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T- Stat Caught Fire

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    #46
    Originally posted by SEO View Post
    Did 240 volts get applied to the furnace? That could explain the 46 v reading at the thermostat. (Open grounded conductor ?)
    The heat pump is 240V, so there is no grounded conductor

    Originally posted by Gibbs View Post
    I've seen this before. A thermostat in a children's bed room probably means it is zoned with dampers. Triacs in the thermostat run the dampers. They can over heat and start a fire.
    The thermostat was moved from the hall that was open to the 1st floor. No zone involved.

    Originally posted by MAK View Post
    I don't suppose it could have been a Chinese counterfeit thermostat could it?
    Maybe cheaper components more likely to catch on fire.
    It was a Honeywell-- that's all I know.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky

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      #47
      Dennis, If you ever get to the bottom of this, please do tell

      and just an FYI, a transformer with shorted turns on the secondary will increase the turn ratio and result in lower voltage out, a transformer with shorted turns on the primary will lower the turn ratio and result in a higher voltage out. Almost all class 2 transformers manufactured in the last 20 years or so have a thermo fuse embedded in the windings that should open if the transformer gets hot, and it opens the primary.

      The above is what make this post such a mystery
      Wayne A. From: N.W.Indiana
      Be Fair, Be Safe
      Just don't be fairly safe

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        #48
        Originally posted by hurk27 View Post
        and just an FYI, a transformer with shorted turns on the secondary will increase the turn ratio and result in lower voltage out, a transformer with shorted turns on the primary will lower the turn ratio and result in a higher voltage out.
        Correct, unless the secondary shorts to the primary.
        Originally posted by hurk27 View Post
        Almost all class 2 transformers manufactured in the last 20 years or so have a thermo fuse embedded in the windings that should open if the transformer gets hot, and it opens the primary.
        Common in wall-warts but I have not come across one in a HVAC unit. Generally they are fused externally.

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          #49
          Again, the final info I got, I probably will get nothing else, is that the contactor in the compressor unit somehow shorted the HV to the LV. My guess is a loose connection that melted the contactor and then shorted the wires.
          They say I shot a man named Gray and took his wife to Italy
          She inherited a million bucks and when she died it came to me
          I can't help it if I'm lucky

          Comment


            #50
            thermostat fire.

            Originally posted by Dennis Alwon View Post
            The heat pump is 240V, so there is no grounded conductor


            The thermostat was moved from the hall that was open to the 1st floor. No zone involved.



            It was a Honeywell-- that's all I know.
            The zoned thermostat fire I am talking about, the damper motors don't have limit switches. The power to the damper motor is shut off when the damper blade hits the stop, and the thermostat control senses the motor current spike.

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              #51
              ...the contactor in the compressor unit somehow shorted the HV to the LV. My guess is a loose connection that melted the contactor and then shorted the wires.

              Ok, that makes the most sense. It wasn't the compressor itself but the outdoor condensing unit that they are talking about. I can see the LV coil wiring in contact with the line voltage wiring around the contactor. When it heated up the insulation melted on both.

              Kinda makes you appreciate the restrictions on LV wiring in the same enclosure as line voltage.

              -Hal
              Last edited by hbiss; 01-18-11, 12:51 PM.

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