Thermostat Connon wire question

Discussion in 'HVAC Heating & Cooling' started by brunetmj1, Oct 14, 2012.

  1. brunetmj1

    brunetmj1 New Member

    Messages:
    11
    Location:
    New York
    My current thermostat uses a simple red wire/white wire system used for just the gas furnace. I recently purchased a wi/fi thermostat which also requires a common wire for power. It is very clear in the directions that this thermostat will not work without it. I hooked up a third wire on the furnace logic board labeled C (which I assume means common) and put everything back together. The red and white wire were also attached to this board.
    I then stripped back this third wire to measure voltage assuming it would read 24 volts , but I got no voltage at all.
    Note: I cannot voltage check the connector itself because this panel has a rocker switch which prevents operation without the panel being attached.
    Did I have the correct wire connector on the logic board?
    Any suggestions as to what else I could do?
    Thanks in advance.
    oops title should read common wire.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,896
    Location:
    New England
    You maybe measuring zero volts because the new wire is in parallel with one of the other leads - 24vac to 24vac would read zero. And, if the thermostat was calling for heat, all three would be connected to the same point. You need to be on both sides of the step-down transformer to power the new thermostat, and you may need to look at the schematic (often there's one inside the panel) to determine which terminals you need to be on to provide that.

    As I understand it, the C terminal would get connected to the red terminal when the thermostat closes, which then powers the (often it's a relay) to turn on the heat, so you need to find the other side of the transformer to connect your new lead to.
  3. brunetmj1

    brunetmj1 New Member

    Messages:
    11
    Location:
    New York
    Thank You
    This is not as straight forward as I thought it might be. So I will hire a professional to look at this.
  4. brunetmj1

    brunetmj1 New Member

    Messages:
    11
    Location:
    New York
    Well it turns out my furnace man has never run into a wireless thermostat before so he wasnt really sure but offered some suggestions.
    One suggestion was to call honeywell which I will do later today.

    Question for Jim: When you say "I need to to find the other side of the transformer" do you mean the connector on the logic board that closes the C connector loop?
    So one connection goes to the C connector and the other connector which completes the circuit (gleemed from the wiring diagram) plus the red wire and white wire?
    So I would have a four wire run..
  5. brunetmj1

    brunetmj1 New Member

    Messages:
    11
    Location:
    New York
    Ok well I thought I would answer my own question in case anyone comes along with the same question.
    I have a sense that people are not yet familar with wireless thermostats. I found a series of videos on u tube which gives several
    suggestions on how to do it. If you do a u tube search for C wire or similar they should appear.
    There are a number of choices depending on how many wires your thermostat uses. Since greater detail is provided in those videos I am only going to give an overview of what the choices are. The first step is to turn off power at the circuit breaker box and check your furnace logic board to insure the color wires are consistant .
    Anyway you can jumper the green wire (assuming it's fan power), you can run a new wire or use an unused wire with a tie in to the C terminal or you can even use a add on 24 volt transformer. Again i am just listing choices leaving the details to the u tube videos. Since I only have a two wire system I am going to run a new set of thermostat wires using two of them as how I found the current ones (red and white) and use one wire hooked to the C terminal. If this works as the video suggests I will post back.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,896
    Location:
    New England
    I'm in the UK on vacation...haven't been on the internet...so, responses will be slow to non-existent for awhile!

    But, the HVAC system's schematic should show the transformer, and show you where it is available. Essentially, any electrical circuit needs a complete loop for the current to flow. WIth a transformer, that's from one end of the coil to the other (NOT from the input to the output coil windings!) - in this case, the 24vac windings. The LOAD must be between the two ends, as if you short them together, you'd create what is called a short-circuit, and burn things up.
    Last edited: Oct 20, 2012
  7. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
    Cave Creek, Arizona
    haven't read your length answer yet, but the neutral wire for the thermostat had to be connected to the "other" terminal on the transformer, (in a 24 volt transformer system, the terms hot and neutral are relative. Whichever one you use as hot, the other is neutral). The "C" on the panel board probably was not part of the 24 volt side, or was on the same leg as your "hot" wire.
  8. DaMan

    DaMan New Member

    Messages:
    11
    Location:
    Vancouver, Wa

    Your meter might not measure 24 volts. You have to make sure your meter measures down to 24v. Some do, some don't.
    Last edited: Oct 23, 2012
  9. DaMan

    DaMan New Member

    Messages:
    11
    Location:
    Vancouver, Wa
    The third wire connected to C on the logic board is correct. Take that same wire and connect it to the C on the thermostat. Remember, you have a digital programable thermostat. The soul purpose of the C wire is to provide power from the logic board to your thermostat. The C wire takes the place of batteries in the thermostat. Before you run a bunch of wire, hook up the C wire and see if you get any power to your WIFI thermostat.


    If you are not getting power from the C on the logic board, your logic board might need to be replaced. I tried to power my thermostat from the C wire with no results. I replaced my logic board with a new better board and hooked up the C wire and everything worked. I had power to my digital programable thermostat, now I do not need to use batteries.

    Some logic boards just go bad or part of the logic board goes bad. I have seen logic boards do strange things. Sometimes a single resister can break connection causing most of the functions to work, but other functions not work.
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,896
    Location:
    New England
    To power anything, you need two wires from the a/c (power) supply (in this case, the 24vac transformer). They usually do not provide both sides of this when wiring the thermostat. The C wire is one side. This gets connected to heat, cool, and/or fan by the thermostat when required and the other side is connected to them in the HVAC unit, not in the thermostat to complete the circuit...so, to power the thermostat, you often need to either provide it with batteries or find the other side of the transformer and run it to the thermostat. Some older electronic thermostats will momentarily connect one of these loads and power itself from the current running through - not holding the power on long enough to pull in a relay, but many newer HVAC units do not use relays for all power switching, and it will burn the systems control board up. To work with any system, you want to find the other side of the C connection to the transformer and provide continuous power to the thermostat. You want to be in parallel with the transformer's secondary. If you can read the HVAC's schematic, you should be able to find it. Look for the 24vac step-down transformer, look at the 24vac side, find where C is, then look for a place to connect to the other end of the coil and bring it to your thermostat. Then, you'll have clean, uninterrupted power to make the thing work.
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