Designing efficient K-12 schools: HVAC

In a digital age, children across the U.S. are more technologically advanced than ever—and they require educational facilities that can keep up. Here, engineers with experience working on K-12 schools share views on HVAC.

By Consulting-Specifying Engineer March 26, 2018


Maxwell Chien, PE, LEED AP BD+C, Associate, Kohler Ronan, New York City
Chuck Dale-Derks, PE, LEED AP, Principal, McClure Engineering, St. Louis
Evan J. Hammersmith, PE, LEED AP BD+C, CGD, Associate, Mechanical, Karpinski Engineering, Cleveland
Michael Lentz, PE, CPD, HFDP, Director of Operations, Baltimore Office, Setty, Baltimore
Rodney Oathout, PE, LEED AP, CEM, Principal | Energy + Engineering Leader, DLR Group, Overland Park, Kan.
Michael Rader, PE, CEM, Vice President and Chief Operating Officer, Barton Associates Inc., York, Pa.

CSE: In your experience, what unique HVAC requirements do K-12 school buildings have that you wouldn’t encounter in other buildings?

Michael Lentz: One unique aspect of K-12 buildings is the wide variety of program requirements encountered. The program for each school is different and brings about unique challenges. Each individual school program defines different HVAC systems. For example, a school with an arts and humanities focus requires a much different type of HVAC system than a school that focuses on the sciences.

Hammersmith: K-12 schools have a number of unique conditions when compared with other commercial buildings. In schools, students migrate in groups to different areas of the building throughout the day and year. The HVAC system needs to be able to operate efficiently and react quickly to the changing loads to maintain comfort. In competition gyms, the HVAC system has to support hundreds of people during a sporting event, as well as a couple dozen students during gym class. Lastly, the HVAC system needs to be able to maintain humidity levels in spite of the exterior doors being propped open every morning and afternoon.

Rader: The variable occupancy of the K-12 building presents challenges in maintaining both temperature and humidity control. Some facilities are essentially vacant during summer months while others are fully occupied. Understanding how an owner will use their facility is critical in designing a system that will meet their needs.

Chien: Typical K-12 school buildings require many different HVAC systems that are typically designed for restaurants, theaters, laboratories, and fitness centers, as a school typically requires all those functions. Gymnasiums and locker rooms are provided with 100% outdoor air units with energy recovery, while special exhaust systems with kilns and science fume hoods are designed for art and science classrooms. Kitchens and prep areas require grease removal and fire suppression. And lastly, smoke control and pressurization life safety systems are required at schools with large spaces and/or are considered high-rises.

Oathout: We continue to stress the importance of the human element on building performance. Equipment and system designs have become more complicated to achieve the code, energy performance, and sustainability requirements of a project. Engineers need to fill the information gap with their owners by staying involved with projects after completion to ensure the stakeholders understand the design intent and maximize the performance of the facility.

CSE: What unique or innovative HVAC systems have you specified on such facilities? What unusual or infrequently specified products or systems did you use to meet challenging HVAC needs?

Lentz: The most innovative HVAC systems that we see in K-12 schools are those that employ variable refrigerant flow (VRF) or geothermal well systems.

Chien: We have used many different HVAC systems to provide thermal comfort to classrooms, including the use of a dedicated outdoor-air system (DOAS) along with a VRF system to provide a smaller footprint and better comfort for the occupants. We also have designed classrooms with an active four-pipe chilled-beam system to provide both heating and cooling. In this example, the classrooms are required to have inoperable windows and the fresh air must be extremely dry. By passing the air through two enthalpy wheels and a cooling coil to make sure water condensation does not occur in the space, a successful end result was achieved.

Rader: We recently renovated an existing elementary school. However, the renovations did not extend to the building envelope outside of window replacement. The owner wanted to achieve LEED Silver certification and take advantage of available increased state reimbursement that is associated with high-performance buildings. This put a lot of emphasis on the HVAC system design to achieve the desired building energy performance. We looked at the building and HVAC system in a holistic manner and tried to incorporate elements of the building structure to support the HVAC system. This resulted in specifying a transpired wall to allow preheating of outdoor air in the winter and convective cooling of exterior walls in the summer. Additionally, we incorporated a thermal chimney in a tall-volume space to take advantage of natural stratification within the space. When outdoor conditions are suitable, the HVAC system can be turned off in the space, which can then be conditioned using 100% outdoor air without any mechanical cooling or fan energy. These two strategies helped us in achieving increased energy performance and ultimately LEED Silver certification.

CSE: K-12 schools have many spaces that serve different purposes (classrooms, gymnasiums, cafeterias, theaters, etc.) all in one building. What HVAC systems are you specifying to ensure each separate portion of the school remains environmentally independent?

Rader: The diversity of spaces within a K-12 facility is both a challenge and an opportunity, as each space has unique characteristics including its own thermal load profile. Looking at the building holistically, we can take advantage of this diversity and use the HVAC system to transfer energy within the space with minimal input of energy. An example of this would be a water-source heat pump system in a building with both exterior and interior spaces. The exterior spaces may require heating while the interior spaces require cooling. The condenser-water loop can transfer energy between the spaces to meet their needs without having to input heat via boiler or reject heat via a fluid cooler.

