Improving indoor air quality and energy efficiency in K-12 schools

K-12 insights

• Amidst the COVID-19 pandemic, public school districts are prioritizing indoor air quality assessments and HVAC system upgrades.
• Best practices for designing efficient HVAC systems in K-12 buildings involve establishing energy usage benchmarks, conducting energy modeling analyses and collaborating closely with the school district.

Courtesy: WTWH Media

Respondents:

• Keith Hammelman, PE, Principal, CannonDesign, Chicago
• Sean Holder, P.E., Principal, Salas O’Brien, Houston
• Steven Mrak, PE, Vice President, Peter Basso Associates, Inc., Troy, MI
• Johnny P. Wood, PE, LEED AP BD+C, CxA, CPD, Associate Vice President, Regional Market Segement Leader, Dewberry, Raleigh, NC

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

Steven Mrak: In the midst of COVID, we were approached by several public school districts that wanted help evaluating their existing building heating, ventilation and air conditioning (HVAC) systems, especially regarding air filtration and ASHRAE recommended minimum air changes. Our efforts consisted of field verification, review of existing airflow balance reports and ventilation calculations. Through our efforts, these districts were made aware of specific HVAC systems and rooms that did not meet current ASHRAE recommendations. They now have the ability to selectively target these spaces for improvements.

Johnny P. Wood: We recently have had the opportunity to help Durham Public Schools (DPS) in North Carolina replace the building automation system in several of their schools, along with a test and balance to make sure the proper amount of ventilation air is being provided throughout the building.

What unique heating or cooling systems have you specified into such projects? Describe a difficult climate in which you designed an HVAC system for a K-12 school building project.

Johnny P. Wood: We have had the privilege of including many different types of systems in our K-12 projects. Air cooled chillers, water cooled chillers, cooling towers, water source heat pumps, ground source heat pumps, hot water boilers, variable refrigerant flow (VRF) systems, built up air handling units and direct expansion cooling systems. Each of these systems have applicability to K-12 buildings, depending on the school systems they serve. In North Carolina, for instance, the climate is sometimes a bit challenging for HVAC systems. High temperatures in summer, low temperatures in winter and high humidity require proper system design and equipment selection/application for the systems to function properly.

What types of unique building pressurization have you designed in K-12 schools? Describe the project.

Steven Mrak: Building pressurization control can vary from simple gravity relief hoods to more complex variable speed relief systems that can provide makeup air accommodations for kitchens, general restroom exhaust or process exhaust. As these needs come and go throughout the day, it becomes hard to define a “steady state” condition regarding building pressurization. To help accommodate these different scenarios, differential building pressure control can be used to vary the speed of relief fans. This uses atmospheric pressure and compares it to the pressure inside the building, controlling the speed of the relief fans to provide a consistent, usually slightly positive, differential pressure.

Johnny P. Wood: Science classrooms are designed to be negative in order to contain the air within the room. Kitchen and dining rooms are designed to be negative in order to contain food smells within the preparation and eating areas. The overall school building is designed to be positive pressurized to prevent infiltration. Building pressure is normally controlled by a differential pressure sensor inside the building. The pressure sensor is typically utilized to control the speed of a building exhaust or relief fan.

What unusual or infrequently specified products or systems did you use to meet challenging heating or cooling needs? This might include active chilled beams, variable refrigerant flow, etc.

Steven Mrak: Like most systems, VRF has its place and applications. While maybe not a solution for all situations, one instance where it brings value is with limited ceiling space. The relatively small refrigerant pipes can be routed in tighter spaces than an insulated supply duct carrying the same cooling capacity. The VRF system can also provide occupants with the level of individual control expected. Using the heat recovery type VRF systems (also referred to as a three-pipe system) can increase the overall system energy efficiency by recycling waste heat/cool around the system.

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

Keith Hammelman: HVAC systems are just one of many systems in a building that use energy within a building, but they seem to be the primary focus on how we look to reduce energy use without first looking at other passive energy users first. Even before we start looking at the type of HVAC system to install within a K-12, building we are working with the architectural team to understand building massing orientation and envelope first by conducting early modeling exercises. Once this is complete, we move on to other energy users within the building such as lighting or process loads to work with those designers on how to reduce the heat produced by these processes. From there we then also focus on the ways to reduce energy consumption and introduce energy recovery methods into the design of HVAC systems. We are pushing this system selection into the earliest stages of the design process and making sure that we are bringing the owner into these discussions.

Sean Holder: Through a balanced approach between first costs, maintenance costs and energy efficiency, we have implemented more energy recovery equipment, higher efficiency equipment and innovative HVAC system control sequences to increase a building’s energy efficiency.

Steven Mrak: Implementing energy recovery devices in HVAC units has become more common practice. Beyond energy code requirements, energy recovery devices like enthalpy wheels, energy cores and plate heat exchangers are being used on smaller and smaller systems, trying to capture as much energy as possible, even down to the individual classroom unit level. While the energy savings at each unit may not be substantial, when multiplied by 30 or 40 classrooms, the savings can become real.

Johnny P. Wood: Spaces that are used off hours should be provided with independent systems and not share systems with the larger building. We make an effort to locate air handling units toward the center of the area service. This allows the ductwork sizes to be smaller, resulting in smaller fan horsepower and fan energy use.

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

Keith Hammelman: In order to ensure that an efficient HVAC system is designed for K-12 buildings, we start first looking at establishing an overall energy usage index for the project that will be used as a guiding factor in the design of our building systems, which includes the HVAC system. We also look at the relevant energy code and determine what is the minimum prescriptive approach HVAC systems that are stated within the energy code. From this we will use basic energy modeling to evaluate the systems as a whole, in combination with building orientation, envelope and lighting systems.

We also begin conceptual cost estimating of systems to determine how the different systems fit within the building cost models. Included in this process we work with the design team to understand the impacts to program space and square footage of the building. The impact to square footage is also included in the conceptual cost models to have a full picture of what each option would cost. Once we know the total picture of the systems, we can then make an informed decision with the district on the HVAC system.

Sean Holder: One best practice is to ensure that the school district is able to support an energy efficient system design. While many systems are highly efficient at initial installation, without continual maintenance the systems performance is degraded.

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

Keith Hammelman: A challenge we see in K-12 Facilities is providing for water efficiency, minimizing environmental impact and minimizing maintenance and operations while still meeting the needs of students and staff. An example of this is the design for the use of showers in a gymnasium locker room. The number of showers is often provided to meet a maximum load of occupants, community use during a disaster or other criteria that will rarely be achieved. The challenge we often see identifying how large of a water heating system is required, and we work with client to further understand the needs of the day-to-day occupant and the frequency of rare use cases to come up with a balance that utilizes resources effectively.

Johnny P. Wood: A frequent challenge is when water flush volume is reduced, flushing becomes difficult. This should be discussed with the owner before specifying these types of fixtures. On-site water reuse systems require the addition of more equipment and additional maintenance. Simple rainwater collection for irrigation has proven to be cost effective for our K-12 projects.

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