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Reverse Osmosis Water in Solar Hot Water System

Island resorts and commercial laundries spend a fortune for propane to heat laundry water. I am concerned that the DI water will attack the copper in solar collectors. Suggestions have been made on using sodium carbonate / washing soda [affil links]. Is there a system to dispense this in proper amounts? This would be most useful for my applications

Tony Woicekowski
manufacturer - Sacramento, California, USA
February 26, 2008

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I am not sure of what you are asking. You do not want super pure water for your use. 100 Kohm to 1 Mohm water is not going to harm your copper. You do not want any more calcium or sodium cations than you absolutely have to (cost of removing) . You also do not want sulfides and sulphate anions. Carbonates affect pH, so you probably do not want to add any of those.

A reverse osmosis system that you take very good care of is probably the cheapest source of a quality of water that you would like to have. If you do not operate it correctly, it can get expensive. It is not difficult, but you need to follow instructions on operations and maintenance.

James Watts
- Navarre, Florida
February 29, 2008

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Either RO permeate (which usually has a pH below 7) or DI water will slowly attack copper pipe, so it is good that you are concerned. To know for sure what to do, you need a water analysis and some help from a water professional.

There are two indices that can give you some guidance on this, the Agggressive Index or better yet, the Langlier Saturation Index (LSI). The same lab that does the analysis should also be able to calculate the LSI.

If your water is too aggressive, then sodium carbonate may be a good solution to the problem, but without an analysis, there is no way to know ho much to add. It is also common to use food grade calcite in a fiberglass pressure vessel to raise the alkalinity of RO permeate back to a pH of 7-8.5. This also adds back some hardness, but it solves the potential corrosion problem. If you have corrosion, copper will stain white laundry.

Lyle Kirman
consultant - Cleveland Heights, Ohio
March 7, 2008