School Ventilation Update

School Ventilation Update
Posted on 09/02/2021
School Ventilation Update

OCDSB Ventilation and Indoor Air Quality Summary

Since the pandemic began, we have been very active in ensuring the school environment is safe for the staff and students. Our engineers have consulted with the HVAC professionals and public health organizations to ensure we are following their guidance. There is a broad variation of HVAC system complexity and design in our facilities (over 150 schools). Each facility's HVAC system was reviewed independently during the summer of 2020 to ensure it was operating as designed and to determine how to best optimize the ventilation and indoor air quality for that facility. Our focus is on increasing ventilation and filtration at every school.

Actions taken before September 2020

The District prioritized work on ventilation during the summer of 2020 to ensure that schools would be ready to re-open in September 2020. The work included:

  • Recommissioning of all heating, ventilation and air conditioning (HVAC) equipment, which included operational testing/inspection, repairs as needed, thorough cleaning and filter changes
  • Reprogramming of ventilation systems to comply with the ASHRAE standards and public health recommendations, including: creating purge cycles that exchange the building air with outdoor air; increasing ventilation rates during other school hours to a fixed, constant rate; and operating ventilation systems longer (2 hours before & after occupancy). The OCDSB investments in our central Building Automation System enabled us to modify our HVAC systems programming to accomplish this.

Actions taken during the 2020-2021 School year

  • Short-term measures focused on enhanced air purification using high-efficiency particulate air (HEPA) filtration. The District ordered 1,250 units. Of these, 800 units were distributed in the fall of 2020 and the remaining 450 units were received and distributed in the spring of 2021. Details of the deployment of the HEPA units can be found below.
  • Long-term measures were initiated to implement infrastructure upgrades at various sites in order to provide increased ventilation capability and/or add permanent filtration options. 

Actions taken for the 2021-2022 School year

  • An additional 1260 HEPA filtration units will be deployed for the start of the September 2021 school year.
  • Depending on system type, all schools will be provided with upgraded filtration systems either by augmenting existing filters to MERV-13 type or by introducing permanent UV filtration. This work is ongoing and will continue through the fall.
  • Additional CO2 monitors continue to be installed across the District and by late fall 2021, all schools will have central CO2 monitoring capabilities.
  • In addition, Facilities staff continues to complete and implement various ventilation, filtration and COVID-19 related projects across the District. These projects include HVAC equipment replacements and new HVAC system installations. Many projects will continue through the fall and are anticipated to be completed by January 2022

HEPA Filtration Units

Portable HEPA units have been deployed throughout the District to support the ventilation systems. The allocation of the units was based on criteria from the Ministry of Education and summarized below. We have kept a surplus of units on hand for quick deployment should additional areas of need be identified, should there be a community outbreak or in the case of a ventilation system failure.

Phase 1 (August 2020) and Phase 2 (winter 2021)

  • 1116 Units deployed based on the following criteria:
  • Classrooms with passive ventilation systems where increased ventilation measures were not possible due to the nature of the HVAC equipment;
  • Isolation Rooms with a higher potential for airborne virus particles;
  • Classrooms where the more important steps of wearing masks, hand washing and distancing may not be achievable. This included all special program classrooms and all junior and senior kindergarten rooms across the District.

Phase 3 (August 2021)

  • 1260 to be deployed based on the following criteria:
  • All portable classrooms
  • Any classrooms with HVAC system upgrade projects ongoing
  • Any rooms or areas where increased ventilation measures were not possible due to the nature of the HVAC equipment or are subject to gathering of staff/students. This may include libraries, lunchrooms, staff rooms, admin and staff offices, child care spaces, multipurpose rooms and gymnasiums.

HVAC System Ventilation Rates

The ASHRAE standard 62.1 “Ventilation for Acceptable Indoor Air Quality” provides detailed calculations for the required ventilation rates in buildings. In our case this equates to approximately 3 air changes per hour (ACH). At the rate of 3 ACH, 95% of airborne contaminants are eliminated.

During the pandemic, ASHRAE issued guidance for the re-opening of schools. This guideline recommends ensuring that systems are providing 3 ACH as a minimum, and increasing outside air to the maximum allowable without compromising indoor thermal comfort. Other guidelines have similarly recommended increasing ventilation rates up to 6 ACH where possible.

All of our HVAC systems are able to provide 3 ACH per hour as a minimum with most capable of the target rate of 6 ACH. We were able to achieve this through the reprogramming of our building automation systems.

Systems that were not able to be increased up to 6 ACH are supplemented with higher efficiency filtration systems such as HEPA, MERV-13 or UV filtration.

Carbon Dioxide (CO2) Monitoring

CO2 readings can serve as a proxy measure of overall ventilation but are not a measure of Covid-19 transmission risk.

Many of our schools have carbon dioxide (CO2) monitoring performed on an ongoing basis through our Building Automation Systems. Before COVID, CO2 levels were used to balance ventilation rates throughout the day (ie: increase ventilation when CO2 levels are high and decrease ventilation rates when CO2 levels are low). 

When COVID started, we adopted the ASHRAE recommendation to remove CO2/demand-based ventilation controls. What this means is that while CO2 levels are still being monitored, CO2 levels have been disconnected from ventilation rates, which are set at higher levels to encourage the flow of outdoor air through schools.

Our monitoring shows that during our purge cycles, the CO2 levels indoors match the outdoor levels of approximately 450-500 ppm. During other hours, with our increased ventilation rates we are typically maintaining 700-1200 ppm which is well within the recommended levels and meets the ASHRAE standard. Any rooms with higher levels are provided with supplemental filtration, typically HEPA or Ultra Violet systems to further reduce the risk of airborne transmission.

The intent of the CO2 monitoring is to help identify possible ventilation issues within a building so that the proper resources can be quickly dispatched to physically identify and correct any deficiencies. Ventilation systems are also continuously monitored for proper operation through our central Building Automation System.


  • Recommissioning, testing and any necessary repairs to HVAC systems were completed in the summer 2020. For all facilities, our technical staff again verified the operation of ventilation equipment in the summer of 2021.
  • Preventative maintenance programs are ongoing.
  • The chief custodians also verify operation of the equipment and submit work orders as issues arise.

Testing and Monitoring

  • Visual inspections of our HVAC systems are performed daily by our Operations staff.
  • HVAC system status and performance are also monitored continuously with our building automation systems which send alarms if a system failure is detected.
  • As noted above, CO2 levels in many schools are monitored to help identify possible ventilation issues. Additional CO2 monitors continue to be installed across the District and by late fall 2021, all schools will have central CO2 monitoring capabilities.

School Specific Information
The OCDSB has created a
Standardized School Ventilation Measures Report (
Excel file), which provides a snapshot of school ventilation and filtration measures taken at each school. In addition, we have also compiled additional details according to school at the links below.


Guidelines and reports referenced include:

ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality

ASHRAE Guideline - Epidemic Task Force for the Reopening of Schools.

ASHRAE Journal – June 2021 Managing Aerosols Using Space Flushing

SickKids (Ontario Science Table) – Covid-19: Guidance for School Reopening.

Operational Guidance from The Ministry of Education (August 2021)

PHAC & OPH published guidelines for the reopening of buildings

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