Ventilation To Improve IAQ In Schools
The author heads to the blackboard to break down fundamental outside air and sensor strategies (and how they can work together).
Providing high-quality education for our nation’s children in grades K-12 has never been more important than it is today. The world is changing quickly, and we need to provide an educational system that prepares them for this fast paced world in which we live.
One often overlooked aspect of providing quality education is IAQ and its impact on learning. Over the years, many different studies have shown that good IAQ in the classroom contributes to better learning. One of the best ways to improve IAQ in the classroom is to increase the amount of outside ventilation air delivered to the space. Increasing the outdoor air ventilation rate in the classroom provides an environment where the students are more alert and engaged in their work. Teachers, as well, are more energized, focused, and able to do their best work. These same studies show that disease transmission and absenteeism is also reduced when more outside air ventilation is provided to the classroom.
ASHRAE WEIGHS IN
This study data is supported in ASHRAE Standard 62.1-2010. In reviewing Table 6-1 of the standard, you will note that K-12 classrooms have some of the highest required outdoor air ventilation rates of all of the spaces listed. The writers of the standard have obviously recognized that better ventilation provides a better and healthier learning environment.
Table 6-1 of Standard 62.1 identifies two separate K-12 age groups, but in each case, the outdoor air ventilation requirement is the same. A standard K-12 classroom requires 10 cfm of outdoor ventilation air per person plus an additional 0.12 cfm of outdoor air per sq ft of classroom space. Art, science lab, and shop classrooms require the same 10 cfm of outdoor ventilation air per person but increase the classroom space requirement to 0.18 cfm/sq ft. A standard classroom for grades 1-12 is typically between 900 and 1,000 sq ft and is designed for a normal occupancy of 30 students and one teacher. What this means is that each standard classroom requires between 418 to 430 cfm of outdoor air for proper ventilation.
Now that we have identified the outdoor air ventilation requirements for a typical K-12 classroom, the next step is to identify the best and most efficient way to deliver that air to the classroom. As noted previously, K-12 classrooms have some of the highest outdoor air ventilation requirements of any spaces. Depending on the climate and location of the school building, the costs associated with heating, cooling, and/or humidity control of this outside ventilation air can be significant. There are a number of ways that designers can meet these ventilation requirements. The best approach can only be determined after careful consideration of all of the factors driving the design. Is lowest first cost a priority, or is maximum energy efficiency a higher priority? Identifying these factors early in the design process will help to determine the best design options moving forward.
This brings us to our discussion of the systems and technology that are currently available to meet these classroom ventilation requirements.
STANDARD VAV AHU’S
Most school buildings designed and constructed today utilize some type of VAV delivery system to provide recirculated and outdoor ventilation air to the spaces. For discussion purposes, we will assume that our typical school building uses a system of central station AHUs that provide supply air to groups of VAV box terminal units. Each VAV box terminal unit serves one occupied classroom space in the building. It’s important to note that many, if not most, of the control technologies we will be addressing here can be applied with equal effectiveness in HVAC systems that utilize other types of air delivery systems.
A basic design approach commonly used for a central station AHU system with zone VAV boxes is as follows. The minimum outside air ventilation rate for each AHU system is determined using the procedure identified in ASHRAE 62.1-2010. The minimum outdoor air ventilation rates are determined for each space served by the AHU using Table 6-1. The ventilation rates for each space are then added together to determine the minimum outdoor air requirement for the AHU system. The AHU system is then sized and configured to provide the required amount of outside air, supply, return, and relief air, heating, and cooling capacity; and any other HVAC functions the system needs to provide.
The basic VAV AHU system described here typically uses a fixed position outdoor air damper to regulate the amount of outside air drawn into the system. During startup of the VAV AHU system, the TAB contractor is typically the person that measures and sets the minimum outside air damper position for each system. Per ASHRAE 62.1-2010, this minimum outside airflow setting must be performed when the variable volume AHU is at its “minimum system primary airflow.” The purpose of this is to ensure that the minimum required outdoor airflow is being delivered to the spaces under all system operating conditions. One problem with this fixed-position outside air damper in a variable flow AHU system is that as system primary airflow increases, the amount of outside airflow can also increase, which over-ventilates the spaces and uses unnecessary additional energy.
