Can Water Be too Soft? Our Experts Discuss the Issues with Having Zero Hardness

Donald Taylor, CPD, GPD, a Senior Plumbing/FP Designer with CRB in Dallas, is working on a project that has 16 to 20 grains per gallon of water hardness, so he is providing a water softener. The vendor asked what level of hardness Don wanted to achieve, so he’s wondering if there is an issue with having zero hardness.

The general consensus of our experts on ASPE Connect was yes, water can be too soft, but as with most other plumbing design considerations, it depends on the application and the piping materials.

First of all, though, you need to have an understanding of soft water.

What Is Soft Water?

It is important to distinguish between soft water and softened water. Naturally soft water, reverse osmosis (RO), or deionized (DI) water is hungry for minerals, the universal solvent. Softened water, where ion exchange has replaced hardness mineral ions with ions that are more soluble, won’t be aggressively corrosive.

Soft water is not the same as pure water. The TDS (total dissolved solids) of zero soft water is about the same as the original (unsoftened) supply.

Material Matters

Some metals readily react with water, so pure water can be corrosive to certain systems. That typically applies to copper and lead in plumbing systems, which will leach into the water if there isn’t already a certain level of dissolved metals. Copper also needs some scale formation to prevent erosion; otherwise, pinholes will develop.

If the system is almost entirely PEX, CPVC, or stainless steel, then corrosion is minimal for these materials. This should be confirmed with the material data sheets or the manufacturer of the pipe being specified. Keep in mind the materials of valves and other fittings as well.

What Is the Recommended Value?

Taking these considerations into mind, the consensus was that 10 ppm would be appropriate for most applications.

For more information on softener sizing to achieve the desired water hardness, visit the discussion thread on ASPE Connect. Also, consult Chapter 11: Water Treatment, Conditioning, and Purification of Plumbing Engineering Design Handbook, Volume 2 and Chapter 10: Water Treatment of Plumbing Engineering Design Handbook, Volume 4.

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