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  1. #53
    Join Date
    Nov 2006
    Location
    Southeastern Pa
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    19,289
    Quote Originally Posted by Ac_man View Post
    Well I went back to this call earlier today and double checked all my ground connections. i removed one that was connected to the frame of the transformer and placed it with the rest of the ground on the furnace cabinet and it tested fine. I cycled the furnace 3x and everything was fine after that, man I'm happy I'm involved with hvac talk. I will def learn a lot from seasoned techs. Thanks guys
    Please, tell me you took a picture before you removed that wire, so I can see what you did....
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  2. #54
    Join Date
    Nov 2006
    Location
    Southeastern Pa
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    19,289
    Quote Originally Posted by Hvac216 View Post
    We had a goodman tech at our shop today for a training class and he says you can check between 24v com and 120v n. you should get less than 2v with the furnace running. Is there any validity to this?
    What was the reason that was given for that check?

    A properly grounded (bonded) appliance should have the cabinet and the neutral at the same potential, and since the transformer secondary is bonded to the cabinet (24 volt common) you should show no voltage from neutral (the grounded line conductor) to the cabinet. I would expect that value to be close to zero, and not as high a two volts.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  3. #55
    Join Date
    Apr 2012
    Location
    Cleveland
    Posts
    611
    Quote Originally Posted by timebuilder View Post
    What was the reason that was given for that check?

    A properly grounded (bonded) appliance should have the cabinet and the neutral at the same potential, and since the transformer secondary is bonded to the cabinet (24 volt common) you should show no voltage from neutral (the grounded line conductor) to the cabinet. I would expect that value to be close to zero, and not as high a two volts.
    His reason was for board and flame sensor issues. I agree that the closer to zero the better. I was unsure if 2v is acceptable or not. I haven't had a chance to try it out yet. With all the ground and flame sensor talk in this thread it seems like a good place to get some input.

  4. #56
    Join Date
    Aug 2004
    Posts
    41
    Be sure to check the air intake and vent pipes for partial blockage. Also, the reason not to use sand paper is that small particles of sand will get embedded in the sensor and then the flame will cause them to turn to glass and insulate the sensor. I use a 3M scouring pad like you use to clean dishes. This works as well as steel wool but you can actually keep it in your pocket.

  5. #57
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,113
    Quote Originally Posted by timebuilder View Post
    I would expect that value to be close to zero, and not as high a two volts.
    I just personally did this on Ruud furnace about two weeks ago, I got .210 volts, there is a point there "pun intended".
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

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  6. #58
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    19,289
    Quote Originally Posted by Mr Bill View Post
    I just personally did this on Ruud furnace about two weeks ago, I got .210 volts, there is a point there "pun intended".
    This is an opportunity to remind the guys here about grounding and bonding.

    The main panel cabinet is the only place in the system where the neutral is connected to the equipment grounding (bonding) conductors. The two should never have another connection in an appliance, or a sub panel. That is no longer an approved practice. You still see it in old dryers and ranges, with the copper strip used to bond the unit cabinet to the neutral. Now, a four wire scheme is used, with a separate neutral and equipment grounding conductor.

    If you find voltage between the unit cabinet and the neutral, you need to repair the grounding circuit going back to the main.

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