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Black oily substance clogging boiler screen

I'm a home owner searching for answers on why there is a black oily substance that keeps clogging the screen on our
Rinnai bolier model V2532FFU. The system has shut down 4 times with a code of LC ,scale build-up in exchanger. The heating contractor that installed the system 8 years ago has been out twice and has done a one hour and a two hour decalcification and has clean the screen each time finding a black oily substance. There are three Bell Gossett pumps on the system and I have talked with them and they say there is nothing in the pumps that would release a black oily substance. The system was filled with rural water from a lake that has chlorine and fluoride added. I don't want to replace the Rinnai if its not needed or without finding out what is causing this black oily substance.
Would filling the system with glycol help?
Any help will be greatly appreciated.


  • TwoSilos
    TwoSilos Member Posts: 2
    My wife sent me a picture of this substance and its basically a collage of black slivers and small black balls the largest being about an 1/8" in diameter and she says its more like a tar consistency oppose to an oily substance from what I was first told.
    Any ideas?
  • hot_rod
    hot_rod Member Posts: 16,097
    is it connected to radiant tube? Non barrier tube can form iron ferrite in a system. USA;ally a fine black powder, could be oily if there is glycol in the system.

    Put some in a clear plastic cup or glass, let it settle and take a magnet and see if it sticks.

    You may need to power flush, run a cleaner, and fill with good quality water or pre-blended glycol.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • icesailor
    icesailor Member Posts: 7,265
    How did you get a lake with chlorine and fluoride already added?

    Do you have rust stains in your toilet tanks? Look inside when you take a cover off.
  • RobG
    RobG Member Posts: 1,850
    Isn't that a tankless water heater? And are you talking about the little screen on the inlet?