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High School Air Quality/Odor Investigation

November 2021


Due to reported noxious odors, Indoor Environmental Concepts (IEC) was engaged to perform an assessment of the air quality at a local high school, specifically evaluating office space serviced by a forced air heating, ventilation and air conditioning (HVAC) system. Initial inspection of the system and ceiling space by the HVAC service provider did not identify any unusual conditions.


IEC’s review of the HVAC system design, which services the area of concern, revealed the Air Handler Unit (AHU-2) is in the boiler room with the fresh air intake located in the west wall adjacent to an exterior courtyard. The gas fueled  boilers were ruled out  as the source of the odor by visual inspection and measurements from a photoionization detector for VOCs. 


Interviews with staff indicated odors had been experienced several times in the area and odors had been reported in the auditorium on various occasions. Further discussion with the facility personnel indicated continuous HVAC system operation based on normal occupancy on weekends and increased percentage of fresh air to mitigate the spread of COVID-19 by dilution ventilation.


From the roof, IEC inspected the boiler exhaust stack, immediately adjacent to the courtyard where the AHU fresh air intake is located.  The smoke stack housing is a brick structure which elevates approximately 5 feet above the roof membrane. Two (2) individual exhaust pipes protrude up above the brick structure approximately 3 feet. Both of the individual exhaust pipes are covered by “cone caps.” The two (2) roof-top AHUs that service the auditorium are approximately 25 feet to the northeast of the stack.


In accordance with the ASHRAE Standards, “the goal of stack design is to specify the minimum flow of the exhaust, exhaust velocity, and stack height that ensure acceptable air quality at all locations of concern.” In other words, the goal is to exhaust the contaminants at a height and velocity sufficient to allow atmospheric dilution ventilation which will not be re-entrained into the building via nearby fresh air intakes or cause detrimental effects to the community. Exhaust velocity of approximately 2600 feet per minute (fpm) prevents rain from entering the stack. Exhaust above 2000 fpm is the goal to provide sufficient plume rise and dilution. With these considerations, ASHRAE does not recommend installing cone caps or hinged covers over discharges since these can minimize exhaust velocity and dilution.

Source: 2019 ASHRAE Handbook-HVAC Applications

Based on the assessment, the following recommendations were presented:


  1. Engage a qualified firm to evaluate the exhaust stack for appropriate exhaust velocity and discharge height as well as the removal of the caps over the exhaust pipes, which potentially decrease dispersion of the contaminants.

  2. Evaluate dampers in the supply air duct system for the even supply of air distribution to all areas being serviced by AHU-2.

  3. Verify and evaluate weekend boiler setback parameters for energy efficiency, building use and COVID-19 mitigation.


Yellow Arrow: Location of boiler exhaust stack. Red Arrow: Location of fresh air intake for the AHU servicing the area of concern. Green Arrow: Roof top AHU’s which service auditorium


Fresh air intake of AHU-2 and intake duct at west window of boiler room in courtyard.


Left: Roof top exhaust stack with cone caps affixed to the top of both exhaust pipes.


Right: Overview of exhaust stack housing with west courtyard in background, location of fresh air intake for AHU-2.


Looking north on roof: Exhaust stack housing with capped exhaust pipe on left and roof top AHU's which service auditorium in background (red arrow).

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