Closed-Loop System Chemical Water Treatment and Maintenance: A Comprehensive Guide for HVAC

Is your closed-loop system water clean and clear? Or is it dirty and filled with corrosion by-products, sediment, and suspended solids? 

While closed-loop systems offer protection against many external contaminants, they aren’t immune to internal challenges like rust, scaling, and debris from new construction. Neglecting these can lead to costly disruptions and replacements. 

The key to longevity and efficiency in these systems lies in chemical water treatment, monitoring, and maintenance. With the right treatment program, you can combat internal contaminants, reduce maintenance costs, and extend equipment life. 

And, if your loop is suffering from neglect, the right treatment program can help reduce further corrosion damage and clean-up your closed-loop system water. 

Dive into our guide to learn how to maintain and treat your closed-loop systems to ensure optimal performance, lower operating and treatment costs, and extend equipment life. 

Overview and Importance of Closed-Loop Systems

Closed-loop systems often fly under the radar in many facility management plans. Whether for heating or cooling, these systems are integral to commercial, industrial, and residential applications. 

Building occupants expect consistent comfort. Poorly treated and maintained closed-loops lead to disruptions for individuals and equipment. Given the embedded nature of these systems within the building, repairs or replacements are costly. Treating closed systems to a high standard is not just a recommendation—it’s a necessity. 

Typical Closed System Temperatures

From hydronic heating designs in residential spaces to commercial heating water systems, temperatures can fluctuate between 50ºC (120ºF) to 150ºC (300ºF). Typically, hot water systems hover around 80 – 85ºC (175ºF – 185ºF). Chilled water systems, on the other hand, operate at cooler temperatures, usually between 4 to 10ºC (40 – 50ºF). 

Why does temperature matter? System temperature affects the level of dissolved oxygen in the water, potential for general and pitting corrosion, and may affect the selection of chemical water treatment inhibitors. 

Heat Exchange Fluid: Water and the Role of Glycols in Closed Systems

Water is the primary medium for heat exchange in closed-loop systems, but glycol/water combinations are also common, especially when freeze protection is paramount. Where glycol is used, ethylene glycol and propylene glycol are most common and typically used in concentrations of 30% to 50%. 

Glycol concentrations below 25% are susceptible to biological degradation, introducing the potential for microbially induced corrosion. It’s important to note that corrosion products within the system can accelerate the breakdown of glycols. Breakdown is influenced by factors like temperature, metal presence, and fluid pH. 

Glycol condition and concentration should be monitored and tested at minimum on an annual basis. 

Comprehensive Chemical Water Treatment for Closed-Loops

A holistic approach to closed-loop water treatment is essential. The ideal scenario starts with pre-operational cleaning to remove construction debris, surface corrosion, and any oils/greases that can act as micronutrients. 

This is followed by chemically treating the heat exchange fluid whether it’s water or glycol. For optimal results and long system life, closed-loops must be chemically treated to control: 

  • scale
  • corrosion 
  • microbiological growth 

And they should include proper filtration to tackle suspended solids and suspended microbial contaminates. 

Common Closed-Loop Water Treatment Formulations 

Common closed-loop formulations may contain nitrite, molybdate, phosphate, or silicates. Filming amines are less common but have proven to be a superior technology. To learn more about the results that can be achieved with filming amines such as our Aquaguard 810 or 819 (for systems containing aluminum) and how they clean and protect dirty or corroding systems, check out these case studies: 

Biological Control: Microbiocides and Biodispersants

In closed systems, conditions should ideally deter microbial fouling. However, contamination and the breakdown of organic material including glycols can lead to fouling. When this occurs, the presence of nutrients like nitrite, a common closed-loop treatment can actually encourage microbial growth. 

Microbial activity can compromise system performance, making it essential to control bacteria levels in closed-loops. Oxidizing biocides such as chlorine or bromine aren’t typically used in closed systems, instead, non-oxidizing biocides, which harm microbial cell walls or disrupt their metabolism, are preferred.  

Polyquats are a superior non-oxidizing option. They may offer the functionality of a biocide, but also act to: 

  • coagulate (bind-together) suspended solids 
  • clump microbes 
  • act as biodispersants 

This helps to move suspended solids and any biological material to filters for removal. Polyquats like Aquaguard 630 are excellent biocide options to ensure closed loop systems remain clean and efficient. 

The Economics of Treatment

Initial costs for treating a closed-loop system depends on the chemical treatment formulation and the condition of the loop and water within it. Ongoing costs of treating are minimal. With proper maintenance and annual water losses under 10% of the initial volume, ongoing chemical water treatment costs are low. Given the potential risks and the benefits of treatment, the choice is clear—all closed systems should be treated to achieve the lowest possible corrosion rates. 

Learn more about closed-loop treatment costs in our article “Protecting Your Investment: What Influences Your Operating and Water Treatment Costs in Closed Loop Systems? Costs/Prices. 


Closed-loops systems are the arteries of your facility. They keep building occupants comfortable and equipment operating smoothly. While these systems inherently protect against many external contaminants, they can still face internal challenges like corrosion, suspended solids, and microbial contamination. Failure and repair are often major operations—so it’s better to invest in proper chemical treatment and maintenance to ensure a long and efficient system life.  

So, what’s the next step? 

When you’re ready, contact Guardian Chemicals to learn how you Create a Better Future with our comprehensive closed-loop chemical treatment and service programs.

Closed Loop Water Treatment FAQ

What is a closed-loop system?

A closed-loop system, in the context of facility management, refers to a waterrecirculating system where water or glycol/water circulates continuously without being exposed to the atmosphere, typically used for heating or cooling purposes.

Why is chemical water treatment essential for closed-loop systems?

Chemical water treatment helps control scale, corrosion, and microbiological growth to ensure system efficiency and prolong its lifespan.

How does temperature influence closed-loop systems?

System temperature affects the level of dissolved oxygen in the water, potential for corrosion, and may influence the selection of chemical water treatment inhibitors.

What role do glycols play in closed-loop systems?

Glycols, especially ethylene and propylene, are used in closed-loop systems to prevent freezing. However, they can degrade biologically, leading to potential microbially induced corrosion.

How often should glycol condition be monitored in a closed-loop system?

And what happens if glycol has degraded? Glycol condition and concentration should be monitored and tested at minimum on an annual basis. If glycol is starting to degrade, depending on condition inhibitors may be added or glycol replaced to ensure system protection and efficiency.

What are common chemical formulations used in closed-loop water treatment?

Common formulations may contain nitrite, molybdate, phosphate, or silicates. Filming amines are sometimes used in newer systems or those in need of cleaning due to their superior technology and enhanced capability to clean and protect systems.

What are the costs of not treating a closed-loop system?

Without proper treatment, closed-loop systems can face increased operational costs, higher water losses, and reduced equipment lifespan, leading to more frequent and costly repairs or replacements.

What are the financial benefits of treating a closed-loop system?

Proper treatment and maintenance can reduce operational costs, minimize water losses, and extend equipment lifespan, offering significant long-term savings.

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