We previously noted that the ASHRAE minimum ventilation rates are based on the occupancy of the spaces served. In a K-12 school environment, the actual occupancy of classroom spaces can fluctuate significantly over the course of the day. A particular classroom can be fully occupied with 30 students for several hours, but then the same space can be unoccupied if the students are at lunch or attending classes in other specialty areas of the building such as the gymnasiums, science labs, or music classrooms.
ASHRAE allows several options for determining and controlling outside air ventilation rates in these instances. Section 6.2.7 of the Standard 62.1 allows what is called “dynamic reset” of the outdoor air ventilation rates as operating conditions change. These demand control ventilation (DCV) strategies allow the use of occupancy schedules, occupancy sensors, CO2 sensors, and other similar controls to reduce or eliminate outside airflow to the classrooms if the rooms are empty or not at their rated occupancy.
Fortunately, technology is available today that allows system designers many options for control of these systems to significantly reduce the energy costs associated with providing the required level of outside air for ventilation. Let’s look at how some of those technologies can be applied to our classroom VAV system.
Because ASHRAE 62.1 is an occupancy-based standard, the only time we need to provide substantial outdoor air for ventilation is when the space is occupied. Modern DDC systems have multiple capabilities to control VAV boxes based on occupancy schedules, and this is usually the first step in any DCV strategy. Knowing what times of day a particular space is unoccupied and programming the DDC system to reduce or shut down outside airflow, and even total airflow, to that space during those times is an effective control strategy for saving energy. A problem with this approach in the K-12 environment is that classroom use in a school is typically very dynamic and can change not only from semester to semester but from day to day. This makes it very difficult to program classroom occupancy schedules that are workable and satisfy the needs of the students and staff.
OCCUPANCY SENSOR DEMAND CONTROL VENTILATION
A better and more effective approach is to utilize space occupancy sensors to control ventilation airflow to the classrooms. Many schools are already using occupancy sensors to control lighting in the classroom. Lighting control occupancy sensors are available with extra sets of dry electrical contacts. These extra contacts can be used to send occupied/unoccupied signals to the local VAV box controller. In the unoccupied mode, the controller can be programmed to reduce or completely shut down the airflow to the space. This type of occupancy control is automatic and responds to the actual occupancy conditions of the classroom.
The next level of control uses CO2 sensors in the classrooms to measure the actual ventilation effectiveness of the supply air being delivered to the space. The control system then adjusts the supply and/or outside airflow to the space to most efficiently meet the space ventilation requirements. People breathe in air for its oxygen content and exhale CO2. The more people you have in a room, the greater the CO2 concentration in the air. Good IAQ from a CO2 perspective is generally understood to be less than or equal to a 1,000 ppm CO2 level. Using CO2 sensors to control space CO2 to a value of approximately 1,000 ppm will result in the system being able to maintain the required outside air ventilation rate, while at the same time using the least amount of energy. There are several ways to accomplish this.
In the basic VAV system, the approach might be as follows. If the space CO2 is below the 1,000 ppm setpoint, the VAV box serving the space is at the minimum airflow required to maintain the space temperature setpoint. If the space CO2 increases above the 1,000 ppm level, the VAV box damper begins to open and allows additional supply air into the space. Once the CO2 level in the space drops below its setpoint, the VAV box damper modulates towards the closed position to maintain the CO2 level at setpoint.
This control sequence can also be extended to the central AHU where the AHU minimum outside air damper position is reset based on the space ventilation requirements. In the VAV box control sequence noted above, it is possible that once the VAV box reaches its maximum design airflow, the room may still be above the 1,000 ppm CO2 control setpoint. At this point, the control system would modulate the AHU outside air damper open so that additional outside air is drawn into the system. This additional level of CO2 control would allow for a more energy-efficient system, due to the fact that the default AHU outside air damper minimum position can be set at a lower cfm airflow than would be required if this control sequence were not available.
Maximum system efficiencies can be obtained by using a combination of these control strategies. Programming the optimum time-of-day occupancy schedules for central equipment such as AHUs and using both occupancy sensors and CO2 controls in classroom spaces will provide maximum efficiency while at the same time providing the good IAQ needed for learning. As noted previously, these same control strategies can be used and applied with other systems such as unit ventilators, fancoil units, fan-powered VAV terminal units, and even active chilled beam systems.
In conclusion, this article is only the tip of the iceberg when it comes to designing and fine tuning HVAC systems for good IAQ and energy efficiency in K-12 schools. Always remember to consider the owner’s requirements for overall cost and energy efficiency, and use that information to develop a system design that meets those needs and gives our children the healthiest and best learning environment possible.