Oathout: K-12 facilities, especially high schools, have many different spaces with variable occupancy types and use patterns that can change throughout the year. We have found success grouping the areas with similar use patterns on common central plant systems so those areas can be conditioned independently from other areas. For instance, there is a strategy that includes serving the classroom areas with a four-pipe system and activity areas like gymnasiums and performing arts with package equipment. This approach recognizes that the classroom portion of a building is commonly occupied at the same time, and the heat and cooling loads are repeatable with minimal spikes. This approach also provides flexibility as to when and to what level the activity areas are conditioned. The recent advancement in compressor technology allows the packaged solutions to have similar energy-performance criteria as the central plant systems.

Chien: I typically specify single-zone, variable-speed air handling units to handle spaces with large assemblies. This can greatly increase energy savings, and while the space is not in use, the unit may be set back or off to reduce unnecessary energy use. Classrooms are served with a DOAS and local controls for cooling and heating to provide a more targeted thermal comfort range.

Lentz: Typical classroom and office areas are being specified with VRF or geothermal systems, while areas with large occupancy loads (e.g., gyms, cafeterias, theaters) are being designed as VAV systems.

CSE: Have you specified variable refrigerant flow systems, chilled beams, or other types of HVAC systems into one of these structures? If so, describe the challenges and solutions.

Dale-Dirks: Yes, VRF, which has its place and application (such as in the administrative offices) but is not the proper system for every facility. I believe other systems are more efficient and easier to maintain than VRF for classrooms. We have measured as much as 12 degrees difference in supply-air temperature from Side 1 to Side 4 on a VRF classroom cassette. Students and teachers have complained about the noise, temperature swings, drafts, and poor air mixing out of some models of VRF. It is my opinion that maintenance staff will be plagued with difficulties in troubleshooting problems as components begin to wear or fail.

Chien: We have specified VRF systems and chilled beams in schools, and they each present their own challenges. Refrigerant piping systems are required to not exceed certain concentrations based on the refrigeration type. This can have many impacts on routing locations depending on the size of the room and whether or not the space is used for emergency egress or as a corridor. They will require an engineer to design and calculate the correct length of piping required. Chilled beams are definitely a price premium, and many projects have requested us to reduce the number of control points, control valving, etc., in order to keep the system. Since the active chilled beams are working in tandem with the DOAS units, the controls for the DOAS also had to be adjusted to match the reduction.

Lentz: We have specified VRF systems for multiple schools. The challenge is typically the location of the outdoor units along with the location of the DOAS unit. These units can usually be placed on the roof, but in most cases require some type of screening for aesthetic amelioration.

Oathout: We have designed many K-12 projects with VRF systems. The systems coupled with a DOAS can provide a cost-effective, energy-efficient approach to the HVAC design. VRF systems have a significant advantage as the HVAC system of choice for renovation projects where space available for piping and ductwork is commonly limited. There are a few challenges associated with these projects that need to be avoided. For one, there continues to be an issue with satisfactory training for the installers of this equipment. There are many similarities between most brands available in the United States, but manufacturers will have unique details about the installation that are critical to the successful operation of the VRF system. If the details are not followed and cleanliness is not maintained during installation, system operation will be problematic and the lifespan of the equipment will be compromised, resulting in lots of frustration amongst the stakeholders.

CSE: What types of waste-heat recovery, combined heat and power, or other systems have you designed for office buildings? Please describe the challenges and solutions.

Rader: We typically do not see these types of systems on our K-12 projects primarily due to first-cost considerations.

CSE: What types of DOAS are owners and facility managers requesting to keep their facility air fresh?

Lentz: Owners and facility managers are requesting high-efficiency DOAS with high minimum-efficiency reporting value (MERV) ratings in the filtration.

Rader: In humid climates, it is critical to control and condition outside air. We typically see a DOAS with energy recovery via enthalpy wheels. This allows us to adequately pressurize the building to mitigate infiltration and dehumidify outside air before it enters the building. It also allows us to operate the building in a summer mode, if the building is essentially unoccupied but still air conditioned.

Dale-Dirks: The “type” makes no difference. Understanding how building area zones fit operating schedules and how the facility is used by outside clubs or venues is important to be able to control these areas to their actual occupancy rather than over-ventilating (spending money).

CSE: What types of air balancing or environmental balancing do you include in your design? Describe the project.

Chien: School laboratory classrooms require delicate air balancing to ensure student safety and precise sash control in order to provide energy savings when the hood is not in use. With the use of fast-reacting pressure-independent air valves, we were able to maintain necessary air changes in the classrooms of one of our recent projects while diverting the fume hood-exhaust air to general exhaust.

Lentz: We include an air-balance summary set of documents that depict the supply air, return air, exhaust air, and transfer air for every space within the building.