Proper ventilation is an important part of maintaining a comfortable, healthful, productive environment for students and faculty. Yet, one study found that 86 percent of classrooms had incidences of inadequate ventilation, and a California study found that classrooms had inadequate ventilation 42 percent of the time. With statistics like these, the question becomes: “What is the best way to monitor and control ventilation in the classroom?”
CO2-based ventilation control (also called demand-controlled ventilation or DCV) is one solution. This building-control strategy optimizes the outside-air intake based on measured ventilation rates. Optimum means the building will not be under- or over-ventilated. The result of under-ventilation can be poor indoor air quality. Over-ventilating wastes energy because the air often must be conditioned before being sent into the building.
Indoor carbon dioxide (CO2) levels form the basis of ventilation control. There is a clearly defined relationship between indoor CO2 levels and per-person ventilation rates. This relationship is recognized by ASHRAE, ASTM and the EPA.
Students, staff and faculty breathe in oxygen and exhale CO2. Outdoor air or ventilation has a very low and typically constant CO2 content and, when introduced into a room, dilutes the CO2 exhaled by people. High indoor CO2 levels mean there is not enough ventilation entering the room. Low CO2 levels indicate over-ventilation.
The indoor CO2 reading allows a HVAC system's outdoor-air intakes to modulate based on the building's actual load. Maintaining the proper indoor CO2 level ensures required ventilation rates are met.
CO2-based ventilation control should be applied in spaces such as classrooms, offices, auditoriums, gymnasiums, cafeterias and lobbies. The benefits can include:
Helping ensure a comfortable, healthful indoor environment
Numerous studies link proper ventilation to a healthful indoor environment. For example, a Lawrence Berkeley National Laboratories research paper on indoor air quality, ventilation and health symptoms in schools found that headaches, dizziness, drowsiness, respiratory and throat irritation, and lack of concentration symptoms increased with high CO2 concentrations (i.e., low ventilation rates). A recent EPA report stated that student use of inhalers dropped 50 percent after IAQ improvements were made in two San Francisco schools. Ventilation also has a significant effect on sick building syndrome symptoms and perceived air quality.
Many K-12 school districts receive state funding based on student-days or average daily attendance (ADA). Inadequate ventilation has been shown to increase absenteeism by 10 to 20 percent. Using CO2 control to maintain proper ventilation can reduce absenteeism, which increases state funding. Even a small reduction in absenteeism can substantially increase funding.
Improved student performance
According to a recent study published in the ASHRAE Journal, student scores increased significantly when the indoor CO2 level was kept at or below 1,000 ppm. This is backed up by a European study in which student scores were lower and health symptom responses higher in classrooms with high CO2 levels (i.e., low ventilation rates).
Having a comfortable, healthful environment reduces the possibility of an illness blamed on poor indoor air quality.
Second, there is a clearly defined and recognized relationship between indoor CO2 levels and ventilation rates. Documenting indoor CO2 levels shows the building is in compliance with codes and standards.
How does school administration respond to an illness blamed on poor indoor air quality if there is no means of measuring ventilation in the building? Performing an IAQ study weeks after the reported incident does little to show compliance. However, using CO2-based ventilation control gives administrators a way of proving that ventilation codes and standards were being met during the time in question.
CO2-based ventilation control delivers energy savings compared with a fixed-ventilation approach. Fixed ventilation assumes that the building is always fully occupied, so the maximum prescribed amount of outside air enters the building during all equipment operating hours. A CO2 ventilation approach is based on the actual ventilation load of the building.
For example, let's examine a classroom designed to hold 25 students. A fixed ventilation approach will provide enough outside air for 25 students, whether there are 5 or 25 present. When only 5 students are in the room, the fixed-ventilation method brings in more outside air than is required.
In this example, CO2-based ventilation control would reduce the outside air intake to bring in the correct amount for 5 students. On a hot summer day or cold winter morning, the opportunity to reduce the amount of outside air saves money because it does not have to be conditioned. The typical payback period is between 6 and 18 months.
Several companies offer energy-analysis programs to calculate the expected energy savings when using ventilation control vs. fixed ventilation. Also, schools should check with their local utility company about rebates for using CO2 sensors in an education facility.
CO2-based ventilation control offers other benefits:
CO2 control doesn't care where the outdoor air enters the building. For example, most schools have doors that open and close constantly, allowing outdoor air into the building. With CO2 control, this additional source of ventilation is accounted for. Fixed-ventilation approaches, such as using outdoor airflow monitoring stations, cannot detect such natural ventilation, and the facility receives additional over-ventilation.
Space CO2 sensors measure the ventilation that reaches where students are situated. Thus, ventilation effectiveness is taken into account.
CO2 control detects problems with a ventilation system. For example, improper CO2 levels can indicate a broken damper motor or linkage.
LEED (Leadership in Energy and Environmental Design) certification program points are available for projects using CO2-based ventilation control.
ASHRAE's 90.1 energy-efficiency standard requires the use of CO2 control in certain applications.
ASHRAE's Humidity Control Design guide shows that outdoor air ventilation is the source of 60 percent of the humidity inside the building. Using CO2-based ventilation control to reduce the outdoor air intake when the building is not fully occupied results in reduced humidity control because less moisture enters the building.
Welch is sales engineer at GE Sensing. He can be reached at [email protected].
To read more about energy-management topics, visit the Energy & Indoor Environment section at ASUmag.com.