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Hot Water Heater: Scale build-up at inlet/outlet of all plastic tank

<span style="font-size:12pt">I am looking for a permanent solution to a recurring problem. The junctions to my two 85 gallon electric Rheem Marathon Hot Water heaters routinely develop a thick scale build-up which reduces the internal diameter and restricts the volume of flow. In the five years these have been in-service, it has been necessary to disassemble and clear the sludge every 18 months. I’ve been here for twenty years using conventional hot water heaters with no experience of scale. I also currently have a five year old steel 30 gallon heater servicing a kitchen in the same unit – this one remains clear too.</span>

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<span style="font-size:12pt">So what is causing this scale at the Marathon heaters? The two Marathon MR85245B feature ALL PLASTIC tanks; they (and the steel kitchen tank) are installed on the twelfth (top) floor of a high-rise residence, the plastic tanks were chosen specifically for their resistance to leaking (an occasional problem in the building). All plumbing into and out of the tanks is copper. The plastic tanks are installed in tandem (they feed a giant Jacuzzi, among other things). The copper plumbing is joined to the brass tank extensions by dielectric fittings. Build-up is at/around the dielectrics/extensions.</span>

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<span style="font-size:12pt">Note - the installation manual for the heaters does NOT call out dielectric adaptors to separate brass extensions from copper – this was determined by the installing plumber. Also of note…the external plastic housing of the tanks occasionally demonstrate a static charge. I had attempted to close the electric circuit by crudely placing some conductive metal across the cold and hot copper pipes – but perhaps not in the correct location? Water source: City of White Plains (here is a link to their most recent analysis: <a href="http://www.cityofwhiteplains.com/dataimages/annual_water_quality_report.pdf">[u][color=#800080]http://www.cityofwhiteplains.com/dataimages/annual_water_quality_report.pdf[/color][/u]</a> )</span>

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<span style="font-size:12pt">What causes the scale buildup? Recommendations for a permanent fix (short of ripping out the heaters)? I look forward to your posts.</span>


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Get rid if the dielectric unions....

    THEY are the source of more problems than they avoid.

    In the case of a plastic tank, there is absolutely NO protection being provided. The unions themsleve set up their own electrolysis process and generate teh corrosion that you are seeing.

    The code requires a means of disconnect (union) at every vessel that will eventually require replacement, and personally, I do NOT put them in, because the weekend warriors view them as being the equivalent plumbing velcro. It's just ASKING for trouble. Mismatched tanks set on bricks to make up the difference, etc. In reality, they do NOT reduce replacement time, because they are usually so corroded that they too end up being replaced.

    Believe it or not, the group that got this provision pushed through and into the code was the Apartment Owners Association. They wanted the ability to set their less than skilled maintenance people up so that they could replace failed water heaters, etc.

    Personally, I think that if you can't solder, you shouldn't be messing with pipes. :-)

    As for the static electric discharges, it is going to be pretty tough to bond anything electrically conductive to the tank to discharge the static buildup. I think the factory should be made aware of this potential though. Not that it would really cause their plastic tank to fail, but getting zapped stepping into the shower cold start an arythmia of the heart...

    If you MUST put unions on it, go with brass unions. THey are neutral and benign to most metals typically used in potable water circuits.


    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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