Can a pool heater have both corrosion AND scale?

If you've ever stared at your pool heater wondering how it could possibly have both scale buildup AND corrosion damage at the same time, you're not alone. This seemingly contradictory scenario has been puzzling pool owners and professionals alike, leading to countless questions about the chemistry behind these opposite conditions. The answer might surprise you: not only is it possible, but it's actually quite common – and understanding why could save you thousands in equipment replacement costs.

The Science Behind Simultaneous Scale and Corrosion

To understand how both scale and corrosion can occur in the same heat exchanger, we need to go back to the fundamentals of water chemistry. The Langelier Saturation Index (LSI) is the key to understanding this phenomenon, as it measures water's tendency to either corrode (negative LSI) or scale (positive LSI).

The answer is actually yes. And I want to go back to the original reason that Dr. Langlier came up with the saturation index back in 1936. His entire purpose of doing this was to protect the pipes in the drinking water system from corrosion. He wanted to figure out the saturation point of calcium carbonate because calcium carbonate is corrosion resistant, and it can coat the inside of the pipes.

— Eric Knight, Rule Your Pool Podcast

The critical insight here is timing. Corrosion and scale don't necessarily happen simultaneously – they can alternate based on changing water conditions. When water chemistry swings from corrosive to scale-forming conditions repeatedly, both types of damage can accumulate over time in the same equipment.

The Startup Scenario: Setting the Stage for Problems

One of the most common ways this dual problem develops starts right at pool startup. The traditional "hot start" method – adding large amounts of acid to a newly plastered pool – creates the perfect conditions for future heater problems.

During startup, many pools receive significant acid doses to balance the initial high pH from fresh plaster. When circulation systems run during this period (despite recommendations against it), this acidic water flows through the heat exchanger, beginning the corrosion process early in the pool's life.

As the pool transitions to regular operation, the natural tendency for pH to rise (especially with alkalinity levels of 80-120 ppm) creates scale-forming conditions. The hottest areas of the system – primarily the heat exchanger – become the first places where scale deposits form, potentially covering already-corroded surfaces.

The pH Bounce Cycle: A Recipe for Equipment Damage

Perhaps the most insidious cause of combined scale and corrosion is the common practice of "chasing pH" – repeatedly adding acid to bring pH down from its naturally rising state, only to have it rise again.

This is a real scenario that can happen. And we see it quite often where heat exchangers will be both scaled and corroded underneath. It's pretty wild. And this is why that happens. It's the bounce. It's chasing pH. It's chasing range chemistry. Something you can never actually catch because it's physically impossible.

— Eric Knight, Rule Your Pool Podcast

Here's how the destructive cycle works:

• pH naturally rises to scale-forming levels
• Pool owner adds acid to bring pH back to 7.4
• Undiluted acid flows through the system, potentially dissolving existing scale and causing corrosion
• pH rises again due to natural water chemistry
• The cycle repeats, creating layers of damage over time

This bounce between corrosive and scale-forming conditions explains why many heat exchangers show evidence of both types of damage when examined closely.

The Counterintuitive Effect: How Acid Can Cause Scale

One of the most surprising revelations is that improperly applied acid can actually accelerate scale formation in heat exchangers. When concentrated acid hits cement-based pool surfaces, it etches the surface and dissolves calcium hydroxide from the plaster.

You can actually accelerate a scale forming condition by column pouring acid or just pouring acid without diluting it. Because it's going to force that high pH, high calcium stuff that just came out of the cement into the main drain. And then it goes to the heater. Falls right out. Isn't that crazy? Acid can cause scale in a heat exchanger? Yes, it can.

— Eric Knight, Rule Your Pool Podcast

This dissolved calcium creates a high-pH, high-calcium solution that gets pulled into the main drain and flows to the heater. In the hot environment of the heat exchanger, this calcium-rich water immediately precipitates as scale, even though the original problem was caused by adding acid.

Beyond Chemistry: Electrical Issues and Galvanic Corrosion

Not all heater corrosion stems from water chemistry imbalances. Electrical problems can create equally devastating results through galvanic corrosion and stray current issues.

Improper bonding can create electrical pathways between different metals in the pool system. In one documented case, stray current flowed from a downstream salt cell backward to a copper heat exchanger, using it as an anode and causing severe corrosion.

Come to find out, it was not properly bonded, and there was a stray current between the salt cell, which was downstream, but the electrical current was going backwards against the flow of water to the heat exchanger, because that was the nearest anode that it could pull electrons from, and it did.

— Eric Knight, Rule Your Pool Podcast

This type of corrosion can be so severe that copper oxide flakes break off and flow back through the system, appearing as black debris in the pool water. The solution often involves installing sacrificial zinc anodes that provide an easier target for stray electrical current.

Prevention Strategies: Breaking the Destructive Cycle

Understanding the causes of simultaneous scale and corrosion leads to clear prevention strategies:

Proper Chemical Application: Always dilute acid before adding it to the pool. Most residential pools need far less acid than commonly assumed – often just 15 ounces for a 20,000-gallon pool rather than the half-gallon many people use.

LSI Balance Management: Maintain proper Langelier Saturation Index balance year-round, using tools like the Orenda calculator to determine correct dosing.

pH Ceiling Awareness: Instead of chasing current pH levels, understand where pH is naturally heading based on alkalinity levels and other factors.

Pool pros listening to this. If you are servicing a pool once a week, and the vast majority of you are, you need to be aware of where the pH is going. Because if you're just focused on today, it's going to move... You need to know where the pH is going.

— Eric Knight, Rule Your Pool Podcast

Proper Equipment Installation: Ensure correct electrical bonding and avoid placing trichlor feeders upstream of heaters.

Regular Monitoring: Consistent water testing and gradual adjustments prevent the dramatic swings that cause equipment damage.

The Path Forward: Sustainable Pool Chemistry

The revelation that pool heaters can indeed suffer from both corrosion and scale simultaneously shouldn't be discouraging – it should be enlightening. By understanding the mechanisms behind these problems, pool owners and professionals can implement strategies that protect equipment while maintaining beautiful, safe water.

The key lies in working with water's natural tendencies rather than fighting against them. Water will always seek its natural equilibrium state, and attempts to force it into unnatural conditions through aggressive chemical additions inevitably lead to equipment damage.

Instead of the traditional approach of reacting to current conditions, successful pool management requires anticipating where water chemistry is heading and making gentle corrections to guide it toward balanced conditions. This proactive approach not only extends equipment life but also reduces chemical costs and maintenance requirements.

Whether you're a pool professional servicing multiple accounts or a homeowner managing your own backyard oasis, understanding these principles can save thousands in premature equipment replacement while ensuring years of trouble-free pool enjoyment. The chemistry might be complex, but the solution is surprisingly simple: measure carefully, dilute thoroughly, and always consider where your water is going, not just where it is today.

Episode Chapters

• 00:00 Introduction and Episode Purpose
• 02:30 Can Scale and Corrosion Coexist?
• 05:45 The Pool Startup Scenario
• 09:15 The pH Bounce Cycle Problem
• 13:20 How Acid Can Cause Scale
• 16:40 Electrical Issues and Galvanic Corrosion
• 19:30 Prevention Strategies and Best Practices