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Water Maintenance in Unpressurized Storage?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Rory, Oct 22, 2013.

  1. Rory

    Rory Member

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    I read on another thread that you must be vigilant regarding the water in an unpressurized system. Are you talking about the water in the tank, or the system water? If the former, it seems curious, as the dealer I bought my Tarm from recommended sealing the tank. Mine is not sealed, and I have a cover on a pulley for easy access. I added baking soda in the beginning, but I'm in my 5th or 6th season without doing anything more than adding water occasionally.

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  2. peakbagger

    peakbagger Minister of Fire

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    If you have an unpressurized system, you have to have it unsealed as the water expands and contracts when it is heated or cooled. Most tanks have a vent on the top cover. No need to treat the water as its gets sterilized whenever you heat it up and during the summer, there is no real "food" for bacteria to move into the tank. When I drained mine after a year or so the water was clearer than when I put it in.
  3. tom in maine

    tom in maine Minister of Fire

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    I believe folks with EPDM liners are adding baking soda to manage the pH. I suspect it helps with longevity, although I have not seen any research to back that up.
    Our tanks, with high temp PVC liners do not use additives. In a worst case, degradation is the liner gets brittle. That is not an issue unless you move the tank. I have not seen any embrittled PVC liners to date.

    Not sealing a cover has the potential to add unwanted humidity in the house. I have only had two people in over 30 years who have sealed tanks too tight. Those tanks had an issue when the homeowner boiled the tanks and popped the covers.
    These were very unusual situations.
    My sense is that it is imprudent to not seal the cover. An airspace of 4-6" between the top and water level is adequate for thermal expansion.
  4. jebatty

    jebatty Minister of Fire

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    There are several important factors, two of which are oxygen in the water and pH. Oxygen + iron/steel = rust. If the unsealed tank results in admission of O2, then rust will occur. Same reason you use oxygen barrier pex. You might consider an oxygen scavenger chemical to add to the system.

    There is lots of research and info on pH. One thing you do not want is acidic water (pH < 7.0). Acidic water is very corrosive. I have seen different mfrs of boilers specify pH as low as 7.0 and all the way up to about 11.5 (base).

    My personal experience informs me further on pH. My first system was open and I did not use an oxygen scavenger nor did I test the water pH. The amount of rust that occurred in the system was tremendous. I was alarmed by this, tested the pH and found it to be 6.5 from the well, still OK for drinking water. I refilled the system and added a strong base and brought the pH up to somewhere between 8-9. I also added sodium sulfite as an oxygen scavenger. The water went in clear and stayed clear. The base I used was sodium hydroxide (lye) which is quite dangerous if not handled properly. Use at your own risk and research carefully the proper way to use it you choose that route.

    Back in 2008 I did some research and came up with the following (I didn't make a note on the source):
    * Sodium sulfite - used in water treatment as an oxygen scavenger agent, pH 9
    * Sodium carbonate (soda ash) - a relatively strong base in various settings; used as a pH regulator to maintain stable alkaline conditions; common additive to neutralize acid effects and raise pH
    * Trisodium phosphate - a highly water-soluble ionic salt; dissolved in water has an alkaline pH; used as an acidity regulator (buffering agent).
    * Sodium silica - soluble in water, producing an alkaline solution; binds to heavier molecules and precipitates them out of the water.

    Swimming pool test kits are pretty good to give you a water analysis, pH and total alkalinity.
  5. tom in maine

    tom in maine Minister of Fire

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    I'd better add to my comments. In our tanks, which are lined with plastic and have nothing to corrode, the water chemistry is almost always not an issue. If you are using an unpressurized metal tank or a tank with ferrous metals, testing is prudent.
    It is never bad or imprudent to check the pH.
    As I said, we almost never see an issue with our tanks when used with our heat exchangers and as such do not see much necessity for water conditioning for potable water which is used to fill our tanks.
  6. Rory

    Rory Member

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    I have an EPDM liner and a whole bunch of copper in the tank, but that's the only metal outside of whatever's in the solder. Revision Heat did recommend greasing any copper above the water line, but the slacker in me skipped that step.

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