Classroom Ventilation: Meeting Today's Challenges
Budgets, codes, and technologies are constantly shifting and evolving. These days, even the classrooms are often portable. Staying one step ahead will depend on your knowledge of current design options and how to integrate them into the space.
It doesn’t get much more sensitive than a discussion about the learning environment for the youth of America. Optimizing the classroom HVAC so that students are not too hot or too cold while also providing proper outside ventilation air with energy efficient systems can be a challenge for engineers. This is especially true in a time of growing construction costs and pressure by underfunded school districts. The codes and standards are also getting more stringent, which starts to eliminate some of the older low-cost technologies that have been used for years in classrooms.
There are studies that show that proper ventilation in classrooms does indeed make a difference in learning. In addition, there are studies indicating fewer absences when more ventilation air is provided to the classrooms.
Back in the old days of the one-room country school house, before electricity was available for ventilation fans, the ventilation was “controlled” by operable windows that could be opened and closed for comfort control.
Now, engineers have more sophisticated technologies for control of ventilation air in school design. Engineers have the ability to measure ventilation air quantity and quality, too. Engineers also have a variety of technically enhanced systems for heating, cooling, and humidification to meet energy codes. All the while, there is a growing focus on comfort.
Codes and Standards
Meeting the code criteria is required. Meeting industry standards is generally optional, yet good prudent engineering embraces appropriate industry standards as well. The rationalization that a standard will only be used in a design if the budget allows is becoming more of a liability risk for professionals and school districts. More and more standards are being included in codes. The ASHRAE Standard 62 is the overriding standard for ventilation in occupied facilities. This standard has been used for HVAC design as of 1973. ANSI/ASHRAE Standard 62-1973, Standards for Natural and Mechanical Ventilation, presented minimum and recommended ventilation rates for 266 applications and became the basis for most state codes. This standard has been updated throughout the years and is currently published as ANSI/ASHRAE Standard 62.1-2016, Ventilation for Acceptable Indoor Air Quality.
Standard 62.1-2016 has incorporated a variety of changes including, but not limited, to these as noted on the ASHRAE summary page at www.ashrae.org:
Revised operations and maintenance requirements to better align Standard 62.1 with the requirements in ASHRAE/ACCA Standard 180-2012
New requirements to the Indoor Air Quality Procedure for determining minimum ventilation rates by considering the combined effects of multiple contaminants of concern on individual organ systems
A change to allow ventilation to be reduced to zero through the use of occupancy sensors for spaces of selected occupancy types
Changes related to demand control ventilation to make clear that the standard is intended to be used for calculations for code review and also for physical operation.
Designing an HVAC system to consistently bring in the appropriate theoretically calculated outside air can be an engineering challenge. Distributing that outside air properly within the classroom such that each student’s desk is in a position to get their code required minimum ventilation rate is a totally different challenge and requirement. Architects and interior designers must work with engineers more than ever to help integrate the ventilation system components into the space design and have this as a top priority.
Maybe one of the hardest challenges an engineer has these days is proper air diffuser selection to get the ventilation air in a classroom properly distributed at all times of occupancy, and documenting these selections. If the system is a VAV as opposed to a constant volume, the challenge is even greater. Research is ongoing to provide a documentation procedure that can be validated in the commission process.
The unfortunate fact is that some engineers still don’t even attempt to evaluate air diffusers beyond the rule-of-thumb simplified jet velocity method of terminal velocity of 50, 100, or 150 FPM to the nearest wall or opposing air stream at peak cooling design airflow, let alone working to adjust these throw values for heating when catalogued data is based on isothermal conditions. In this litigious society, this can be a point of concern for even the most diligent engineering firms who may have never given a simple diffuser selection much deeper consideration. The ASHRAE 55 Standard is raising the bar on how comfort is evaluated, and this has a direct impact on product selection and documentation of those selections.
First cost is always a factor of the engineering decision making process. However, it is not the primary focus when codes that dictate ventilation rates for schools are involved. Industry professionals who attempt “value engineering” to reduce first costs at the expense of proper ventilation put everyone at risk, including the school districts who agree to such compromises.
Many older rural schools, and even schools in big cities, for years had simple PTAC units on the perimeter walls which often didn’t (and likely still don’t) distribute ventilation air properly to meet current codes and standards. Some schools simply had, and still do have, perimeter finned tube or electric convectors for heat and operable windows for cooling with outside air.
More sophisticated and larger schools generally have central air conditioning systems with ducted air distribution for heating and cooling and ventilation. VAV systems are very common in schools. However, these have the same challenge as in commercial office buildings when it comes to ensuring the outside air is properly distributed throughout the breathing zone of the occupants. Dedicated outside air systems have helped ensure compliance with minimum required ventilation effectiveness with constant volume diffuser selections.
Newer classroom HVAC systems deploying VRF, displacement ventilation, and chilled beams provide new challenges for engineers who are looking for LEED compliance while keeping ASHRAE Standards 55, 62, and 90 system performance criteria. Other popular systems like water source heat pumps and fan coil units also must take into account proper ventilation air distribution. It may be that some of these product types don’t provide sufficient air distribution performance data to evaluate the ventilation effectiveness in the room during the design. Much focus has been placed on energy, possibly at the risk of not analyzing the ventilation effectiveness in classrooms. This could be problematic for engineers to defend in court.
