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.

By Consulting-Specifying Engineer March 17, 2014


  • 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?

CSE: What indoor air quality (IAQ) challenges have you recently overcome? Describe the project, and how you solved the problem.

Hedman: The International Building Code allows the use of operable windows as the source for ventilation air for classrooms. However, while fresh air can be introduced via operable windows, the air is not filtered. Filtering the air reduces the quantity of pollen, which is a major catalyst to allergies. Furthermore, noise and air pollution is not filtered when operable windows are used. In designing a new private school in New York City, all windows were fixed and fresh air intakes serving mechanical units were located at the upper levels.

Najafi: This was encountered in the design for the renovation of an existing school, in which the desire to replace existing systems included an upgrade of ventilation requirements to current code standards. The desire to maintain limited equipment space use, yet design for increased amounts of outside air, yielded a humidity concern that was best addressed by employing demand ventilation techniques.

CSE: In your experience, have alternative HVAC systems become more relevant? These may include displacement ventilation, chilled beams, etc.

Jefferson: Nontraditional systems have certainly been more relevant in K-12 design because the fundamental challenges that districts have with operational/energy budgets remain in place. I think school districts will continue to be willing to explore new ideas and concepts that decrease their energy costs over the life of the building. But beyond just energy, the systems have to be responsive to educational needs, so that means great thermal comfort and air quality too. While certainly not qualifying as a new technology, I’m really excited about how many of our projects are using natural ventilation strategies. Early in my career, getting a school district to consider operable windows or fresh air louvers was an uphill battle.

Oathout: The new “big thing” in HVAC system design for K-12 facilities is variable refrigerant flow (VRF) systems. The VRF systems offer a compelling combination of energy efficiency and affordability. There is still work to be done by the manufacturers to make these systems more occupant- and operator-friendly.

Najafi: We are very impressed with the energy performance approach of distributed VRF with ground coupled (geothermal) thermal sinks. This employs a three-tier energy recovery approach, with localized direct exchange of heat and low transport energy (as with refrigerant flow only), a building level exchange via water based transport, and a seasonal exchange via the ground coupled heat sink. Also, we are seeing displacement ventilation play a large role in energy savings when applied appropriately.

We are currently using displacement ventilation in our design of the Duke Ellington School of the Arts. The large 850-person theater at the school provides an excellent opportunity for displacement ventilation and cooling by capitalizing on the natural stratification that occurs as warm air rises because of the high ceilings and the high occupancy of the space. Cooling and ventilation will be supplied at very low velocities along the floor level and will be returned higher in the space. As a result, contaminants are forced away from occupants and more efficiently removed from the space as opposed to being mixed throughout the space. This approach also reduces the amount of energy required to cool the space as the supply air is provided first to the occupied zone prior to mixing with the entire volume of air. The displacement approach reduces energy, and provides high thermal comfort and better indoor air quality, all of which contribute to an enhanced design.

Hedman: We see chilled beams becoming more relevant in school projects. With the noise criteria for classrooms set forth by ANSI 12.60, it is becoming difficult to design classrooms with unit ventilators and fan coil units similar to how classrooms were designed in the past. Chilled beams generate minimal noise in the space, allow for smaller duct runs for ventilation air only, and require little to no maintenance.

CSE: Do you find it more challenging to retrofit HVAC systems on older buildings than installing on new?

Najafi: Existing facilities do pose a retrofit challenge that adds another level of complexity compared to new building design, especially in the arena of coordinating with limited passageways (for equipment replacement) and hidden obstructions, but there are advantages that can be employed with some of these older, mass-based school structures that support lower differential temperature approaches, such as in radiant heating and cooling.

Hedman: Retrofitting older buildings is more challenging due to the existing structure. Systems in older schools were mostly designed for heating and ventilation only. Schools that are being retrofitted are now designed to incorporate air conditioning throughout. Existing ductwork cannot be used because it was sized for lower airflows (based on heating load) and not necessarily insulated. Therefore, as a design engineer on a retrofit project, we need to coordinate with the existing structure and architecture to incorporate larger ductwork.

Jefferson: Dealing with existing space limitations in older buildings can be a challenge. Too often though, designers don’t look at why it’s a challenge to retrofit these buildings. Most of these buildings didn’t use centralized, forced-air systems originally, so putting one in place today is going to create headaches. Instead, we look at the original design team’s approach, which typically involved steam or hot water piping (but no cooling). Thermodynamically, water or refrigerant is so much better than air for heat transfer, so we can move small piping around a building in much more compact spaces than what a ducted, air-based system would use. We still need to introduce ventilation air, but that system is usually much easier to accommodate than a centralized forced-air HVAC system.

Roy: It is almost always easier to design from a clean sheet than to try to integrate with existing systems. However, the relative ease isn’t necessarily a function of the building’s age. Many times, it’s easier to work with older buildings because they were built much more soundly and the existing MEP systems are generally much simpler. Whether gutting the existing and starting from scratch with an empty shell or retaining some or all of the existing systems, older (pre-1950s) systems are generally better documented, when original drawings are available, and it is easier to determine whether they are still viable or should be scrapped. Many times, there is greater pressure to reuse systems that are 1970s and 1980s vintage because they appear to be in functional condition, whether their operational efficiency justifies reuse or not. Most times, it is safe to say operation efficiencies trump functional conditions in determining whether to remove older systems. Also, code issues such as refrigeration availability and ozone depletion potential (ODP) calculations drive the elimination of older systems. However, project construction budgets many times require maintaining old, inefficient equipment. This is where the greatest challenges come up with retrofitting or renovating existing building systems: establishing capacity and condition, whether HVAC or electrical, when the budget doesn’t allow upgrading. Most times, it is easier to determine the capacity and condition of existing HVAC than existing electrical infrastructure. However, the challenge is always the same to establish the condition of concealed distribution systems like ductwork, piping, conduit, and wiring. If these distribution systems have to remain, they limit design options to less than optimal solutions.

Hammelman: It is more of a challenge to retrofit HVAC systems within older facilities, especially those built between 1950 and 1980, over retrofitting newer buildings. Often the buildings were not planned or designed to account for air conditioning, and now owners want to incorporate it. These facilities also possess environmental issues ranging from asbestos-containing materials to lead paint, which need to be remediated prior to construction, or they may not have adequate/existing electrical infrastructure and structural capacity to support the new HVAC equipment without significant upgrades and additional costs.