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How do outdoor reset sensors fail?

My W-M Ultra 3 boiler is not providing enough heat this winter.
When I checked the reading of the outdoor sensor it was revealing a temperature about 10F higher than the actual outdoor temperature, so the boiler is operating in the wrong part of the reset curve.

While it is possible to adjust the controller to read a more accurate temperature, I should not have to do this, especially so far as 10F.

The sensor is just a negative temperature coefficient gizmo. As it gets warmer outside, its resistance goes down. Therefore, if I am getting high readings, it is not due to a bad joint in the wiring as that would give lower temperature readings, not higher.

Do these sensors frequently fail? In particular, do they fail by lowering their resistance?

Do I just get a new sensor?


  • Zman
    Zman Member Posts: 7,569
    Although they are pretty reliable, like all things, they do fail.
    You could try to fix the one you have but I think it would be easier to get a new one. :*
    It does not have to be from the boiler manufacture, any sensor with the same resistance will work.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Jean-David Beyer
  • BobC
    BobC Member Posts: 5,477
    Back when i had to use these sensors in military gear I was struck by how fragile the actual sensor is. We countered that by dipping the raw sensor in epoxy but some of them still had reliability problems.

    We solved the problem by using the temp coefficient of a signal diode to monitor temperature - problem solved. I'm surprised the manufacturers have not made a similar move.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • SWEI
    SWEI Member Posts: 7,356
    That's basically what an LM335 does (with some nice electronics wrapped around it.)
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I stuck in a new sensor and the problem is fixed. So I guess it was the sensor. I wonder why it was too low in resistance.

    Unfortunately I do not have a high enough ladder to put it on the north wall of my house, so it is on the east wall. The sensor is in a white box (part of the sensor), and the sun does not hit it in winter until it is a fair bit above the horizon after sunrise, and moves to the south wall by a bit after lunch, so I should be OK. Maybe I can put something there to shade it.

    The sensor was shipped with a short cable. I cannot imagine why. At one end is a tiny connector that probably would not fit the P10 connector on the control board. At the other end is a terminal block so I could connect a pair of wires to it.

    I wonder what it is for.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    It was not OK: Once the sun came up, it heated the sensor about 20F over the outside air temperature, so I made a shield for it: solid on the three sides (and the wall behind, but open at the top and bottom for air circulation. Now it is reading correctly.
  • mogel
    mogel Member Posts: 2
    Can the sensor temp cut off level be adjusted?
  • mogel
    mogel Member Posts: 2
    Weil McClain ultra3 outside temp sensor. Shuts down boiler when it reads 70. Would like to turn it up to 80 due to direct sun light issues
  • lchmb
    lchmb Member Posts: 2,997
    Just put a box over the top of it...leave the bottom open so the air can get into it...