Many of these types of systems were not as prevalent in years past as they are becoming today. The option of balancing first cost of a school HVAC system with the requirements of industry standards for energy, comfort, and IAQ is not as much a matter of rationalizing the justification of lowest first costs anymore, as these standards are becoming more and more codified.
Parents of students who are not performing well are taking a more serious look at how improper ventilation is affecting their child’s learning and they are becoming more aware of their legal resources to challenge the professionals who design the buildings. School boards and school districts need to stay focused on the health of the students as impacted by the ventilation effectiveness of the HVAC system and to not allow the aesthetics and function of the building to dominate the budgeting of the facility construction project.
Each system type has pros and cons when the goal is to provide a high-performance school as desired by the Collaborative of High Performance Schools, which believes “kids learn better in schools with good lighting, clean air, and comfortable classrooms.” The first three objectives in the CHPS definition of a high-performance school is a school that is “healthy” (ASHRAE Standard 62), “comfortable” (ASHRAE Standard 55), and “energy efficient” (ASHRAE Standard 90). Engineers must work with the school districts to fully understand the total building objectives and integrated building design approach that incorporates all aspects of the building sciences.
Outside Air Measurement
It is essential to have a system that is designed and documented on paper with numbers that meet codes to get a permit. What matters more is ensuring the proper amount of outside air is consistently brought into the building and distributed to the right places. Airflow measurement of outside air is now a common and essential part of ensuring the building is truly performing as designed. Verification of outside air quantities coming into the air handling system is simply not that expensive with today’s technologies, and it is a small investment when compared to the liability of not doing so.
There are more and more applications in schools where energy recovery is being used to help meet the energy codes while at the same time meeting ventilation codes. Energy recovery is not new to the HVAC industry, but it has become more common practice as energy costs rise, and the paybacks of these HVAC system enhancements start to make more economic sense.
Part of the challenge of incorporating energy recover devices is allowing for the space required and determining the type of energy recovery system to use. There are air-to-air ERVs and fluid coil type pump-around systems. Some ERVs are fixed-plate, while others use a rotating wheel. There are pros and cons to each type that need to be evaluated based on whether sensible only or total energy recovery is desired. Location of the units and ongoing maintenance also need to be evaluated. Some units can be mounted inside the mechanical rooms while others are mounted outside on roofs for space saving consideration.
There are some baseline criteria integral to the minimum outside air rates in the codes on the cleanliness of the outside air. There may be times when the specific site has air that is not as clean as these criteria, and filtration systems may be needed to ensure the outside air is clean enough. This type of analysis is not normally done as often as it probably should be. Local code officials may or may not review the ventilation requirements based on the building site outside air quality, but they should if the intent of the code is to be met.
It is important to fully integrate the HVAC system layout with the architectural functional layout. The outside air intakes need to be incorporated into the building design such that outside air contaminants or exhaust from other areas of the building such as laboratories for science, toilets, lockers or kitchens are not re-entrained into the outside air intakes. Sometimes just meeting the minimum code separations are not sufficient when wind currents are taken into consideration.
Recent improvements and understanding of laboratory exhaust calculations procedures by ASHARE allow better analysis with design wind velocities for the location of the school and determination of the plume height desired. Just assuming an up-blast type fan will “throw” the exhaust air high enough is not sufficient anymore. Wind velocity data is available just like temperature data is available. Using average wind velocities is not sufficient, nor is just looking at the prevailing wind directions. Overlooking the exhaust re-entrainment can be a huge risk.
System Maintenance and Operation
Designing systems to meet the codes and standards must also focus on the practical utilization of the systems once the owner takes over. The operation of the systems must be clearly understood by the facility manager and allow a level of control that is appropriate for the staff knowledge being challenged to operate it.
It does no good to install sophisticated controls that allow the design to take advantage of decreasing ventilation based on CO2 levels or occupancy levels if these controls are not maintained properly and understood by the facility manager and maintenance staff.
According to the EPA, one of the common problems with portable (or “relocatable”) classrooms is poorly functioning HVAC systems that provide minimal ventilation with outside air. This type of classroom is used by many school districts because they enable quick response to demographic changes and provide the ability to be moved from one school to another as demographics change.
The EPA also found that the recent surge in student population has fueled an explosion in the use of portable classrooms in many parts of the country. Engineers who design portable classrooms must be held accountable to the same codes and standards used for permanent school building design because, according to the EPA, portable classrooms are seldom moved and become permanent fixtures of the school.
The EPA states that “The effects of poor indoor air quality in portable classrooms are no different than those in permanent classrooms. All school buildings use similar construction and furnishing materials, so the types of chemicals present in indoor air are not likely to be different for portable versus permanent classrooms. However, pressed-wood products which may contain higher concentrations of formaldehyde are used more in the factory-built portable units than in buildings constructed on-site. As a result, levels of airborne chemicals may be higher in new portable classrooms, especially if ventilation is reduced.”
Ventilation control and ventilation effectiveness are not optional. It is essential for our youth to have the best opportunity to learn in an environment that allows them to function at their best. The design team is challenged to have sufficiently integrated knowledge of the building science disciplines to ensure that classroom ventilation is executed as a high priority in every new and retrofit project.
Taking time at the beginning of a project to incorporate and discuss classroom ventilation with the entire design team will ensure that this important aspect of the HVAC system is not overlooked. Having a serious discussion with the owner of the school about classroom ventilation is a good place to start any project discussion. ES