PJM was called in by a New Jersey hospital with a 100% outside air DX system to diagnose problems. The hospital was experiencing elevated temperatures on high-temperatures design-days, and attempts by the facilities staff to address the issue with the equipment and building controls manufacturers only resulted in finger-pointing between the two factions. PJM stepped in and determined that trend logging was just the diagnostic tool needed to get to the root of the problem.
Trend logging is a powerful method of diagnosis in which building or system parameters are recorded at regular intervals over a period of time, usually several days to a week, to identify irregular patterns and anomalies. Readings can be taken as often as every minute, and multiple trends can be plotted on one chart to measure how they interrelate. On this particular project, PJM set up trend logs to record five different variables – space temperature, air handler discharge temperature, outside air temperature and humidity, cooling coil discharge temperature, and exhaust humidity – at one-minute intervals over a period of one week in late July.
When technicians returned a week later to read the logs, they showed no abnormal readings during nighttime hours and cooler days. But on hotter days, when outdoor temperatures reached into the 90s, space temperatures and AHU discharge temperatures showed a “saw tooth” pattern, spiking and then dropping about every five minutes. This led the techs to examine the system’s four compressors, and it was soon determined that one of them was short-cycling. Faulty microprocessor controls had caused the compressor to cycle on and off, leading to electrical problems and damage to the stator core. PJM fixed the problem by replacing the microprocessor and the failed compressor. They also implemented discharge air reset to better control temperatures and humidity in the space.
PJM Mechanical Contractors, Inc. is a multi-faceted mechanical contractor providing construction and installation, service and maintenance, and design/build services in a comprehensive range of mechanical disciplines that include HVAC, plumbing, controls, refrigeration, custom metal fabrication, piping, fire protection, and backflow preventer testing. PJM primarily serves clients in pharmaceuticals, manufacturing, healthcare, institutional, and other various commercial and industrial sectors.