Hybrid Approach to Decentralized Systems
Reading time: 5 minutes
Poor indoor air quality in schools has shown to induce symptoms including coughing, eye irritation, headaches, and allergic reactions while aggravating asthma and other respiratory issues. These problems can affect students’ attendance and grades as well as reduce faculty performance. When schools work to eliminate indoor air pollutants, they house healthier faculty and students, performance improves, and students are more attentive and retain information better.
When designing a new school or engineering the retrofit of an existing school, a one-size-fits-all approach to heating, cooling, and ventilation could cause more problems in the long run than developing an approach to fit all areas of the school building.
Developing systems that run throughout the building and from classroom-to-classroom are inefficient and potentially harmful to faculty and students.
Instead, the best option often is developing a case-by-case solution. When first designing an HVAC system for an older building, it can be very expensive and sometimes not even possible to add central duct distribution without current ventilation. A way to counteract this issue would be to build a hybrid HVAC system. Each classroom would have a HVAC unit ventilator, common areas such as gymnasiums, theaters, and cafeterias would use a stand-alone AHU, and a central HVAC system would cover the hallways, offices, and breakrooms. Offices and breakrooms, connected to the central HVAC system, would employ the use of a portable air cleaner with a HEPA filter for cleaner, recirculated air.
Measuring Central and Classroom Units
Based on classroom default densities in ASHRAE Standard 62.1-2019 (25 students/1000 sq ft for 5- to 8-year-olds), the Harvard T.H. Chan School of Public Health recommends classrooms need at least 5 air changes per hour (ACPH).
Researchers at the university performed a study measuring the ACPH in two different classrooms: one classroom that shares a central HVAC system with other classrooms and another classroom that uses a unit ventilator. The classroom that shares a central HVAC system had 1.3 ACPH, well below the recommended limit of at least 5 ACPH. The classroom with a unit ventilator had 1.4 ACPH.⁵ This is a slightly better result, but still not the targeted 5 ACPH.
With both classrooms experiencing inadequate ACPH, the experts recommended the following three options:
Improving ventilation both naturally (opening windows and doors) and mechanically (setting the unit ventilator or the central HVAC system to the maximum outdoor air supply percentage the unit can handle).
Use MERV13, or better, filters on recirculated air. Recirculated air that passes through a higher efficiency filter can be added to the CFM calculations to reach ACPH targets.
Add portable air cleaners with a HEPA filter to classrooms or other rooms that do not meet the recommendation.⁵
Applying a Hybrid Approach to Schools
With these recommendations, we can take the same data, and according to this study, upgrade ventilation in our schools.
Using the central HVAC system example, we calculate that using a MERV-13 or better filter will achieve the recommended ACPH of 5. For schools with unit ventilators, those without the capability to install MERV-13 filters would have to resort to natural ventilation or a portable air cleaner with a HEPA filter. Researchers found that this approach [KT1] resulted in 6.7 ACPH, well above the recommendation.
For smaller rooms in schools, such as breakrooms, or the offices of faculty members, the Harvard T.H. Chan School of Public Health recommends a portable air cleaner with a HEPA filter for rooms that do not meet the recommended ACPH. While there is no formal recommendation by the CDC for use of portable air cleaners with HEPA filters for decontamination of COVID19 air particles, prior CDC guidance of the SARS outbreak from 2002- 2004 suggests theoretical efficacy for HEPA filters to remove COVID19 air particles; however, direct studies have not yet been performed.⁶
In larger, common areas, such as gymnasiums, theaters, cafeterias, and hallways, bigger AHUs are needed. Naturally ventilating larger areas like these is difficult.
By using a hybrid HVAC system, classrooms are heated, cooled, and ventilated by a unit ventilator, while a separate HVAC system would cover the gymnasiums, theaters, and cafeterias. A portable air cleaner with a HEPA filter in each office and breakroom would be installed to improve the IAQ of the recirculated air, and a new/upgraded central HVAC system would cover the hallways, offices, and breakrooms.
The figure (right) shows how this system would work. New central HVAC systems can be used to bypass ducts, and create or connect to an existing Variable Refrigerant Flow (VRF) system. Also, installing new MERV13 filters into the current central HVAC system could improve the IAQ vastly.
Adequate ventilation increases student performance and decreases the amount of indoor air pollutants, thus improving IAQ. Better temperature control can increase comfort and increases student performance.
A hybrid HVAC system will keep air separated in rooms, and where air cannot be separated, portable air cleaners with HEPA filters can be installed to provide cleaner, recirculated air.
Read an extended version of this article, including how to measure and calculate ventilation and the full breakdown of the hybrid system study by the Harvard T.H. Chan School of Public Health, by downloading our white paper.