Enhancing learning in K-12 schools: HVAC systems

Doug Everhart, PE, LEEP AP, K-12 education practice director|principal, Henderson Engineers, Lenexa, Kansas

Keith Hammelman, PE, principal, CannonDesign, Chicago

Brian A. Hummel, PE, LEED AP BD+C, mechanical engineer, senior associate, DLR Group, Phoenix

Richard Sparozic, PE, mechanical engineer, Kohler Ronan Consulting Engineers, Danbury, Conn.

Casimir Zalewski, PE, LEED AP, CPD, principal, Stantec, Berkley, Mich.

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Describe a recent project in which you addressed indoor air quality issues to account for COVID-19 health concerns.

Keith Hammelman: As a member of the ASHRAE Epidemic Task Force for reopening schools, I have been working with clients to ensure that HVAC equipment provides the maximum amount of outside air without sacrificing temperature and humidity levels within the building. We have also been evaluating existing equipment filtration systems to provide a minimum of a MERV 13 filters.

In nursing suites and offices, we have designed in dedicated exhaust systems to ensure that these spaces are maintained at a negative pressure while also providing HEPA filtration systems for air which may be recirculated. We have also been employing the use of devices to measure the air quality of the rooms to validate that the adequate ventilation is being provided to these spaces. These devices include the measurement of particulate matter, CO₂, temperature and relative humidity within the space.

Richard Sparozic: Recently, many clients have requested that preventative measures be considered for our HVAC systems. Construction began on a particular project in late 2019 that presented unique challenges. The design team was tasked with implementing improvements to air quality in an effort to reduce exposure to students. It required careful consideration of equipment and air handling units that had already been purchased. In coordination with AHU manufacturers, we assisted in implementing ultraviolet lights and improved filtration systems without significant system changes.

Casimir Zalewski: We recently completed the design of a 162,000-square-foot middle school infrastructure replacement project. The school used large central station air handling units with limited filtration and air movement to each classroom. We replaced these systems with multiple dedicated outdoor air systems using MERV 14 filters and energy recovery to pretreat the outside air. The dedicated outdoor air systems used purges on the energy wheel to protect against cross contamination. Each classroom was provided with variable refrigerant flow cassettes with energy recovery that mixed fresh air with classroom air shifting heating and cooling energy from one space to another through the refrigerant system.

The overall HVAC system was designed to limit room-to-room mixing of supply (ventilation) air. Additionally, the central systems were designed with additional ventilation capacity and the systems were programed through the building automation system to be able to quickly increase ventilation rates and provide either an optimum start or pre and post-ventilation mode of the spaces.

Doug Everhart: During COVID-19, many of our largest districts in the Kansas City metro system have come to Henderson for guidance for existing building assessments with the focus of providing safe learning environments to facilitate in-person learning. Since many districts are not on the timeline for major HVAC system changeouts, our assessments during COVID-19 have focused on the review of air distribution, ventilation and filtration to determine what systems can accommodate increased filtration and increased ventilation to mitigate the virus transfer in learning spaces.

We are also looking at occupant load and mechanical system zoning to determine what modifications may be necessary to allow for spaces to be used differently than perhaps first intended with the need for students and staff to spread out and occupy more flexible spaces with room to socially distance. Many efforts are also revolving around limiting the number of devices or fixtures that need human touch and plumbing retrofits are including touchless/automatic faucet and flush valve change-outs.

Because many schools were closed for extended periods due to COVID-19, how have you addressed the water issues, such as bacteria growth in pipes? Other health and environmental concerns?

Doug Everhart: We recommend thoroughly flushing domestic water piping and ensuring hot water recirculation pumps are operating correctly before occupants return to the building. Per the Centers for Disease Control and Prevention, Legionella growth is most conducive within the 78°F to 108°F temperature range. We recommend setting hot water tank setpoints to 140°F and verifying thermostatic mixing valves are set properly to prevent scalding.

Keith Hammelman: With the building systems and buildings being closed for an extended period of time we have been using the guidelines prepared by the ASHRAE Epidemic Task Force Building Readiness Guide to prepare these facilities. This guidance has been important to use because it focuses on procedures to maintain a safe facility during the extended unoccupied periods and also provides guidance as the facility opens back up.

Richard Sparozic: Typically, Kohler Ronan implements chemical treatment on closed loop building systems (heating hot water and chilled water) via the use of chemical shot feeders. In open systems, such as the toilets and lavatories, it is recommended that these systems are flushed. Water heaters should be checked to confirm that they are maintaining a minimum of 140°F water temperature to prevent Legionella. If possible, water heaters should be drained and refilled.

What unique heating or cooling systems have you specified into such projects?

Doug Everhart: Gymnasiums located in K-12 buildings present some unique challenges to the HVAC designer specifically. They have high latent loads resulting in low room sensible heat ratios and require large amounts of ventilation air. The dramatic difference in load when occupied versus unoccupied also presents challenges. To successfully design HVAC for gymnasiums in K-12 applications in climates with intense fluctuations such as the Midwest, we’ve developed some best practices over the years to help us meet the diverse needs of the occupants.

