A new 112,000-square-foot K-8 school was completed on schedule, allowing the facility to open as planned for the fall semester. The building’s HVAC systems had undergone comprehensive commissioning, including installation verification, sequence testing, and performance and trend log verification during cooler months.
However, as temperatures began to rise in the spring, staff reported indoor humidity levels exceeding acceptable limits on humid days, prompting a call to Mechanical Service Corporation (MSC) to investigate.
The building was equipped with multiple package air handling units with DX cooling and several direct outside air systems (DOAS) providing 100% conditioned outside air. The DOAS units relied on chilled water to cool the air to 52°F or lower. Two 400-ton air-cooled chillers served the building, each equipped with four compressors designed to maintain water temperatures between 40-42°F to meet cooling and humidity control requirements.
The MSC team conducted a thorough assessment of the facility's HVAC systems on a warm, humid day. Two issues were quickly identified:
Elevated chilled water temperatures
Higher-than-optimal DOAS unit discharge temperatures
Further investigation revealed the root cause of the humidity issues lay in the chilled water system and the building automation system (BAS). We found that:
The BAS was operating the chillers as two independent units rather than as an integrated eight-stage cooling system
A lack of isolation valves allowed return water from the inactive standby chiller to mix with water from the operating chiller, raising overall water temperatures
Based on our analysis, the following solutions were implemented:
Established a communication network between the chillers to operate as a single coordinated system
Installed isolation valves with electronic actuators to prevent return water mixing between active and inactive chillers
Modified the BAS programming to integrate start-stop signaling, alarm functionality, lead-lag control, runtime tracking, and emergency changeover
These modifications allowed the school’s DOAS units to successfully achieve temperature and humidity levels to design parameters during critical design days. Post-implementation monitoring confirmed that indoor humidity levels consistently remain within the building's specified parameters.
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