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Your Closed-Loop HVAC/R System May Have a Dirty Secret

Closed-loop HVAC/R systems, which include chilled water, hot water preheat, hot water reheat, condenser water, glycol economizer run-around loops, and many others, provide a variety of advantages in commercial and industrial applications. Because fluids are isolated from the atmosphere, closed-loop systems are protected from outside contamination, require less maintenance, conserve water, and are more efficient than most other types.

Sediment found in a closed-loop HVAC/R system by an MSC service technician

These very benefits, however, can sometimes lull facilities managers into neglecting the maintenance of closed-loop systems. A number of things can go wrong with closed-loop systems lacking in PM. Make no mistake, preventive maintenance is an absolute necessity for closed-loop systems.

One common misperception about closed loops is that newly-installed and commissioned systems are pristine and well-shielded from any possible problems. Typical commissioning requirements include circulating sodium phosphate as a cleaning agent for 24-48 hours while occasionally opening high-point vents and low-point drains, followed by draining, flushing, and refilling the system with clean soft water and a rust inhibitor. Some commissioning efforts, however, are lax when it comes to verifying that this process actually occurred according to specifications. This is evidenced by the occasional fairly-new closed-loop systems our technicians come across that have silt, rust, and other sediments settled in low points of the coils where there is low-to-no velocity. This condition has a detrimental effect on heat exchange, and the bottom of these coils are also susceptible to freezing and breaking.

A more alarming issue is insufficient glycol in poorly-maintained closed-loop systems. Our technicians’ spectrometer tests often reveal levels of 10-20% glycol-to-water, which is significantly lower than what is needed for freeze protection, leaving systems vulnerable to broken pipes and coils. This condition can also affect heat exchange rates and pumping capacity. Other problems that often occur in closed-loop systems are the presence of welding slag, improperly filling the system with hard water after maintenance or repairs, and air pockets that lead to rust forming in the system, just to name a few.

Closed-loop systems should undergo routine preventive maintenance on an annual basis at a minimum. This should include checking and cleaning strainers, blow-down of all low points, and verifying that air is properly vented from the system at high points. Glycol levels should be checked and added as needed to maintain design parameters.

Please contact MSC for more information on closed-loop HVAC/R system maintenance, or if your system is excessively dirty or plugging heat exchangers and/or coils.

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