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Home furnace different leg voltage

  1. Oct 24, 2016 #1
    Ok im having a problem with my furnace in my home so I called a friend that is a certified HVAC technician to come over and help you work on it so upon doing some tests we come to find out that one leg of the 220 had 220 volts and the other leg had 10 what could be causing that
     
  2. jcsd
  3. Oct 25, 2016 #2

    Averagesupernova

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    Best guess without knowing more specifics would be a weak or open neutral.
     
  4. Oct 25, 2016 #3

    jim hardy

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    It is important to know where the voltmeter probes were connected.

    Was its black wire hooked to the neutral wire , to metal frame of the furnace, ????

    Is this a US or European installation ?

    What did your HVAC technician friend say about it ?
     
  5. Oct 25, 2016 #4
    He had it grounded to the metal on the box and was touching the wire directly
     
  6. Oct 25, 2016 #5
    Sounds like this could be dangerous, assuming this is North American 220, which is 110-neutral-110. If you get 220 from one of lines to the chassis, that could indicate the chassis is sitting near 110 volts of the other leg. But also, voltmeters are very high impedance, a little leakage current could provide a high voltage reading.

    But, the chassis should be grounded, so very near the neutral in potential. Each leg should measure 110V to neutral, and 110 V to the chassis.

    It is sometimes helpful to repeat that with a 110V filament bulb (25W, 40 W 60W, no matter) across the meter leads to provide a load.

    I had an electric stove top with some burnt food that bridged from one hot line to the chassis, and it turned out the chassis was not grounded. We were getting shocks from the chassis until I investigated, grounded the stove, and when I turned the breaker back on - POW! - the burnt food blew out from the rush of current.

    No more shocks though.
     
  7. Oct 26, 2016 #6

    jim hardy

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    That's scary.
    Ranges have an exemption in the code regarding grounding, they are allowed to use the neutral wire for ground. If some amateur brought only two "hot" wires to your cooktop , well, shame on him.
    Glad that you found and fixed it.

    You're right about his voltage readings. If it's a US type hookup what's worrisome is the 220 one not the 10.
     
  8. Oct 26, 2016 #7
    The stove definitely was supposed to be grounded. I know this forum has members worldwide, so I'll repeat that this is North America, and 220V to a stove/oven is two opposite phase 110 lines. The stove had a 110V outlet for convenience, and standard 110V lights, so it also used the neutral to provide stable 110V sources (something like an electric water heater may just use 220V, no neutral required). I don't know what code or common practice is today, I would hope they add a ground leg and neutral to the plug and outlet (4 pins), but this stove required a separate green ground wire from chassis to ground (we have conduit here in Chicago-land, and the conduit is used for ground). So whoever installed the stove, just never installed the ground wire. There were tags on the chassis instructing to add the ground wire.

    The only time we noticed it, is if one of us made a rather long reach and touched the metal faucet at the same time we contacted something metal on the stove. You would get a tingle. Yes, it could have been fatal under the right (wrong?) conditions. I get scared when I hear about people dealing with house wiring that don't understand how to deal with neutral and ground (even if they don't understand the 'why', they should understand the 'what' to do).

    The OP should get that furnace fixed. It should be pretty easy, like adding a ground wire was on my stove, and someone touching that furnace and a water pipe or something might be alone when it happens - could be fatal.
     
  9. Oct 26, 2016 #8

    Averagesupernova

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    Not anymore. No new construction in the US allows sharing of the ground and neutral except prior to the main service panel. Range and clothes dryer outlets are all 4 prong. Mobile homes have been like this for many many years.
     
  10. Oct 26, 2016 #9

    jim hardy

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    I knew new dryers are 4 wire
    but is there still an exemption for installing them in an older house ? Last one i bought had instructions how to use both a 4 wire cord for new construction and a 3 wire cord for existing buildings.

    Glad to see things moving slowly in direction of "better". As an old guy i generally resent change,
    but as a retired nuke guy i believe in doing things right.
    Exactly right .
    Then double check.

    old jim
     
  11. Oct 27, 2016 #10

    Averagesupernova

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    You can purchase new appliances and install the old 3 wire cord. In other words they cannot make you rewire your house simply because you wanted a new dryer. The appliance is not considered permanent wiring so the NEC has no jurisdiction over it.
     
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