace as shown and the black wire coming from the same load side of the furnace is not connected to anything also. 
  • David_S
    David_S Member Posts: 12
    Should t the common be on the opposite side of the load or hot side of the transformer?
  • Jamie Hall
    Jamie Hall Member Posts: 20,416
    edited November 2022
    what is the yellow wire connected to at the thermostat? Remember that electricity is colourblind -- it doesn't care what colour the insulation is, all it cares about is what it's connected to.

    So.

    If someone hasn't messed up the wires in the furnace, you should have 4 low voltage wires going to... mostly the thermostst. These should be one black wire, which is connected to one terminal of the transformer along with two other wires. The other two are returns from two relays in the furnace controls (blower relay and main power contactors). You should have a green wire, which is the feed wire for the blower relay. It should show no particular voltage unless the thermostat or bleed air control box is calling for the blower to run. There should be a black wire which should show 24 VAC referenced to the red wire. The red wire, in turn, is the common power feed from the transformer to the thermostat and bleed air box. The white wire is the feed wire for the two main power contactors.

    Now at the thermostat, there are, again, four wires. The one connected to the Rh terminal is the hot power feed from the furnace, and should be connected to the red wire at the furnace. The one connected to the G terminal should in turn connect to the green wire at the furnace and energise the blower relay and turn on the blower. The one connected to the W terminal should connect to the white wire at the furnace. The one connected to the Y terminal is a common return, and should be connected to the black wire at the furnace.

    If you have any doubt AT ALL about what is what, before you connect anything to anything place a 2 amp fast blow fuse in both the red wire and black wire external wiring.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • David_S
    David_S Member Posts: 12
    So at first install of the thermostat, I connected the wires up the same way they were in the old thermostat. Red to R, green to G, white to W, and yellow to Y. After the error on the nest, I looked further into the wiring in the furnace and found the the yellow wire is actually not connected to anything in the furnace. The black wire coming from the furnace is also not connected to anything. I am unsure if the red and black on the same side of the terminal is 24v in and 24v comm. 
  • David_S
    David_S Member Posts: 12

  • David_S
    David_S Member Posts: 12
    I am also unsure where this 240v system is getting 24v to the thermostat. Shouldn’t a 240v furnace require a 240v thermostat and just be 2 or 3 thicker gauge wires going to the thermostat?
  • David_S
    David_S Member Posts: 12
    This is where the red going to the thermostat and back wire going 3 ways is connected. That back wire next to the red should be my 24v common I need to power the thermostat?
  • ratio
    ratio Member Posts: 3,331
    You have everything you need to make it work. First, 'electricity don't know no color'. The colors of the wire are only there for your convenience. Since you are not using the yaller (which is traditionally the first cooling stage), you can use that for your common. Put it under the C terminal on the Nest, and connect the other end to the black wire with the wire nut that's circled in your picture above. ASSUMING there's no internal fault (a reasonable assumption if it's working) and that the wiring going to the stat is undamaged (a somewhat less reliable assumption—but still quite likely), it'll work.
  • Jamie Hall
    Jamie Hall Member Posts: 20,416
    Power for the thermostat (and internal relays and contactors) is coming from a built in 240 VAC to 24 VAC transformer in the furnace itself. There are three black wires connected to one of the 24 VAC terminals; one of these is our lonely black wire. The other terminal has only one wire attached -- the red wire. That line is protected by an internal 3 amp fuse (which may have blown -- if you measure between the black and the red wires leaving the furnace, you should get 24 VAC; if you don't, that fuse may be blown).

    The black wires are common.

    Now I just noticed that the wiring diagram is for a heat/cool thermostat, and it appears that the Y terminal on the thermostat is intended to control a cooling unit contactor. There may not be, in fact, a common wire connection to the thermostat in the wires you have. If there isn't, or you have any doubt as to what that yellow wire is or was connected to, you'll need to connect the black wire from the furnace to the C terminal on the Nest.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • David_S
    David_S Member Posts: 12
    There is no a/c here so That yellow wire was never hooked up to anything. I don’t think the black wire was either. I was hoping that I could just connect the black and yellow together for a common wire but I’m not too knowledgeable on this so I wanted some advice. Thanks guys I appreciate the help.
  • HVACNUT
    HVACNUT Member Posts: 4,856
    The black wires on the load side of the transformer is Common. 

    Shut off the power and splice to any one of the black wires its attached to. 
  • Jamie Hall
    Jamie Hall Member Posts: 20,416
    Try it and use it as the common -- but put a fuse in it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ratio
    ratio Member Posts: 3,331
    The black with the wire nut hanging out beside the others where the stat ties in is almost certainly the common, the only way it isn't is if someone fooled around with the internal wiring. The yaller is almost certainly usable as a common to the stat, the only way it isn't is if the wire is damaged or improperly spliced somewhere.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,913
    edited November 2022
    From what I can see this is your existing or old thermostat wiring.
    This is the nest thermostat wiring.

    I believe the unused black wire is the one you want. Sometimes pictures are better


    This is easy. Use the unused yellow wire from the thermostat wire to connect the Nest C terminal to the unused black wire on the transformer.
    Edward Young Retired HVAC Contractor & HYDRONICIAN Services first oil burner at age 16 P/T trainer for EH-CC.org
    ratio
  • HVACNUT
    HVACNUT Member Posts: 4,856
    @Jamie Hall and @ratio are correct. The lone black wire with the wire nut outside the cabinet is the Common. 
    Now, about that Nest...
    EdTheHeaterMan