• Do not overestimate the load; because of the drastic difference between peak and minimum loads in a gym, artificially inflating the load can potentially exacerbate the problem of turndown and could shorten the life of the equipment.
• Use an indoor summer design condition higher than what’s used for other areas; since simultaneous peak load and peak occupancy won’t occur often and if they do will be for fairly short durations, you can design to a higher indoor temperature and humidity with minimal risk of issues.
• Select rooftop units with good load-matching capability and do not oversize them; it’s better to have a slightly undersized unit so the compressors operate for a longer time period, thereby providing more effective dehumidification
• Use a dehumidification sequence; specify the unit with a dehumidification sequence that uses hot gas reheat, if there is not enough hot gas reheat, the space may overcool during a dehumidification sequence.
• Communicate with the owner; every gymnasium is different and thousands operate successfully at lower and higher humidity than recommended but it’s important that the owner be informed via a dialogue about first cost, risk and long-term operation.

Richard Sparozic: The firm has implemented numerous types of HVAC systems in K-12 school. We have specified radiant floor heating/cooling, variable refrigerant volume (VRV), dedicated outdoor air Systems, active chilled beams and more conventional variable air volume systems. Each project presents unique challenges in selecting building HVAC systems and should be considered on a project-by-project basis.

Many building renovations, for example, will typically have lower floor-to-floor heights. Systems that primarily use water as a heat transfer medium will assist with reducing ductwork sizes. As a result, dedicated out door air systems, paired with either VRV or chilled beams, would be important to consider.

Keith Hammelman: While this may not be unique to certain parts of the country, we are seeing a continued focus on geothermal heat pump installations with a dedicated outdoor air system to provide ventilation air to the classrooms. We like to provide the decentralized heat pumps located in closets that are remote from the classrooms so that acoustics can be addressed in the classroom along with maintenance of the equipment. In areas of the country with higher humidity levels, we have been employing the use of a dehumidification cycles to maintain humidity levels within the spaces within acceptable levels. We continue to see a larger focus on not only maintaining an adequate temperature within the space, but also controlling to maintain humidity levels of the spaces.

What types of unique building pressurization have you designed in K-12 schools?

Casimir Zalewski: In big box spaces such as an elementary school cafeteria, gymnasium or media center, we typically will provide a rooftop unit with a variable-speed relief fan controlled off building space pressure. In larger middle and high schools with larger rooftop units or central station equipment, we normally recommend more volumetric tracking of outside, supply and return air. The decision to use one over the other typically relies on project size, construction cost and the reliability of the technology and control programming available in the local market.

Richard Sparozic: Building and zone pressurization is important to consider when hazardous gases or chemicals are present in a room. Kohler Ronan previously designed a high school that housed lab classrooms as well as an autobody shop and paint spray booth rooms. Each space demanded careful consideration and expertise. It was important to ensure that noxious gases did not migrate to other areas of the building. We implemented dedicated exhaust systems that adjusted based on heating/cooling demands of each zone and ensured that the space remained at slightly negative pressure in relation to adjacent spaces.

What unusual or infrequently specified products or systems did you use to meet challenging heating or cooling needs?

Keith Hammelman: With the increased desire of clients to provide facilities which rely on electrical power for both the heating and cooling demands we are seeing a greater use of the variable refrigerant flow technologies within schools. The systems which are being deployed within K-12 schools rely on dedicated outdoor air systems to provide the necessary ventilation air to the buildings. These dedicated outdoor air systems are provided with means to monitor and control the amount of outside air delivered to each space to increase energy efficiency of the spaces.

The VRF technology for the classroom environment employs the use of a heat recovery system, which allows for the VRF system to provide both heating and cooling to different zones within the building. This technology is being employed in the majority of our projects which used VRF. For rooms with a consistent cooling need such as main distribution frame, intermediate distribution frame or electrical rooms we will employ the use of a traditional VRF heat pump system.

Casimir Zalewski: The Michigan climate can make it difficult to design HVAC systems. The climate has large swings from subzero temperatures in the winter to over 90°F degrees in the summer and fall. The proximity to the Great Lakes also provides for high humidity, depending on location.

Recently, we designed multiple dedicated outdoor air systems with energy recovery and dehumidification to provide neutral air to in-room variable refrigeration cassettes. The central DOAS units are sized for all space ventilation and latent requirements while the sensible heating and cooling requirements are handled by the cassettes and the refrigerant coils. In this varied and moist climate, there are typically a mixture of zones requiring heating while others need cooling.

Richard Sparozic: Kohler Ronan has designed several successful projects using active chilled beams to meet room heating and cooling loads. These types of systems afford individual control with the ability to heat or cool at the room level. A proper application for the use of chilled beam systems would be to help counter issues with reduced ceiling cavities due to either existing conditions or high ceiling requirements. Careful consideration of the building’s envelope must be given when designing systems such as chilled beams as condensation is a concern. A de-coupled chilled water loop with slightly higher supply temperatures, as well as space humidity and condensation sensors in the ceiling cavity, are often use to reduce this risk.

