Softeners typically run in the background and are easy to forget about. When softener performance is critical, such as when a softener protects an industrial boiler, operators are typically more vigilant of hardness making its way downstream. Hardness concentrations of 1 mg/L (1 ppm) or less can cause scaling in some conditions.

If you’re looking to track down the root cause of hardness leakage at the outlet of your softener system, read on.

3 Types of Issues That Cause Hardness Leakage

Barring feed water issues, hardness leakage in softeners is usually caused by at least one of the following:

1.     Regeneration cycle issues

2.     Flow issues

3.     Resin issues

Regeneration Cycle Issues

Since the softener resin needs to be exposed to a concentrated brine for a specific amount of time in order to be properly regenerated, any issues affecting either brine concentration or purity, flow or the duration of brine exposure will have an impact on the regeneration’s effectiveness.

In order to confirm an effective regeneration cycle, technicians will perform a “brine curve”, measuring salinity at the drain regularly throughout the brine draw and slow rinse cycles. The salinity is then plotted against time and the shape and height of the curve is used to confirm that the regeneration cycle is effective or to identify potential issues.

How to do a brine curve

Here's the procedure on how to do a brine curve on your water softener!


As you might imagine, there are a number of ways that things can go wrong during the regeneration cycle. If you need a refresher on how softeners work before diagnosing your softener’s issue, check out our article on basic softener theory.

Issues That Disrupt Regeneration Cycles

The following are potential root causes of a disrupted regeneration cycle:

·       Too little salt in the brine tank

·       Obstructed or malfunctioning brine valve

·       Obstruction in the brine tubing or piping

·       Obstructed injector or injector screen (in vacuum-based systems)

·       Leaks in the brine line (especially in systems that use vacuum to draw brine)

·       Too little time between regenerations (especially in systems with a shared brine tank with a brine plate)

·       Too little or too much inlet pressure / flow

·       Brine tank too far from suction or brine line too small for the brine flow (too much resistance in brine line)

·       Disconnected or malfunctioning outlet flow meter (regenerations too infrequent)

·       Incorrect programming or malfunctioning controllers (cycle times or regeneration frequency incorrect)

·       Obstructed drain line flow controller (low flow to drain)

·       Control valve damage, obstruction or defect

Flow Issues

If a softener is still leaking hardness once it’s been confirmed that a softener is regenerating effectively and frequently enough, it’s time to consider whether flow issues may be at the root of the problem.


One of the most common reasons for hardness leakage is that untreated water is somehow bypassing the normal flow path and making its way to the softener outlet. Depending on the type of softener system, a bypass can occur in a number of ways:

·       Bypass at the top of the riser due to worn or missing o-rings or riser mis-alignment

·       Bypass through a bypass valve that was left open or does not close properly

·       Bypass within a multi-port valve due to broken flappers or seals

Most maintenance staff can easily diagnose the first two issues if they know what they’re looking for. The third one requires more hands-on knowledge with multi-port valves and is usually best checked by a service technician.

Too Much Flow

As mentioned above, resin needs enough contact time with the water flowing through the softener in order to properly exchange hardness from the inlet water against sodium stored on the resin beads.

Resin manufacturers typically provide target flow rates for softener operation. Our trusted partners at Purolite recommend that softeners be operated with a service flow rate between 1 GPM to 5 GPM per cubic foot of resin and 4 GPM to 20GPM per square foot of cross-sectional surface area. This translates to a flow of 8 to 40 bed volumes per hour and a linear velocity of 10 to 50 m/h. See the engineering guide from Purolite here for more details.

If you’re seeing hardness leakage and your flow rates are above the limit for flow or velocity, there is a good chance that reducing the flow or adding more softening capacity will help solve the issue.

Too Little Flow

Sometimes softener systems were designed and installed to meet a much higher demand than is currently the case. Other times, water demands are simply too variable and sometimes dip below the minimum flow or velocity recommendations.

Reducing water demand is a great thing even if it means your softener could be oversized!

At particularly low flow rates there is a high likelihood that the water flowing through the softener won’t be flowing through the entire diameter of the vessel. The flowing water will take the path of least resistance and, at low flow rates, the differences in resistance can be significant enough to cause channeling.

When channeling occurs, some of the resin in the softener isn’t coming into contact with water flowing through the vessel. This resin therefore remains unused and so it can’t offer up its softening capacity. The resin inside the softener becomes unevenly exhausted and the theoretical capacity of the unit can’t be reached before hardness leakage becomes an issue.

In this situation, you can either accept that they should expect a lower capacity from their system or, if it makes economic sense replace the system with one that is designed to suit your specific flow rates. You may simply need a smaller system or a system with a number of smaller tanks that come into service or go into standby progressively as flow increases and decreases. If your system has multiple vessels operating in parallel, you may choose to reduce the number of vessels operating together to achieve better flow dynamics. A less common but often viable retrofit option is to use a recirculation pump to maintain appropriate flow rates.


As water sits idle inside a softener the hardness that was captured by the resin beads slowly seeps back out into the surrounding water. This happens as the concentration of specific ions in the water and the resin beads seeks to return to equilibrium.

The result of this phenomenon is that the initial “slug” of water at the outlet of a softener that has been idle for a long time will contain more hardness than what you’d expect during normal operation. In cases where demand for water is only occasional, this can be an issue, especially if hardness is a critical process factor (as it is for boiler feed water).

Simple solutions exist here: an automated system can send water to drain for the first few minutes of demand, the softener can be automatically flushed to drain periodically, or water can be recirculated in a closed loop around the softener to reduce the effect of equilibrium leakage.

Resin Issues

When we get called in to troubleshoot an industrial softener, many clients ask us to test their resin to see if it needs to be replaced. This is because, as resin is used over time, it can degrade for a number of reasons:

·       It can become fouled with metals like iron, reducing its operating capacity

·       It can become oxidized, reducing its operating capacity by breaking down its active sites and its physical integrity

·       It can become physically damaged through physical collisions and the repeated shrink-swell cycles that are part of normal softener operation

Resin beads fouled with iron

A good resin analysis will give clear answers as to whether the resin needs to be cleaned to remove metal fouling or if it has reached the end of its useful life. That being said, resin typically has a long service life. We’ve seen softeners operate well without a resin change for over 25 years, although they are far from the norm. For that reason, we typically leave resin analyses as a last option to check after going through a thorough mechanical and functional investigation of the potential root causes of hardness leakage. We DO recommend, however, that you send resin samples in for analysis every two years after the first 5 years of operation. This is best done when maintenance personnel are already opening a softener for inspection and preventive maintenance. Typical resin life expectancy is about 300 service/regeneration cycles.

One last “resin issue” that we often see in industrial applications, is that a good portion of the resin has been simply flushed down the drain due to poorly controlled backwashes, an ineffective strainer system (if installed) or broken internal components. This one’s pretty obvious but usually requires opening the vessel, so it’s often one of the last things that get checked.

We sincerely hope that this article has been helpful in identifying the root cause of your softener system’s hardness leakage. We’d love to hear from you so please leave us a comment below.