MERV 13 air filters are unable to be installed in the HVAC equipment serving most areas at Glenwood Middle School, and parts of Hanover Hills Elementary School, Harper's Choice Middle School, Swansfield Elementary School, Waverly Elementary School, and Wilde Lake Middle School.
The HVAC systems at these schools use Variable Refrigerant Flow (VRF) HVAC systems which utilize 1/2-inch thick MERV 8 air filters which are unable to be upgraded to MERV 13 since 1/2-inch MERV 13 filters are not manufactured. These VRF systems are coupled with separate Dedicated Outdoor Air System (DOAS) units which utilize MERV 13 air filters to create an overall higher-efficiency system to mitigate environmental concerns in the school.
VRF HVAC System Quick Facts:
- VRF systems are high efficiency systems.
- Each classroom has its own VRF fan-coil unit which recirculates air within that classroom only. This prevents cross-contamination between classrooms through the HVAC system.
- VRF fan-coil units have a higher recirculating airflow rate than a conventional HVAC system does, which results in more room air changes per hour than a conventional HVAC system.
- When coupled with separate DOAS units, VRF/DOAS HVAC systems exceed the recommended ASHRAE and Maryland Department of Health equivalent outdoor air change rates.
The following is the list of schools where VRF systems are utilized (that cannot utilize MERV 13 air filters):
- Glenwood MS: 194 fan-coil units serving the entire school.
- Hanover Hills ES: 26 fan-coil units serving the Main Office, Health Suite, and Guidance.
- Harper’s Choice MS: 1 fan-coil unit serving Band, Band Office, and Robotics.
- Swansfield ES: 37 fan-coil units serving the Main Office, Health Suite, and Guidance.
- Waverly ES: 28 fan-coil units serving the Main Office, Health Suite, and Guidance.
- Wilde Lake MS: 27 fan-coil units serving the Main Office, Health Suite, and Guidance.
Equivalent Outdoor Air Changes per Hour (VRF/DOAS w/MERV 8 versus Conventional HVAC w/MERV 13)
The high-efficiency variable refrigerant flow (VRF) HVAC system at Glenwood Middle School and the other schools listed above are dual-path HVAC systems. That is, the outdoor air ventilation and recirculated heating/cooling air for occupied spaces are delivered by two separate HVAC systems. Each classroom has its own VRF fan-coil unit that filters the air for just that classroom. Each classroom receives outdoor air from a dedicated outdoor air system (DOAS) unit that delivers 100% outdoor air to multiple classrooms. A more conventional HVAC system, such as the HVAC systems at Cradlerock Elementary School / Lake Elkhorn Middle School, is a single-path HVAC system where the outdoor air ventilation and recirculated air from multiple classrooms combine upstream of the air filters in a centralized HVAC unit, and the supply airflow to the occupied spaces is the sum of the outdoor and recirculated airflows.
Both the VRF/DOAS HVAC system (with MERV 8 filters) at Glenwood Middle School and the conventional HVAC system (with MERV 13 filters) at Cradlerock Elementary School/Lake Elkhorn Middle School exceed the 4 to 6 equivalent outdoor air changes per hour recommended by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) and the 3 to 5 equivalent outdoor air changes per hour recommended by the Maryland Department of Health.
If an air filter were able to capture 100% of particles in the 1 to 6 micron size range (where most SARS-CoV-2 is assumed to be present), the air passing through that filter would be considered the equivalent of outdoor air. A high-efficiency particulate air (HEPA) filter captures 99.97% of particles in this size range; therefore, one room air change per hour of air passing through a HEPA filter is considered the equivalent of one outdoor air change per hour. A MERV 13 air filter is 86.5% efficient at capturing particles in this size range; therefore, one room air change per hour of air passing through a MERV 13 air filter is the equivalent of 86.5% of one outdoor air change per hour. Similarly, a MERV 8 air filter is 35% efficient, so one room air change per hour of air passing through a MERV 8 air filter is the equivalent of 35% of one outdoor air change per hour.
The following is a comparison of the equivalent outdoor air changes per hour delivered by the VRF/DOAS HVAC system serving a typical classroom at Glenwood Middle School and the conventional HVAC system serving a typical classroom at Cradlerock Elementary School / Lake Elkhorn Middle School. The data was obtained from the architectural and HVAC drawings for both schools.
For Classroom 25 at Glenwood Middle School (MERV 8 air filtration efficiency):
- Room air volume = 23’-0” x 30’-0” = 690 sq ft x 9’-6” ceiling height = 6,210 cu ft.
- Outdoor air ventilation = 375 cu ft / min (cfm) = 3.62 air changes per hour (ACH).
- Recirculated airflow = 1,059 cfm = 10.23 ACH.
- Equivalent outdoor air changes per hour (EOACH) = 3.62 + (10.23 x 35%) = 7.20 EOACH.
For Classroom 122 at Cradlerock Elementary School / Lake Elkhorn Middle School (MERV 13 air filtration efficiency):
- Room air volume = 24’-0” x 28’-0” = 672 sq ft x 10’-0” ceiling height = 6,720 cu ft.
- Outdoor air ventilation = 352 cfm = 3.14 ACH.
- Recirculated airflow = 748 cfm = 6.68 ACH.
- Equivalent outdoor air changes per hour (EOACH) = 3.14 + (6.68 x 86.5%) = 8.92 EOACH.
The reason the VRF/DOAS HVAC system at Glenwood Middle School is able to perform nearly as well as the conventional HVAC system at Cradlerock Elementary School/Lake Elkhorn Middle School is that it has a higher recirculating airflow rate than the conventional HVAC system. As a result, it achieves nearly the same equivalent outdoor air changes per hour as the conventional HVAC system, and both systems exceed the 4 to 6 equivalent outdoor air changes per hour recommended by ASHRAE and the 3 to 5 equivalent outdoor air changes per hour recommended by the Maryland Department of Health.