Designing Efficient Schools: Design Strategies
MEP and HVAC trends for designing a LEED-certified school.
By Sunondo Roy, PE, LEED AP, CCJM Engineers, Chicago
Variable flow systems: It is almost a given that all new air handling systems will be variable air volume, and most energy codes now mandate the same for multizone systems and those with fan horsepower greater than a calculated threshold . However, there are systems such as single zone localized heat pumps, fan coil units, and mini-split units that are almost always constant air volume. Similarly, central hydronic pumping plants are also typically variable water flow as well as variable temperature.
Large super systems or distributed air systems: One increasingly rare design approach is a “super system” air handling unit (AHU) that has been installed to serve the entire building. Typically, these systems are custom-built air handling units with two or three very large (40,000 to 50,000 CFM) variable pitch vane axial fans that feed tunnels or shafts to the various areas of the building with terminal VAV units controlling the supply to discrete spaces. The drawback to this type of system is that equipment redundancy is inherently limited with severe system impact when any of the major components fail.
The prevailing design approach to HVAC systems is distributed air handling units, whether indoors or rooftop, which are configured to serve complementary spaces. Typically, separate AHUs will serve classrooms by floor or wing, the gymnasium, lunchrooms, and auditoriums. Depending on the size and configuration of the gymnasium, lunchroom, and auditorium, it is possible to consolidate the AHUs serving these areas if they are in a single multipurpose use space. This is typically the case in smaller elementary schools. High schools with much large student populations and programmed spaces dedicated to common functionalities invariably benefit from separate AHUs due to reduced impact of any one equipment failure and the ability to select specialized energy recovery equipment for high outside air percentage systems.
Hybrid boiler plants: Another increasingly accepted solution is a hybrid heating hot water plant consisting of a combination of noncondensing and condensing boilers. Conventional boilers operate at efficiencies of 80% to 85% during peak heating loads, requiring higher discharge water temperatures. Condensing boilers also operate from around 80% to 85% efficient at similar peak conditions, but operate near 92% to 95% during shoulder seasons and for loops requiring low operating temperatures, such as underfloor loops and reheat systems. Although a total condensing boiler plant would be the simplest to operate at the optimal efficiency under any given condition, the cost premium for condensing boilers typically makes such a solution untenable for most clients. The compromise is to install a couple of noncondensing boilers to operate during the peak heating season and another couple of smaller condensing boilers to operate during low load periods.
The most energy-efficient operational sequence for these systems requires the fewest number of noncondensing boilers to operate at their highest firing rate to meet the peak load while running all the condensing boilers at the lowest possible firing rates during low loads. This dichotomy is due to the fact that condensing boiler flue gases will condense the most and deliver the highest efficiencies when the return water temperature is the lowest. This is achieved by distributing the heating load over the largest surface area of heat exchangers, i.e., multiple boilers.
Demand control ventilation (DCV): It is reasonably safe to say that demand control ventilation has become institutionalized in most HVAC design, schools and otherwise, in large part due to municipalities adopting some form of DCV within their energy codes (typically based on some variant of ASHRAE 90.1). With the improvements in CO2 sensor technology in the past 5 to 7 years, this strategy to adjust building outside air based on zone CO2 concentration is one of the most reliable ways to reduce overall energy usage in the HVAC heating and cooling plant. The challenge in DCV is properly designing the HVAC systems and controls schemes to ensure all spaces are adequately ventilated without over-ventilating at a system level to satisfy the “worst” zone. A complementary strategy to avoid this situation is to use a DOA system described below.
Dedicated outside air (DOA): A DOA unit is the best solution to ensure every space in the building receives code required ventilation because it is designed to deliver neutral temperature outside air to every space. The drawback to these systems in a conventional centralized air handling unit design is that they require a secondary air distribution system to operate in parallel along with the general heating/cooling air distribution system. An effective HVAC system using DOA units couples this unit with local unitary heating/cooling units such as water source heat pumps, variable refrigerant flow mini-split systems, or other similar equipment that are sized for space skin and internal loads. An effective strategy to further reduce energy usage is to provide local control of the DOA distribution and interlock the local shutoff valve and the local heating/cooling units with a room occupancy sensor that also controls the room lighting. Since classrooms are typically empty for at least two out of nominally eight periods (recess/library/multimedia and lunch), the net energy usage per classroom due to internal loads can be reduced by approximately an additional 25%.
VAV integrated with occupancy: The control strategy noted above for the DOA solution can also be implemented for conventional centralized VAV systems served by AHUs or rooftop units (RTUs). Although the control systems are more complicated and require careful commissioning of all the integrated systems, the overall energy savings of setting back HVAC for the classrooms for one quarter of the school day more than justify the upfront costs.
In-slab heating: This design solution is particularly effective for single-story buildings with high floor-plan-to-wall-area ratios or first floors of multifloor buildings with a high population of toddlers through third graders. These occupants are far more prone to spend much of their time in contact with the ground as part of their structured instruction as well as playtime. Underfloor heating and carpeted or cork mat flooring can provide a warm, comfortable environment during the heating season in the “occupied zone.” This design solution still requires separate cooling and ventilation systems. This solution lends itself well to ground source water-to-water heat pumps to condition the in-slab water distribution system; the low-grade heating developed by the heat pumps is ideal for this application.
Displacement ventilation: Displacement ventilation is not an optimal design solution for classrooms. Particularly, it is not a desirable solution for the toddler through third grade student population for the same reason in-slab heating works well. Displacement ventilation works best when the airflow can “percolate” up from the floor level in a relatively laminar fashion to displace the warmer air without excessive turbulence and accompanying drafts. It is virtually impossible to achieve this type of laminar flow with children on the floor for much of the time. Additionally, there is a high probability that the children will be dropping things into the floor level diffusers, causing a maintenance nightmare. The problem we have anticipated for conventional classrooms with older children is that desks, backpacks, and other floor impediments will generally impede effective airflow distribution, whether through floor grilles or floor base level sidewall grilles.
Enhanced lighting control: A proven strategy that allows energy savings as well as user satisfaction with the space is a high degree of lighting controllability through motion sensors and active lighting controls through photocells that react to ambient room lighting levels and adjust artificial lighting output to maintain a set point level. This effect can be compounded with the use of light shelves, which draw in diffuse reflected daylight without the solar gain and glare of direct sunlight.
Reduced lighting power density: Reducing overhead lighting levels and providing localized task lighting in administrative spaces and teacher work spaces also provide incremental energy saving and deliver a more pleasant lit environment.
--Roy is vice president with CCJM Engineers. He is a cross-disciplinary mechanical engineer who has successfully designed integrated mechanical/electrical systems for LEED-certified schools, as well as commercial, aviation, industrial, and institutional facilities for the past 22 years.