Water in the classroom light fixtures???

Water in the classroom light fixtures???
 By Chad Jones, PE 
The Warren Group
One of my more interesting calls and subsequent investigations was regarding water accumulating inside of 2X4 fluorescent light fixtures in a suspended ceiling of a local high school.
The client called and reported that the metal housings of the lights were sweating and generating enough water to accumulate on the diffuser lenses of the lights.  In addition, it was reported that adjacent ceiling tiles were damp and sagging.  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 lenses in one classroom and an adjoining teacher work room.  The phenomenon was only observed on the first floor of the building.  The building had two stories with concrete masonry unit walls and concrete panels for the floor and roof. 
An inspection above the ceiling of the first story in the affected spaces showed abundant amounts of condensation on the underside of the concrete panels.  The panels were literally sweating, much like water drops forming on a glass holding a cold drink on a hot summer day!  
Measurements taken using a non-contact infrared thermometer showed the temperatures of the area above the drop ceiling in the high 50-degree F range.  A quick check of a psychrometric chart shows that these temperatures are near or at the saturation line for water, explaining why the moisture in the air was condensing on the panels.  

Temperature readings of the concrete panels in adjacent rooms with no apparent moisture problems were in the high 60-degrees 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 excessive moisture present in a conditioned building?

Investigating the cause of the extremely cold concrete panels required inspecting the classrooms above.  The teacher interviewed reported the space to be unbearably cold; so cold that she and the students wore coats in class even though it was late August in the South.  The HVAC system in the building was antiquated and did not have a building management system, only local thermostats housed in locked boxes.  The setpoint in the classroom was 55 degrees F and the space temperature was 62 degrees!  Because the thermostat setpoint was not reached, the HVAC unit continued to supply 55-57 degrees F air to the space in an attempt reach setpoint.  The unit never cycled off, it continually supplied air at saturation conditions.  Temperature measurement of the floor indicated 57-59 degrees 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 concrete panels were cold answered, the next question to answer was to find the source of the excess moisture in the drop ceiling space.  With an older building, infiltration of outside air is always a concern.  While investigating above the drop 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, for example where the walls met the roof on the exterior.  The quantity of air was so large it could be felt on the skin of the back of my hand!  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 interior pressure so negative relative to the exterior? 

The next stop was the roof of the building.  Several large “mushroom” exhaust fans were observed on the roof.  Data plate information was located and recorded to allow analysis of the capacity of the fans.  The fans were obviously quite large and moved a great deal of air.  Further investigation inside the building indicated that the fans exhausted the restrooms located in the core of the building, however based on knowledge of fan horsepower and housing size, the fans were moving air well in excess of code required levels. 

The investigation then took me to the central file repository for the District Maintenance Department where I obtained original drawings for the building.  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 with a louver on the exterior wall to bring in fresh air to the space.  The two-pipe system 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 unconditioned air infiltration observed above the ceiling.

Outside (fresh) air calculations for the building as well as code-required exhaust calculations for the restrooms were performed.  The overall air balance for the space was then analyzed because it is best to keep a building from having a negative pressure relative to the exterior to prevent infiltration of unconditioned air.  New exhaust fans were sized to meet exhaust requirements in the restroom.  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 and there were no further reports of puddles of water accumulating in light fixtures.

The air balance of a building is critical to controlling infiltration.  The anomaly of the thermostat set at 55 degrees F highlighted the infiltration by condensing the moisture in the ceiling space and light fixtures.  Resolution of these issues prevented further damage to the ceilings and lighting of the high school. It is best to always step back and observe all details and apply good scientific and engineering analysis when looking for root causes involving moisture in a building envelope.
If you would like to contact the author, please contact Mr. Jones’ office at (803) 732-6600 or email him directly at chad@warrenforensics.com.  Mr. Jones, PE has a Bachelor of Science in Mechanical Engineering from Clemson University, with over 23 years of engineering experience including mechanical, process, and manufacturing engineering. His work has included equipment design, machine safeguarding, cost estimating and safety compliance. Chad has over 10 years of commercial, industrial, and residential HVAC and plumbing design experience.