By Chad Jones, P.E., CMSE, CFEI, The Warren Group
One of my more interesting calls and subsequent forensic investigations was regarding water accumulating inside of 2X4 fluorescent light fixtures in a suspended ceiling of a secondary school in South Carolina.
A client called reporting an unusual problem. They indicated that the metal chassis of the classroom lights were sweating and generating enough water to accumulate on the diffuser lens of the lights. Seeing is believing, so obviously an on-site investigation was in order!
On arrival at the school the next day, water was indeed observed to be accumulating inside the 2X4 fixtures and puddling on the diffuser lens in one classroom and the adjoining teachers work room. The sweating light fixtures were not observed in the adjacent classrooms or teacher workrooms. The phenomenon was also only observed on the first floor of the building. The facility was an approximately 50-year-old two-story educational facility with CMU walls and precast concrete double tees for the floor and roof.
Inspection above the ceiling of the first story in the affected spaces showed large amounts of condensate on the underside of the double tee panels. The concrete panels were literally sweating and looked like the tile walls of a bathroom after a long, hot shower! Measurements were taken using a non-contact infrared thermometer and the temperature of the areas indicated mid to high 50 degrees Fahrenheit. A quick check of a psychrometric chart indicates that these temperatures are near or at the saturation line for water, explaining why the moisture in the air was condensing on the panels. Test measurements in adjacent rooms not showing moisture problems indicated the temperature of the concrete double tees were in the high 60’s F, certainly above the saturation point as evidenced by the lack of condensation.
These observations led to two main questions, why is the ceiling in the first floor of the affected area so cold and why is there excess moisture present in a conditioned building?
Investigating the cause of the extremely cold concrete double tee in the affected room involved inspecting the classrooms above. When I entered the classroom above our subject one, I noted that it was very cold. From discussions with the teacher, it was discovered that the space was unbearably cold at all times since the start of the school year. In fact, the teacher and students wore coats in class even though it was August in South Carolina, definitely not a normal sight for sure.
The HVAC system in the building was antiquated and did not have a central building management system. Each HVAC unit had a local thermostat housed in locked box. The teachers were not allowed to adjust the setpoint at all. Through the clear face of the locked box, the setpoint in the classroom was observed to be set on 55 °F and the space temperature was 59 degrees!! Because the thermostat setpoint was not reached, the HVAC unit continued to supply 55 – 57 °F air to the space in an attempt to reach setpoint. The unit never cycled off, it continually supplied air at saturation conditions. Temperature measurement of the floor indicated 55 – 57 °F, explaining the high 50-degree temperatures observed in the ceiling of the space below the classroom. Maintenance was notified and the thermostat was reset to mid 70’s per district temperature policies.
With the first question of why the double tees were cold answered, the next question to answer is to find the source of the excess moisture in the ceiling space. As the building is not new, infiltration of outside air is always a concern. While investigating above the ceiling the exterior walls were checked for air intrusion. Large amounts of air could be detected entering the building through the building joints and seams. The quantity of air was so large it could be felt on the skin of the back of your hand and it caused the spider webs above ceiling to sway! A check of exterior doors indicated that the doors had significant resistance to being opened. Once a door was opened, large amounts of air began to rush into the building. The day was calm with little to no wind detectable outside. So why was the building so negative?
Next stop was the roof of the building. Several large upblast “mushroom” exhaust fans were observed on the roof, towards the center of the building. Data plate information on the fans was located and recorded. The fans were obviously quite large and moved a great deal of air. Investigation back inside the building indicated that the fans exhausted the restrooms located in the core of the building but based on experience and knowledge of fan HP and chassis size, the fans were moving air well in excess of code required levels. Where was the makeup air coming from and why were the fans so large?
A trip to the central file repository for the District Maintenance Department was in order. Original drawings were obtained for the building. Reviewing the drawings indicated that the classrooms used to be conditioned by a two-pipe hydronic system with fan coil units located in the ceiling. Each fan coil had an outside air duct tied to the return duct with a louver on the exterior wall to bring in fresh air to the space. The two-pipe system and associated ductwork had been demolished and the louvers sealed. Wall mounted heat pumps had been retrofitted to the school in the past. These units had outside air dampers that would close when the wall mount unit was not running. However, the exhaust fans in the restroom were not replaced with appropriately sized fans considering the HVAC system change. This set up an extremely negative condition in the structure. This was the source of the massive infiltration observed above ceiling.
Outside air calculations and code required exhaust calculations for the restrooms were performed. The overall air balance for the space was then analyzed. New exhaust fans were sized to meet current exhaust requirements in the restroom area. Subsequent replacement of the oversized fans with properly sized fans reduced the infiltration to a level expected from a 50-year-old structure. The space above the ceiling dried up and there were no further reports of puddles of water accumulating in light fixtures.
Design defects can manifest in mysterious places. A thorough investigation should lead us to the root cause and potential responsible parties.
Chad Jones, PE, CFEI, CMSE has a Bachelor of Science in Mechanical Engineering from Clemson University. Chad has over 20 years of engineering experience including mechanical, process, and manufacturing engineering. This work has included equipment design, machine safeguarding, cost estimating and safety compliance. Chad also has over 10 years of commercial, industrial, and residential HVAC and plumbing design experience. Chad is a Certified Fire and Explosion Investigator and IFSAC certified Firefighter II in Greenwood County, South Carolina.
