South Carolina Corrosive conditions in closed loop geothermal

Discussion in 'Maintenance and Troubleshooting' started by Jim O., Oct 29, 2021.

  1. Jim O.

    Jim O. New Member

    Lost the third flow center (2013, 2017, 2021) on 5 ton Trane/WaterFurnace system due to erosion or corrosive conditions in closed loop system. Pumps developed leaks from pinholes. System was flushed the last two times with 1-micron filter sock in flush cart, water had red color and filter sock yielded what looked like rust particles.
    Had water analyzed this last time and the “standout” was a Langelier index (LSI) of minus 2.28 which we are told indicates very corrosive tendencies to cast iron (pump housing). Hard to understand as each time we flushed the system with potable water - no water treatment being used however. The ph was 6.68.
    Now the plan is to replace the water in the system with trucked in “pure water”, whatever that is.
    Anyone had similar experience?
    How would you go about removing the water in the system now?
  2. Jim O.

    Jim O. New Member

    Read some internet comments on dissolved oxygen in the water causing corrosion. Could that come from the potable water used to fill the closed loop wells?
  3. ChrisJ

    ChrisJ Active Member Forum Leader

    I am told my 6.8 ph water is acidic.

    do you add any antifreeze?

    maybe use a bronze pump instead of cast iron.
  4. SShaw

    SShaw Active Member Forum Leader

    Waterfurnace requires a pH range of 7-9. My well water was ~6.7 pH so I had city water brought in at the recommendation of Waterfurnace. Since geo loops are sometimes installed in new homes prior to the utilities being connected, it's not uncommon to bring in water.

    I'd be concerned that the next failure could be the coaxial heat exchanger in the heat pump, as acidic water can dissolve copper.

    You might be able to add a boiler "inhibitor" solution, as some of these contain additives to stop corrosion and neutralize the pH.
  5. dharuch

    dharuch New Member

    Are your pumps on the 2nd floor?
    If so, as Eric pointed out - have you considered pump cavitation rather than corrosion as the cause of the pump volute erosion you are seeing?
  6. gsmith22

    gsmith22 Active Member Forum Leader

    water chemistry can be difficult to understand but a few points to consider: don't confuse low pH and corrosivity. Certainly, a low enough pH is an acid and will dissolve metals but a pH a little below 7 can be fine (or not) depending on other items dissolved (or not dissolved) in the water. Which is what the LSI is attempting to figure out using a combination of pH, temperature, and other dissolved solid measures to determine how corrosive the water will behave regardless of its pH. Water is the great dissolver so a lack of dissolved solids can be just as bad as low pH. Its certainly possible to have very corrosive water with a pH above 7 or noncorrosive water (LSI would call it scaling) with a pH below 7. An LSI of -2.28 is insanely low and can't be explained by just a slightly low pH of 6.68 so there must be little to no other dissolved minerals in the water in combination with the slightly low pH. Where is the water coming from that you are putting into the loops - well or some other source? Did they fill the pipes with deionized water (which would be terrible)? You need to purge and flush the ground loop and refill with a different source than prior. Municipal water is probably the best. I think my company hooked their water truck up to a fire hydrant for initial pipe filling.

    As an aside, LSI is used to gauge quality of swimming pool water and you generally try to stay between -0.6 and 0.6. At -0.6 or lower, there is genuine concern of the water stripping components of the plaster/concrete from the pool and certainly corrosion of any metals. Any metal that -2.38 LSI water has touched would now be suspect including the internals of the heat pumps. Your flow center issues strike me as a canary in the coal mine unfortunately.
  7. SShaw

    SShaw Active Member Forum Leader

    I concur with gsmith22.

    While my pH of 6.7 was outside the recommended range of 7-9, I believe Waterfurnace recommended bringing in water for the loop (and recommended treating the household water to protect the DSH heat exchanger) based on the total water analysis, not just the pH.

    In addition to my pH being 6.7, my well water is "pure," which makes it corrosive.

    My water analysis was: pH 6.7, alkalinity 16, iron 0, hardness 2, mang .04, t.d.s. 41

    Based on that, I calculated the LSI as -2.7.
  8. Jim O.

    Jim O. New Member

    Sorry for not responding sooner - did not get notice on REPLIES - we are in SC so antifreeze not required although the anti-corrosion properties could help - do agree that a bronze pump would likely help, would have to order ahead and hold til next failure - after receiving the water analysis contractor did drain system water and replaced with potable water, also added product GEO-LOOP CONDITIONER which supposedly has anti-corrosive properties
  9. Jim O.

    Jim O. New Member

    Pumps are in crawl space on same level as Geo unit - curious about cavitation as system pressure running at 25 psi and seems like such low “boost” across the pumps would not have enough energy to damage the metal?
  10. Jim O.

    Jim O. New Member

  11. Jim O.

    Jim O. New Member

    Thanks for a very informed reply - system was filled with potable tap water. We do question the very low LSI - the water sample sent to the lab was in a plastic bottle washed and rinsed with distilled water - a “rust appearing sediment” formed in the bottle after sitting overnite and we held the sample several days before sending to the lab - this may have produced erroneously low LSI number?
    Any comments on contractor’s use of the GEO-LOOP CONDITIONER product intended to provide some protection to the pump internals?

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