Adding C wire for Smart Thermostat

Discussion in 'HVAC Heating & Cooling' started by kyddrivers, Dec 17, 2020.

  1. kyddrivers

    kyddrivers New Member

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    We want replace our Honeywell programmable thermostat with a smart thermostat.

    Bad news: I don’t have a C wire in the current thermostat

    Good news: The blue wire is part of the bundle inside the wall. Just not hooked up.

    Not so bad/good news: The blue wire is in the bundle by the board inside the furnace, but not hooked up to anything.

    Where do I attach the blue wire to the board in the furnace so I have a powered C wire for the new smart thermostat? See the attached picture for the board inside of the furnace.

    Thanks for the help!!
     

    Attached Files:

  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Can you picture the entire board? At least from the labeling, it doesn't look like they made the common easy, but it is likely somewhere on the board. If you still have the installation manual, sometimes that has a functional wiring diagram. If not, tell us what the model and brand of furnace you have.
     
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  4. kyddrivers

    kyddrivers New Member

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    Attached are additional pictures of the board. It is an Amana furnace. Still looking for a model number and any paperwork.

    Would the C wire connect at the same contact as the white line going outside to the ac unit on the purple B/C terminal?

    Thanks!
     

    Attached Files:

    Last edited: Dec 18, 2020
  5. kyddrivers

    kyddrivers New Member

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    See attached for serial and model
     

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  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    For HVAC systems, labeling something 'C' SHOULD be the common. If you have a meter, you should measure the nominal 24vac between R-C.
     
  7. DavidDeBord

    DavidDeBord Member

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    Yes, & I believe that Your unit is at least 20 years old, which can be determined from the data on the furnace data plate, via the Serial Number.
     
  8. DavidDeBord

    DavidDeBord Member

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    My Apologies,... I just saw the post with the Data plate, it appears to have been made July 11, 2010
     
  9. kyddrivers

    kyddrivers New Member

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    Thanks David. That makes more sense, we moved in in 07 and the unit was replaced a couple years later.

    I did connect the blue wire to the B/C terminal and then installed the Nest thermostat. The Nest reports 200 milliamps for the power source.
     
  10. DavidDeBord

    DavidDeBord Member

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    " 200 milliamps"?

    How many volts are there between R & C? You should be getting 24 Volts.
     
  11. DavidDeBord

    DavidDeBord Member

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    And,... If that "Nest"begins to give you to much trouble, I'd recommend the demanding of a Refund.

    I don't sell them, due to many Contractors/Friends that have had a huge amount of Grief over them.

    I've been installing the VisionPro 8000 series, & it took awhile to get the "Programing Bugs" worked out of it.
     
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    milliamps = current (charging voltage, running the thermostat which will be higher when it triggers the display on with backlighting - if the battery was low, it might draw more for a bit, but normally less)
    volts = 'pressure' or amount of 'push' of the electrons

    Two related, but different items. 200ma is likely normal with the display on.

    Like many things, if you don't setup and configure an electronic device, it can create issues. I've not had any issues with mine for about 4-years or so now. The thing can handle numerous different types of heating/cooling systems and it does need to be told what it's expected to be controlling AND installed properly.
     
  13. DavidDeBord

    DavidDeBord Member

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    " 200ma is likely normal with the display on."

    Not so jadnashua,... Not for the HO.
     
  14. JerryR

    JerryR Active Member

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  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The tech specs say that the Nest will use less than 1Kwhr/month. That's an average of about 41ma. There will be times when the display is on, it's talking to the internet, charging the battery, and activating the HVAC system, so 200ma might not be unusual, depending on when you measured it...especially after first installing it, as it would be trying to recharge the battery from a likely lower state, at least initially.

    But regardless, the post was more to reflect the difference between current and voltage. Then, throw in many inexpensive meters aren't all that accurate, either.
     
  16. DavidDeBord

    DavidDeBord Member

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    Well Jad, I appreciate the "to reflect the difference between current and voltage" tutorial, but,... I am a Mechanical Contractor, & have run my own business since 1983. For the HO to be continuously receiving a display of "200 milliamps" is again, Not Normal.

    That display should only show up in the Advanced Settings Section.

