Making the grade with K-12 projects: HVAC
With state-of-the-art learning facilities, sustainability concerns, and modern design, K-12 schools can be just as advanced as colleges and universities and—for consulting-specifying engineers—just as demanding. The following focuses on HVAC systems in K-12 schools.
Mike Barbes, PE, LC, Senior Electrical Engineer, AECOM, Atlanta
Ben Hobbs, PE, Mechanical Engineer, CMTA Consulting Engineers, Lexington, Ky.
Timothy J. LaRose, PE, Vice President Development, Education, JENSEN HUGHES, Warwick, R.I.
Jason Moyer, PE, PMP, STS, Senior Mechanical Manager, Brinjac Engineering, Baltimore
Jon Rasmussen, PE, LEED AP BD+C, Energy+Engineering Leader/Senior Associate, DLR Group, Denver
CSE: Have you specified distinctive HVAC systems on any K-12 buildings? What unusual or infrequently specified products or systems did you use to meet challenging HVAC needs?
Moyer: We have completed an induction chilled-beam design for HVAC installation into an existing school that did not have cooling or mechanical ventilation. In conjunction with wall-mounted induction units, new dedicated outdoor-air units were provided to handle the total latent load of the building. This type of system design allowed the school to remove the existing unit ventilators and provide a system that is energy-efficient and improves indoor-air quality. This system design also has minimal impact on the existing structure because the ductwork is being minimized.
Hobbs: We’ve used ECM circulating pumps with a hard-wired flow sensor and a decoupled well field. ECM circulator pumps were used to allow for a distributive pumping system in a geothermal water-source heat pump design. Each circulator could provide constant flow at each heat pump without a restricting flow-control device downstream with varying system head conditions. This allowed each circulator pump to only consume the power required to flow its respective heat pump at the time, regardless of building loop conditions and prevented coil overflow. Variable-speed water-source heat pumps were used to maximize load matching and energy reduction in classroom spaces while also providing improved indoor environment control with long run times.
A water-source variable refrigerant flow (VRF) system was used to provide a geothermal solution to an existing building with limited ceiling heights and without having to locate compressors in the learning environments. As an alternative to using carbon dioxide (CO2) sensors, we have provided an Aircuity system to monitor CO2 levels and control demand-control ventilation. This was implemented as a means to mitigate the large variance we were finding in our CO2 sensor readings when providing a standard wall sensor. Fan arrays have been specified to create redundancy, simplify maintenance, and reduce fan power in large, dedicated outside-air systems. We have also used this technology to acquire a greater turndown than a variable frequency drive can provide on a single fan to address areas, such as community centers, in K-12 institutions that operate at different schedules but still require outside air. The dedicated outside-air unit can be reduced in capacity to provide air solely for the uniquely occupied space.
CSE: Have you specified VRF systems, chilled beams, or other types of HVAC systems into a K-12 building? If so, describe its challenges and solutions.
Moyer: We use VRF systems for office administration and health suites for K-12 schools, as these areas often have longer and different operating schedules than the rest of the school.
Hobbs: I have specified VRF systems, and typical challenges are following the refrigerant code requirements from ASHRAE 15 and ASHRAE 34, heating at de-rated capacity, and cooling and heating setbacks. To adhere to the refrigeration codes, the most effective three solutions are communicating between multiple spaces to increase volume, changing the size of the heat pump system to adjust the quantity of the refrigerant, or increase volume by lowering ceilings. De-rates of VRF systems can be addressed by increasing system capacities, identifying design winter temperature, and setting owner expectations in extreme weather conditions. Cooling and heating setbacks must be manually or intelligently adjusted through controls so that the building recovers by occupied times. Or, I have used a dedicated outside-air unit that can recirculate prior to occupancy in concert with the VRF system to bring the building up or down to occupied temperature. I also have used variable-speed water-source heat pumps that, when a malfunction occurred, presented challenges in acquiring parts in a timely manner. This was addressed by asking the supplier to stock parts locally so we could continue providing this product as a solution to prospective owners and repair units with our current clients.
