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MSC Traces Puzzling Control Valve Problems to Bad Potentiometers

When it comes to diagnosing complex HVAC issues, one thing you can always depend on is finger-pointing. Too much energy is spent on assigning blame while problems go unsolved for weeks, even months. As an HVAC, process cooling and building automation service and diagnostic specialist, MSC’s only objective is to come in and solve problems from above the fray.

On one recent project, electric chilled water and hot water control valves were acting erratically. Roughly half the valves on the project were malfunctioning at random, seemingly with no rhyme or reason. Consulted early on, the manufacturer believed the problem existed within the new system, stating they were certain the problem did not originate with the valves themselves.

To find out why the valves were opening and closing at random, MSC first measured the incoming 0-10V analog signal with an oscilloscope. Readings were clean and rock steady. Next, they used a simulator as a separate input source to rule out issues with the BAS signal. Issues with the 120V power to the valves was ruled out when they continued to randomly malfunction when connected to clean UPS power. A twisted, shielded cable was run directly from the I/O panel to the valves in question eliminated antenna effect and induction in the conduit on the control signal as possible causes. All flows and pressures tested within the operating range of the valves. All testing was recorded on video.

MSC went back to the valve manufacturer at this point with supporting data proving that the problems still persisted after the valves had been isolated from all outside influences. Faced with a mound of solid evidence, the manufacturer finally conceded the possibility of a problem with the valves and sent their lead engineer to study the issue. His conclusion: the control valves were malfunctioning due to a bad batch of potentiometers. Once the faulty potentiometers were replaced and the control valves were recalibrated through the circuit boards, the problems vanished and the valves performed flawlessly.


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