Enhancing the learning experience in K-12 schools: HVAC
K-12 schools are among the most important projects engineers can tackle. Mechanical engineers focus on HVAC systems, including unique heating and cooling options, building envelope, and air quality.
Tony Cocea, PE, Principal, DLR Group, Los Angeles
Michael Do, CEM, CxA, AX TCP, Director of Engineering Sciences and Commissioning, Setty, Fairfax, Va.
James Dolan, PE, CEM, CPMP, LEED AP, Principal in Charge of Energy Engineering Services, OLA Consulting Engineers, Hawthorne, N.Y.
Mark Fisher, PE, LEED AP, Principal, AlfaTech Consulting Engineers, San Jose, Calif.
Douglas R. Hundley Jr., PE, CGD, LEED AP, CxA, Mechanical Engineer, CMTA Consulting Engineers, Louisville, Ky.
Peter McClive, PE, LEED AP, Senior Vice President, CannonDesign, Grand Island, N.Y.
CSE: What unique HVAC requirements do K-12 building projects have that you wouldn't encounter in other buildings?
Cocea: In K-12 buildings, some HVAC requirements include noise issues and the level of air circulation in elementary schools. This is a consistent consideration in the K-12 environment that often requires a custom design solution on a project-by-project basis. Hundley: One unique requirement is the transitional loading of a building. There are different loading requirements, from all students located in a classroom to all students located in a gymnasium or auditorium for a school assembly. Taking advantage of these different loading conditions can result in smaller central plants or outdoor air systems.
Fisher: Most classrooms are served by individual units (no variable air volume, or VAV, system), with higher ventilation rates and tight acoustic requirements.
CSE: What changes in fans, variable frequency drives (VFDs), and other related equipment have you experienced?
Dolan: ASHRAE Standard 90.1, which informs energy-code requirements, has continued to increase performance levels. As states adopt later versions of the standard on their energy code, a key metric is overlooked: brake horsepower/cubic feet per minute (bhp/cfm). Long-term, this metric is important to optimize as it translates to ongoing savings or, if ignored, penalties for the owner. Working early in the process with the owner and architect to put mechanical rooms or rooftop equipment near the end use is important to reduce that fan horsepower. Moving the energy via hot and chilled water is a winning strategy as well. We have encouraged the use of a complete perimeter hot-water heating system to enable fans to be off when the school is not occupied.
Fisher: More efficient systems are generally available, and the size and cost of equipment are smaller than in years past.
Hundley: We have seen a trend toward more direct-drive fans, even in larger air handling units. VFDs are commonplace, and the cost is equal to a typical electric motor starter. We use VFDs on all pumps and fans over a certain size, regardless of constant volume operation. The VFD gives you more flexibility in balancing and a built-in soft start, reducing the inrush current associated with starting a motor. We also see most unitary equipment using electronically commutated motors, reducing energy usage while providing flexible operating conditions.
CSE: When retrofitting an existing K-12 building, what challenges have you faced and how have you overcome them?
Hundley: Older school buildings were built to be two-pipe unit vent schools, so the building structure is very low. We have developed a number of strategies to overcome this, depending on the system type the owner prefers. One example is unitary closets placed in the classroom to house a ground-source heat pump, with a soffit running the length of one side of the classroom.
Cocea: The absence of good record drawings has been a challenge I've faced recently. Field investigations and design coordination meetings that included facilities maintenance personnel helped to overcome those challenges.
CSE: Have you specified any combined heat and power (CHP) systems on a K-12 building?
Hundley: Most K-12 facilities do not have the required minimum base load to provide attractive ROI for CHP systems. Renewable energy systems on high-performance projects reduce this base load further and at times eliminate it altogether.
Fisher: Microturbines, using waste heat to heat pools, have been used on a number of local campuses, although the waste heat is generally not as usable in California buildings' heating systems.
CSE: What indoor air quality (IAQ) or indoor environmental quality (IEQ) challenges have you recently overcome? Describe the project, and how you solved the problem.