    Now, so that kyddrivers can get a great deal of "Help/Advice" I'm recommending that he go & sign up for a membership at HvacTalk.com, (It's membership is the World's largest for HVAC/Refrig, & Electric)which I have been a member of for several years, & there he will have access to more than 150,000 HVAC Professionals, that either uphold the "Nest", or will Hate it with their dying last breath, as well as Mountains of Up to Date Data on that Stat.

    Myself, after seeing all of the complaints about it, at HvacTalk, would only install one for a HO, "IF", they fully understood, & Signed an Agreement, that they would receive only Two Free Service calls as part of their Installation & that the rest of the "anticipated calls", resulting from the "Nest's erratic operation", would have to be paid for.

    Again Sir,.. With all due respect,.... a "200 miliamp Display" for the HO to see is not normal.
     
  17. SShaw

    SShaw Member

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    According to Google, the thermostat will display either 100 mA or 200 mA under the "Settings" menu when a C wire is connected to the thermostat. See this page for the details:

    https://support.google.com/googlenest/answer/9241211?hl=en

    So, the display of 200 mA is normal, but it is buried under a settings display in the thermostat or the App for the thermostat, as you suggested it should be.

    I think what jadnashua meant was that the measured power consumption of the Nest would be higher when the display was lit up, which is typical for low power electronic devices.
     
  18. DavidDeBord

    DavidDeBord Member

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    The Key word is "Settings", which is not the "Normal" Screen display.

    With the "C-wire" employed in the Stat, per that document:

    "If there is a C wire in your thermostat:

    Voc: 29 to 42V
    Vin: 29 to 42V
    Iin: 100 or 200 mA"

    Which is "Normal", again per the document:

    "Troubleshooting power issues

    If the above power specs are outside of their normal range, you may need to troubleshoot. For instance, if your Nest thermostat won’t turn on and is unresponsive, or you see a blinking red light at the top of the thermostat’s screen, your thermostat likely isn’t getting enough power to charge its battery. For troubleshooting, please see the following article:

    Troubleshooting when your thermostat won’t turn on"

    And I also want to point out,... 24Volts operating voltage is the "Normal Standard" with a 10% +/- being acceptable, which with the "Nest" their acceptable voltage supply is "29 to 42V", which can cause problems, & it drops to 6 to 7 volts, which tells me it is what could be called, a"Black Hole for power". I've tested many, many pieces of Equipment,during operation & sitting idle, & I've never seen a voltage drop that large, with the equipment properly running.

    You should see the conversations at HvacTalk, about this T-stat.

    I wish You Luck KYD.
     
  19. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The Nest thermostat can control numerous types of systems. When you get beyond a simple one, it can take a bit of experience and knowledge to make it work properly both in wiring and setup. Some don't want to invest the time to make it work properly and not every system is suitable for use with one.

    The thermostat uses some tricks to power itself when you cannot supply it with a complete circuit (the common lead). That may work on some systems. For reliable, full functionality, you really want the C connection filled, otherwise, the internal battery may not be able to maintain enough charge to enable all of its functionality (like WiFi, and motion sensing display activation). Without the C wire, it can only charge its battery when power is flowing through it to turn something on (like to the fan, heat, or a/c). I think one reason why it may want a transformer output a bit higher is that when it is powering something (fan, heat, a/c), it may divert some of that power into charging the internal battery, and still needs to pass enough voltage and current to activate the desired function in the HVAC system.

    ANyway, mine has been working fine for about 5-years now. It's nice to be able to control things over the internet and monitor what's going on. I used it last year while I was in the hospital and detected that the boiler had malfunctioned. I use it when on vacation and returning to tell it to start to condition the place so it's comfortable when I get home. I was able to have a neighbor do a reset on it and restore functionality while I was in the hospital (had a stroke and crashed my gyroplane). Since it was the middle of January, it kept things from freezing up. Without the WiFi, I never would have known. There are other WiFi enabled thermostats, and they each have their own good and bad points.
     
  20. JerryR

    JerryR Active Member

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    Jim, you’re a lucky man twice. Surviving the stroke and surviving the crash.

    I've had WiFi thermostats for over 10 years in 4 homes so far. When traveling between 2 homes I had a geofence set up so the thermostat would turn on the ac when I was 50 miles from home. Even now, I use my iPad/iPhone to adjust the thermostats. My main house has 3 zones with 3 thermostats and 6 additional remote sensors. I can’t imagine having manual thermostats any more.
     
  21. TKB4

    TKB4 New Member

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    I did the same thing
     
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