Engineering in K-12 schools: HVAC systems
Engineers offer practical advice and best practices on how to design HVAC systems and improve indoor air quality in K-12 schools.
- Keith R. Hammelman, PE, Vice president, CannonDesign, Aurora, Ill.
- Robert V. Hedman, PE, LEED AP BD+C, Senior associate, Kohler Ronan LLC, Danbury, Conn.
- Pete Jefferson, PE, LEED AP, HBDP, Principal/vice president, M.E. Group, Overland Park, Kan.
- Essi Najafi, Principal, Global Engineering Solutions, Rockville, Md.
- Rodney V. Oathout, PE, CEM, LEED AP, Regional engineering leader/principal, DLR Group, Overland Park, Kan.
- Sunondo Roy, PE, LEED AP BD+C, Vice president, CCJM Engineers, Chicago, Il.
CSE: What unique HVAC requirements do K-12 school structures have that you wouldn’t encounter on other structures?
Hammelman: The most unique HVAC requirement of a K-12 school is the variety of uses encountered throughout the building—engineers need to account for an auditorium, performance spaces, offices, classrooms, gymnasiums, cafeteria spaces, kitchens, and possibly even a natatorium. These diverse needs in multiple spaces are not typically found in many other building types.
Roy: Generally, K-12 schools have higher ventilation rates compared to “small-hall” university classrooms since the student population is typically higher. This higher ventilation rate offers the designer greater challenges, but also greater opportunities, to come up with efficient design solutions. Decoupling the ventilation system from the comfort conditioning system can offer a more efficient system, but typically at a higher first cost as there are duplicate systems and more complex controls strategies.
Najafi: The one requirement that stands out is the fact that schools operate for a portion of the year while during the summer months they remain dormant or with minimal use. In a similar way, the daily operation of the school also undergoes drastic changes and sifting of HVAC loads. Classrooms may be fully occupied or not during lunch or gym events, while cafeteria and gym spaces have the reverse occupancy characteristics. This operational uniqueness requires a design that allows for quick sifts of heating/cooling allocation without oversizing the system or the central plant.
Hedman: One area that differs from other building types is the function of rooms such as a gymnasium and the multi-functional uses. A gymnasium can fluctuate in occupancy from 20 to 40 students during a normal school day, to 800 to 1,000 occupants during a game or school event. As an engineer, close attention must be made in designing and selecting the appropriate systems that can satisfy both the cooling and ventilation loads with such a diverse space.
Oathout: One of the biggest differences in HVAC design between K-12 facilities and other building types is the occupant density tends to be much greater and more variable in K-12 facilities. A successful HVAC system design for a K-12 facility must be able to react to these considerations and continue to provide a high-quality indoor environment.
CSE: What changes in fans, variable frequency drives (VFDs), and other related equipment have you experienced?
Najafi: The predominant factor is technological advantage. While the value of VFDs was recognized in the 1990s, their high cost was limiting their use to very specific project applications with a minimum rate of return. Today, the use of low-cost VFDs is automatic, even if they are to be used for balancing purposes only. Similarly, we see the use of multiple direct drive fans in units with high capacity of control, low noise, and minimum maintenance needs. However, the largest change of the last 15 years is the use of direct digital controls (DDC) systems coupled with electronic actuators. They have completely replaced the inadequate and underperforming pneumatic controls of the past.
Oathout: The advancement and reliability of variable air systems have been a great match for HVAC system design in K-12 facilities.
Hedman: Several manufacturers now offer many types and quantities of fans within an air handling system. Fan-wall technology consists of multiple direct-drive plenum fans in an array to deliver air. This type of arrangement allows for a smaller footprint to the overall length of the air handling unit, and redundancy due to the multiple fans and lower maintenance costs as there are no belts requiring changing.
Roy: We have heard over and over again from building engineers that new motors running on VFDs, even when selected properly as invert duty with the proper insulation class, typically do not have the service life of older constant speed motors in constant volume systems. I don’t know if it’s one of those sentimental comments of the “good old days” variety, but considering the number of times I have heard it from very different building engineers, I would suspect there is some truth to the complaint. It would be interesting to see the motor manufacturers provide statistical lifecycle data on constant speed versus inverter duty motors and also for motors installed in the 1960s and 1970s long before the widespread use of VFDs. Is it the VFDs wearing down the motors, or are the motors being built less robustly in this hypercompetitive market?