Cocea: A recent elementary school I worked on was located near the I-10 freeway in Santa Monica, Calif. It was continually impacted by air pollution due to its proximity to the freeway. Outside air (OA) intake filters were provided to arrest pollutants, and economizer controls were used to deliver 100% OA into the spaces based on wind direction.
Hundley: On a recent renovation project, the school was experiencing issues with high humidity levels and mold growth above the ceiling plenum. We performed a building assessment and identified that the building was running at a negative pressure: The building controls were nonfunctional and a large majority of the outdoor air dampers on the unit ventilators were closed. All exhaust fans were running continuously and inducing unconditioned outdoor air through any available opening in the building. The building was renovated from a two-pipe unit ventilator system to a ground-source heat pump system, with dedicated outdoor air units with energy recovery. The project cost was greatly reduced by reusing the existing piping, pumps, ceilings, and lighting. After the renovations, the school energy consumption was reduced by 60% and all IAQ issues were addressed.
CSE: Have you specified more alternative HVAC systems on K-12 projects recently? This may include displacement ventilation, underfloor air distribution, variable refrigerant flow (VRF) systems, chilled beams, etc.
Do: A tremendous amount of our K-12 designs have begun to implement VRF systems in conjunction with dedicated outside air systems supplying shutoff boxes for ventilation purposes only. On the commissioning side, we have seen chilled beams implemented in several recent designs but have not used that strategy extensively in our own applications. We have found that the VRF generally presents a fast and comparatively inexpensive installation as compared with other traditional systems. Elementary schools, in particular, offer the perfect profile to maximize the energy efficiency of these systems while offering the ability to cool and heat simultaneously and still maintain zoning with multiple condensing units.
Fisher: Yes, several. We have specified VRF and chilled beams for community colleges.
Dolan: Displacement ventilation, underfloor air distribution, VRF systems, chilled beams: all of these systems have their applications where they are appropriate. A key for selecting systems for schools is the capabilities of the operators as well as the overall lifecycle cost of the system being considered. Small refrigerant-based systems may not be as robust as a more traditional institutional approach using chillers and boilers. A system such as displacement ventilation is best when there are multiple benefits. Working in combination with demand-controlled ventilation (DCV), this approach saves significant energy consumption. Each building is custom; therefore, each system and how they are applied should be as well. Optimizing the size of the system needed, how it is applied, and what is best for the long-term vision of the school district will result in the most appropriate application.
Hundley: We have specified VRF systems, and used the other system types in non-K-12 buildings. As a standard, our firm is big on collecting data, and from an energy perspective, these systems have not resulted in the energy efficiency we have seen from a ground- or water-source heat pump system, or even a high-performance VAV system. Technology may improve, but for a true high-performance system, you can't beat the ground-source heat pump.
CSE: Describe a challenging building envelope project you recently designed in a K-12 building.
Hundley: The building envelope is a critical part of a high-performance building. We like to be an integral part of the building envelope design with the architectural team. With the increased complexity of building designs, there is a constructability concern. For a building to be truly high-performance, the building envelope has to be well-insulated with a tight air and vapor barrier. To help address some of the system complexities, it is best to reduce the workmanship required to cut and fit insulation. A couple of strategies used include spray foam insulation and insulating concrete form (ICF) walls to accomplish the increased insulation and tight air and vapor barriers. Our best-performing buildings have used spray foam or ICF walls. Window tuning is also important. We have used different types of glazing, depending on the building exposure. West- and south-facing glass result in larger solar loads, and spending the money to increase the shading coefficient at these exposures will produce better results than using it across the entire building.
Fisher: All of the schools in the North Bay Area (California) had heating systems only, and the owner wanted air conditioning units. The challenge was that there was no insulation in the walls and very little on the roof, so the building envelope was key. It was too costly to remove all the interior Sheetrock, insulate the building, and reinstall the Sheetrock and finish it. Instead, we blew in the insulation between the studs to insulate the walls, which is a typical method for residential projects. And rather than insulating under the roof framing, we put the bat insulation on top of the ceiling grid.