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Open Expansion Tank - What water treatment chemicals are used?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by DaveBP, Oct 26, 2009.

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  1. DaveBP

    DaveBP Minister of Fire

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    I don't want to hijack my own thread (though I've never hesitated to put my own foot in my own mouth) so I'll pose this related question in a new one.

    The Euro style open 'pressurised' systems must let in some oxygen, more than closed pressurized systems, at least. So, do they required chemical treatment of the water? Maybe similar to open OWBs on this side of the pond?

    I guess this question is really aimed at Hansson. He's the 'star witness' on this topic, but I would be glad to hear from anyone else who has lived with and maintained this sort of system long enough to know what works and what doesn't.

    My guess is something like Garn boilers use would do the trick.

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  2. Gooserider

    Gooserider Mod Emeritus

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    Not an expert, but from what I've heard, O2 is NOT a big problem for open expansion tanks that are properly set up, as there isn't much actual circulation between the water in the tank and the rest of the system, or a lot of airflow in and out of the tank... Given that ALL systems probably need some level of treatment, I'd expect an open ET system to need about the same chemicals that a closed system would, maybe a little more, but not much.

    Gooserider
  3. Hansson

    Hansson Feeling the Heat

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    I dont have any chemicals in my system.I have never heard of one that have chemicals in the system.

    The only thing I heard that they have is some paraffin oil in the top of the expansion tank.
    But I dont think that is ordinary.

    I have a brass pice on the pipe to the expansion tank to avoid galvanic corrosion.
  4. DaveBP

    DaveBP Minister of Fire

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    Thanks, Hansson. Maybe I should call you 'Hotline Hansson' for all the questions you have helped with that almost nobody over here has any experience with.

    This issue is clearly not serious if many systems running for a long time haven't encountered corrosion problems.

    I don't agree, however, with the common notion that there is little circulation between the expansion tank and the rest of the system.

    Between maximum and minimum system temperatures there is a 'tidal' circulation back and forth of the water that expands into the tank and then contracts back into the system. For a system ranging between 100F and 180F that 'current' is over 2% each cycle. For systems with 1000 gallon storage plus boiler volume and piping that's easily 25 gallons each exchange, maybe more than once a day sometimes. And all that water gets circulated and mixed in to the rest of the water each time.

    If the people that have been doing it a long time don't have a problem, I can believe it's just not a problem. But I think it's more that the amount of air that dissolves into the water in the expansion tank just isn't enough to cause a problem, not that the air doesn't get back into the system.

    OK, now I'm up one nit for the day. I love a good mystery, though.
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