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MSC Service Call: Finding an Elusive Propylene Glycol Leak with Dye

During a monthly HVAC preventive maintenance visit to a large manufacturing facility, an MSC service technician noticed that the level of propylene glycol in the reservoir tank was inexplicably lower than expected. This reservoir tank serves two large glycol systems, one of which was a hot water heating loop serving various equipment including several rooftop make-up air units. The other system was a glycol heat recovery loop capturing exhaust heat and returning it to several preheat coils in the make-up air units. Both systems had over 1,000 linear feet of well-insulated piping both above and below the roof. MSC’s challenge was to locate and seal the glycol leak.

Our service tech’s first step was to check low-point drains and high-point vents for leaks. He then checked all of the coils in the HVAC units and miscellaneous specialty piping, but no signs of leaking were found. Next, glycol freeze protection levels were checked by a spectrometer, and readings proved to be within an acceptable range. Delta temperatures on MUAs were verified for proper heat exchange on both systems throughout the BAS.


After all of the testing and a thorough walk-down of the system failed to reveal the leak, we suspected that it was being concealed by the thick, white piping insulation. Since removing insulation from the extensive piping system wasn’t a feasible option, we contracted the manufacturer and ordered two different colors of concentrated food-grade propylene glycol dye, one for each system. Red-colored dye was added to the hot water loop, and blue was added to the heat recovery loop.


After allowing the glycol to circulate for several days, we walked down the system and quickly came upon a blue-stained section of insulation near a control valve. Beneath the insulation, we discovered a leaking union, which was promptly replaced, and the insulation was repaired to the customer’s complete satisfaction.

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