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  1. #1
    Join Date
    Nov 2009
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    9

    Thermopile mv reading: Can it be too high?

    Recently I replaced the thermopile on a gas log fireplace(White Rogers Valve). The old TP was reading about 140 with only the pilot running. So, replacement seemed like it would do the trick. I replaced it, and fireplace ran fine. Just for kicks I decided to test the mv on the new TP. I put my meter on and it was reading is about 950mv, almost 1 full volt... I adjusted the pilot as low as it could go (the pilot screw didn't seem to affect the pilot flame at all) and got it down to 920mv. The pilot didn't seem overly large anyway.

    After talking to the customer she had the TP replaced a year ago and it had already gone bad. Does this excess of voltage tax the TP to the point where it will deteriorate every year?? Can you have too much voltage out of a TP??

  2. #2
    Join Date
    May 2008
    Location
    Wichita, KS
    Posts
    41
    Most RS are rated at 250-750 mv. I don't know that I've ever seen one read that high.

    If you waited at least 5 minutes before taking reading 2 and if the pilot flame is observed to be correct, you must have purchased "the fabled golden tp generator"!

    It won't hurt the magnet or that circuit as long as you don't get line voltage. If you get line voltage off that tp gen, you might have discovered the first step of solving the energy crisis. If that's the case, email me privately off-line... ;-)

    Bart Ogden

  3. #3
    Join Date
    Jul 2006
    Location
    Virginia
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    392
    Voltage output is related to temperature. If you are getting over 900 mv then overheating may be the reason it is failing so quickly. Is this unit burning LP gas? If it is, there may be a natural gas orfice in the pilot assembly causing the TP to overheat and fail.

  4. #4
    Join Date
    May 2008
    Location
    Wichita, KS
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    He said the pilot looked correct, though.

    If it's got a NG orifice in there, it's going to be a LARGE pilot flame and easily distinguishable.

    The pilot flame should envelope the top 1/3 of the tp gen.

    SOME units have had issues with tc's burning out and they've gone to an industrial jacketed tc instead of the factory-installed. I haven't heard of any tp gens with those issues if the pilot is a lp orifice for lp and it's properly adjusted, other than an occasional bad electrical part. And even that's rare anymore.

    Bart Ogden

    Quote Originally Posted by logdoc_rob View Post
    Voltage output is related to temperature. If you are getting over 900 mv then overheating may be the reason it is failing so quickly. Is this unit burning LP gas? If it is, there may be a natural gas orfice in the pilot assembly causing the TP to overheat and fail.

  5. #5
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,175

    Cool interesting case

    First, check mv output with another meter to be sure of reading. Check your parts source. There are TPs that can put out >900mv http://scholar.lib.vt.edu/theses/ava...cted/chap2.pdf


    We don't want to get into too much detail on testing: see site rules.

    Hearthman

  6. #6
    Join Date
    May 2008
    Location
    Wichita, KS
    Posts
    41
    I'm certainly not disputing that there might be tp gens that can produce higher voltages, but we're discussing fireplaces here and a typical 1950-001 which is designed to produce 250-750 mv.

    I do agree with your suggestion to check the mv output with another meter, but how many techs truly carry a spare meter on their truck? I do carry a little cheap radio shack flip case mm, but only for an emergency if my Greenlee mm would get damaged while out on a service route.

    Bart



    Quote Originally Posted by hearthman View Post
    First, check mv output with another meter to be sure of reading. Check your parts source. There are TPs that can put out >900mv http://scholar.lib.vt.edu/theses/ava...cted/chap2.pdf


    We don't want to get into too much detail on testing: see site rules.

    Hearthman
    Use screen name when referencing another member. Not all want their private info in a public forum.
    Last edited by Senior Tech; 11-15-2009 at 03:07 PM.

  7. #7
    Join Date
    Nov 2008
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    1,673
    Quote Originally Posted by jwgagnon View Post
    I put my meter on and it was reading is about 950mv, almost 1 full volt.
    Did you measure the output while it was open-circuited or while it was connected to the gas valve?

  8. #8
    Join Date
    Feb 2004
    Posts
    2,458
    Quote Originally Posted by WhoIsThat? View Post
    Did you measure the output while it was open-circuited or while it was connected to the gas valve?
    Both & calling for heat

  9. #9
    Join Date
    Nov 2008
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    1,673
    Quote Originally Posted by mike3 View Post
    Both & calling for heat
    There should have been a slight drop in open circuit voltage when connected to the gas valve.

  10. #10
    Join Date
    Nov 2009
    Posts
    9
    The pilot oriface is setup correctly, the house has NG and it is setup for NG. I guess the flame looked a bit on the large side, but not horribly large. The pilot adj screw did absolutely nothing anyway.

    I tested when it was connected to the valve. I wouldn't think this would cause an increase in mv but i could try testing with it unhooked.

    The thermopile I used was one we use on most all our gas fireplaces, mostly staying around 600mv max.

    Not sure if this matters, but the valve location was mounted in the basment and the TP wire was about 5 ft long to reach the terminals on the valve.

    In retrospect, I kind of wonder if the valve has some issues. The pilot screw did nothing to the pilot when i adjusted it. Also, while unit was running, the flame was abnormally large for a gas log. (even with the burner covered with sand and embers). I ended up turning the gas shutoff 1/4 shut to make the flame appear more normal.

    I'm going back to the house later this week with another multimeter and a pressure checker for the valve. I'll let you guys know what I find out. Thanks for all the imput!!!

  11. #11
    Join Date
    Nov 2008
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    1,673
    Quote Originally Posted by jwgagnon View Post
    I'm going back to the house later this week with another multimeter and a pressure checker for the valve. I'll let you guys know what I find out. Thanks for all the imput!!!
    Measure the current into the valve and the valve resistance.

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