How have you worked with HVAC system or equipment design to increase a building’s energy efficiency?

Casimir Zalewski: HVAC equipment has varying levels of efficiency. A first step is typically looking at the air velocity through an HVAC unit. Increasing the casing size from 500 or 550 feet per minute down to around 400 fpm can have dramatic effects on the filter and coil pressure drops and subsequent energy required. Working with manufacturers to specify variable speed refrigeration compressors can greatly enhance part-load efficiencies. Looking at optimum coil and fan selections or pumps in hydronic applications bleeds a few more watts of power out of the system.

The next big step is understanding what energy saving control algorithms are available, such as static pressure reset, demand control ventilation, economizer operation and building pressurization control. Once the equipment and controls are as efficient as possible, assisting the trade professionals in making sure that the equipment is set up and operating maximizes energy savings.

Richard Sparozic: Our engineers work frequently with manufacturers to make sure that the equipment we specify can continually operate at peak efficiency. For example, when determining chiller plant configurations, it is important to consider equipment part-load performance; equipment will operate more frequently in this range than peak.

Keith Hammelman: When it comes to energy efficiency of buildings, we always look at the building orientation and building envelope to minimize the size of the HVAC equipment first. We employ early conceptual energy modeling to complete this effort. After the orientation and envelope is addressed then we review the lighting and plug requirements for the spaces and focus on how to reduce this load as much as practical. From there we focus on providing an energy efficient HVAC system that meets the needs of the building, often employing an energy recovery system for ventilation air conditioning to further reduce the energy use of the building, while providing a healthy facility. With all designs we ensure that the client is an integral part of the decision-making process so that they can ultimately ensure the systems are operating at the highest efficiency levels possible.

What best practices should be followed to ensure an efficient HVAC system is designed for this kind of building?

Keith Hammelman: The best practice to ensure that efficient HVAC systems are designed and installed within K-12 buildings is to understand the local contracting community familiarity with installation of different types of systems. Along with the contracting community ability to install a system we do work to understand a client’s ability to operate and maintain a system. By understanding these factors, we work to provide a variety of options which can be pursued and then work with the team to provide a solution which is successful for the contractor to install and the client to operate over the life of the building.

Casimir Zalewski: One size doesn’t fit all. Engage your client in a dialog to understand their comfort level to maintain and operate the HVAC systems. Besides the facility and maintenance staff, what is the proximity of support from either an equipment representative or distributor for potential parts and service. Besides the equipment, the same type of questions needs to be discussed regarding automation and controls. A district could have the most efficient building and HVAC system, but if it is not operating per the design intent, the investment could easily be lost in expensive utility bills. Understand the project, the trade contractor’s familiarity with the systems and the support for both the contractors and the client during construction and into operation and tailor the design based on those factors.

What type of specialty piping, plumbing or other systems have you specified recently?

Keith Hammelman: In a project that we worked on in the Caribbean where all of the water for the island is produced from desalinization plants, which is a mixture of ground water and ocean water, we have employed the use of a full rain water collection system to use in the flushing as a flushing water system connected throughout the campus. This system also is interconnected into the tanks, which are used to provide emergency water in the case of a hurricane and allows these tanks to be continuously flushed out and replenished.

What are some of the challenges or issues when designing for water use in such facilities?

Casimir Zalewski: Renovations to K-12 buildings’ plumbing infrastructure can be difficult. Most of the buildings were designed to older standards with higher use plumbing fixtures and based on a time when students took showers as part of the regular day. Domestic piping is typically oversized leading to low velocities allowing biofilms to form within piping and increasing hot water wait times. Sanitary piping is also typically larger and less reliant on slope. The reduce flow fixtures, gradual pitch and long runs can lead to reduced performance of the drainage system.

When designing school natatoriums, what challenges have you met and how did you solve them?

Keith Hammelman: In designing a natatorium particular attention needs to be paid to the air distribution in the space to ensure that drafts are not provided on the swimmers within the space. We also need to plan on how the HVAC system will operate year-round for this critical environment, which may necessitate the installation or operation of additional supplementary equipment, such as boilers, which many school districts may not operate year-round. These items may be crucial so that the dehumidification systems within a facility can operate at optimum performance.

Casimir Zalewski: A challenge in most natatoriums is addressing the dehumidification needs. In a northern climate, I am fortunate that we can use outside air to dehumidify for extended timeframes. Another key challenge is chloramine removal that forms just above the water surface. The chloramine issue is typically addressed through fresh air, low returns and fabric distribution systems design to skim the water to capture the chloramines and direct them either to the return or exhaust systems. Making sure the system has sufficient fresh air and that the natatorium is negative to the surrounding spaces address odor issues. Introducing a plate energy recovery device to pretreat (heat) the fresh air reduces large heating costs to maintain the elevated room air temperatures. Radiant floors, walls and benches also help swimmers and divers stay warm as they wait for their next turn to complete.

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