Rasmussen: Specialty-type solutions like VRF, radiant slabs, and chilled beams require extensive early coordination and discussion with all of the stakeholders. VRF has great advantages, such as allowing simultaneous heating zones and cooling zones, reduced distribution space requirements (piping versus ducting), and easy expandability. But a project also needs to have experienced installers and a willingness to support a manufacturer’s proprietary system. Radiant heating and cooling systems can greatly reduce a building’s energy usage, but users need to be educated on the response times and shift their familiarity with airflow. Chilled-beam systems tackle their loads extremely well, but attention has to be made to operable windows and varying dew points. All of these factors are easily managed and accommodated by a design, but only when understood and embraced by the entire project team in the early stages. This eliminates surprises and helps prepare facility operators for their new systems.
CSE: What unique HVAC requirements do such projects have that you wouldn’t encounter in other projects?
Hobbs: The most unique requirements I find in K-12 projects generally are created from the school’s necessity to create flexible-use spaces that can vastly vary in type of use and number of users (i.e., collaborative learning spaces and mixed-use spaces, such as auditorium/cafeteria, gym/large event area, etc.). These nonspecific-use spaces create a wide variety of design solutions that must be carefully coordinated with the owner to meet expectations.
Rasmussen: Schools and the learning environment have a critical focus on noise. Our design must take into account solutions for sound attenuation. By performing acoustical analysis studies, we can implement mitigation strategies so that systems can accomplish their goals without disrupting education.
Moyer: Schools often have various functions and do not simply operate during the day. It can be challenging to design an HVAC system that operates various zones at completely different operation schedules. This impacts the total system capacity and how it operates.
CSE: When retrofitting existing K-12 school facilities, what challenges have you faced and how have you overcome them? When addressing indoor-air-quality issues, what best practices or tips do you have for other mechanical engineers? Describe air change rates, particle concentrations, humidity, and other issues.
Moyer: The biggest challenge of retrofitting an existing K-12 school facility is the school’s operational hours and maintaining occupant safety and comfort during construction. This can be achieved by developing a relationship with school administrative staff during design, which allows us to fully understand a normal day’s activities and then provide a phased design to accommodate that as much as possible. When construction begins, the contractor should also develop a relationship with school staff to maintain safety and comfort during the construction phase.
CSE: What are the current challenges when specifying HVAC systems for educational facilities?
Rasmussen: Flexible and adaptive spaces present a distinctive challenge to a mechanical system. Defining space zones, how they’re used, and the comfort ranges is how equipment is sized. Controls systems drive the equipment to supply the spaces as they’ve been defined. Variety and adaptability are great for space planning and programs, but require versatility for a mechanical system and sophistication in its controls. This doesn’t necessarily mean complexity, but a system needs to be aware that its target environment has changed. These are not impossible requirements to provide—engineers are great at designing systems that can be dynamic and robust. These challenges simply need to be discussed, coordinated, and understood as the process for accommodating the needs of today and in the future.
Moyer: It’s a challenge to provide a system that is simple to operate and maintain for the next 20-25 years while complying with energy-conservation requirements.
Hobbs: Challenges include mechanical spaces and the programming square-footage impact. Also, understanding that generally more efficient equipment requires greater first cost and generally requires more space—i.e., larger ducts and pipes to reduce friction, bigger heat exchangers for better efficiency, larger equipment. Space flexibility is an additional challenge. A one-design-fits-all HVAC system, in terms of space use, can be complicated and expensive. Clear communication with the owner to meet expectations is required.
CSE: How do frequent changes to codes and standards impact the commissioning process for HVAC systems?
Moyer: They can have a significant impact if the changes ultimately result in a cost impact, affecting budgets that are set perhaps almost 2 years in advance of completion of the projects.
Rasmussen: Commissioning of a building’s system to demonstrate compliance has become a necessary part of the design and construction process. As we work toward a more sustainable and energy-efficient world, codes and standards change to meet the challenge. Essentially, codes are becoming more stringent, calling for more insulation in wall and roof systems and increased energy performance from windows and skylights. Codes also require lower energy consumption from lighting systems, including controls that turn off lights when the space is unoccupied. HVAC systems require better control to reduce energy consumption for heating and cooling. Often, the control and operation of these systems become more complicated than in the past. The owner benefits from these code requirements through increased energy savings.