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  1. #1
    Join Date
    Nov 2008
    Northern California, foothills.
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    Millivolt-compatible thermostats for wall units using pilot generators.

    I searched the forum for this topic and didn't get any real hits.

    So a supervisor I had at a previous job, a guy that learned everything
    on the job, he told me that you can use digital thermostats that don't
    explicitly say on the box "Millivolt-compatible", and they will work for
    at least a while, but then flakey behavior eventually turns up.

    I called Honeywell tech support today to check on whether a certain
    model of thermostat was millivolt-compatible. His words were,
    "That model CAN be used with millivolt systems." That didn't sound
    too confidence-inspiring to me.

    This was on a wall unit that used a pilot generator. Loaded voltage,
    pilot and main burner-was about 500 millivolts. And still the
    thermostat had consistency issues. It worked most of the time,
    but then failed in my presence. I quickly jumped the leads at the
    gas valve and the burner lit right up.

    My conclusion is that the thermostat is not just bad, but that
    it is the WRONG thermostat. My boss is going to want some
    supporting argument/evidence of the WRONG part as I know
    one of our senior techs installed it about a year ago.

    Looking through catalogs for thermostats, I of course see that
    some explicitly say "millivolt-compatible", but many popular
    digital ones do not.

    What is the verdict? And can anybody say WHY most digital thermostats
    are not by nature millivolt-compatible (if they aren't)?

    Thank you.

  2. #2
    Join Date
    May 2012
    Upper Michigan
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    500 is a tad on the low side, 600 to 750 is usually normal. Wire size also is a factor. I can't answer why some digitals may work but I know it has to be the right stat, I would check the other stuff also.

  3. #3
    Join Date
    Jun 2003
    Chicago, IL
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    For a digital stat to be Millivolt compatible it must have two characteristics

    It must be entirely battery powered (not power stealing)

    It must have relay switching (as opposed to transistors)

    There are many basic and premium stats made by the major manufactures that explicitly state Millivolt Compatible on the box. I would play it safe and stay with one of them.

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