This page is intended to provide a basic understanding of closed loop water treatment terminology, concerns, treatment methods, and common feed equipment.


Dual Temp Loop – A Dual Temp is a closed loop that encounters the boiler in the winter and the chiller during the summer months. It has isolation valves that are switched over spring and fall to direct the flow of water. Limitation of this system are during the swing months that tend to see drastic back and forth temperatures.

4 Pipe System – A 4 pipe system is 2 separate loops used for both heating and cooling. These tend to be less energy efficient as both are using energy at the same time, but tend to allow for more precise temperature controls.

Geothermal Loops – A geothermal loop is a closed loop that uses piping that is in contact with the ground for heat exchange. Heat can both be collected or expelled through a heat pump system.

Water Treatment Concerns Related to Closed System

Dead Legs – A dead leg is a section of water piping that is not circulating due to valving or pipe capping. Having a dead leg can create bacterial issues, act as a dampener reducing pressure, or collect solids. If possible, it is always best to remove dead legs from the system to eliminate these areas of untreatable water.

Freeze Protection / Burst Protection – Glycol is used in many closed loop systems, both propylene and ethylene. They assist in freeze protection and biologic inhibition. Concentrations below 26% will not only create issues in freeze protection, but will also give bugs a carbon source to feed on. Glycol can reduce heat exchange potential.

Corrosion – Metal loss within a pipe due to ion migration from the anode to the cathode. Typically identified by stalactite like deposition and a corrosion pit and cap at the anode.

Erosion – Erosion is metal loss due to suspended solids impinging on metal surfaces typically seen at fittings. Can also be in direct relation to excess velocity. Easily identified by grooves in elbows.

Freeze – Water if left under-protected can freeze, block and break, creating major water systemic water loss and damage. Ensure glycol freeze protection is as directed and water velocity is appropriate.

Biological – Biological activity within a closed loop can create a multitude of issues, from microbiological induced corrosion, to biofilm fouling, and inhibition of heat exchange. Maintaining a clean system is critical and biological control is part of that.

Treatment Methodology

Corrosion and Scale Inhibitors – typically a filming inhibitor is added to a closed loop to prevent corrosion and scale such as nitrite, molybdate, organo phosphonate, silica, or various other custom additives. From time to time, the use of sulfite is recommended for removal of oxygen.

Biological – typically a non-oxidizing biocide will be added to prevent biological contamination or when disinfection is necessary.

Treatment Equipment

Filters – Filtration can either occur in side stream or full flow filters, depending on need. This is a physical removal of suspended solids that can create erosion and damage fittings and impellers. There are both automatic backwashing systems and basic cartridge systems.

Corrosion Coupons – Corrosion coupons monitor corrosion rates of metals within your system. This is done by a small sample coupon that is premeasured and after being installed for a period of time, remeasured. These act as representatives of your metallurgy so we can provide additional recommendations if needed.

Pot Feeders – Pot feeders are an important component for adding chemical to a closed loop system. Typically in 2-5 gallon sizes, these are side stream vessels that should continually have water flowing through them, or they will act as dead legs and provide areas for bacteria and solids to accumulate and hide.

Glycol feeders – Glycol can be lost over time due to bacterial issues, water loss, or system change. Glycol feeders read and react to pressure changes and can automatically maintain appropriate percentages to continue to protect your system.

Water meters – Systems with automatic makeup valves, sometime water loss is not noticed for extended periods of time. Adding water meters can help identify issues before they become major problems.

Industry Resources

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