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ASHRAE Epidemic Task Force Laboratory Subcommittee Guidance Document
 November 12, 2020

One Page Guidance for Re-Opening Buildings
 October 5, 2020
 em Português

Guidance for Polling Place HVAC Systems
 August 19, 2020
 em Português


General Information: Building Readiness Intent | Building Readiness Team | "Building Readiness Plan | Epidemic Conditions in Place (ECiP) | Systems Evaluation | Building Automation Systems (BAS) | Ventilation Per Code / DesignIncreased Ventilation | Increased Ventilation Control | Building and Space Pressure | Pre- or Post-Occupancy Flushing Strategy | Equivalent Outdoor AirUpgrading & Improving Filtration | Filter Droplet Nuclei Efficiency / Particle Size Expectations | Practical Approach to Increase MERV in an AHU | Calculation Approach to Increase MERV in an AHU | Energy Savings Considerations | Exhaust Air Re-entrainment | Energy Recovery Ventilation Systems Operation Considerations | UVGI Systems | Bipolar Ionization and other Emerging Technologies | Domestic Water & Plumbing Systems | Maintenance Checks | General Recommendations | Heating, Ventilating & Air-Conditioning | Plumbing Systems | Shut Down a Building Temporarily | Systems Manual | Re-opening During Epidemic Conditions in Place | Post-Epidemic Conditions in Place (P-ECIP)P-ECiP: Prior to Occupying | P-ECiP: Operational Considerations once Occupied | P-ECiP: Ventilation | P-ECiP: Filtration | P-ECiP: Building Maintenance Program | P-ECiP: Systems Manual | References | Acknowledgements


General Information

Building Readiness Intent

The following Building Readiness information is meant to provide practical information and checklists for how your building should be operating and how to practically check its operation. Actual conditions at any specific building will vary, and the adjustments that should be made will depend on many factors such as local climate, complexity of systems involved and the use, occupancy and activities that occur in and around your building.

Building Readiness modes of operation for the building should include the following:

  • Epidemic Operating Conditions in Place (ECiP)
    • Occupied - at pre-epidemic capacity
    • Occupied - at reduced capacity
    • Unoccupied temporarily, and
    • Operation during building closure for indefinite periods
  • Post-Epidemic Conditions in Place (PECiP)
    • Prior to Occupying
    • Operational Considerations once Occupied

Please keep in mind that the mode of the building (ECiP or P-ECiP) is a decision by the Owner that should consider the Federal, State, or Local Government or Health Department designation for the location of this building.

Also note, that the ASHRAE Epidemic Task Force has created a single page document for Core Recommendations for Reducing Airborne Infectious Aerosol Exposure that should be referenced to understand the intent behind all of the guidance.

The Building Readiness Guide will provide some of the practical guidance on operating your building systems in these different modes.  The suggested mode of operation during the Epidemic periods are detailed in the Buildings Guidance on the ASHRAE Covid-19 Website

  • Healthcare
  • Residential Healthcare
  • Residential
  • Multifamily Residential
  • Commercial
  • Schools & Universities
  • Laboratory
  • Assemblies of Faith
  • Transportation

In addition, this document will cover specific recommendations from the Building Guidelines such as:

  • System Anaylsis
  • Ventilation
  • Increased filtration
  • Flushing calculations
  • Energy recovery ventilation systems operation considerations
  • Building exhaust air re-entrainment

This document assumes that the Owner and Facility Operators have completed their Epidemic Preparedness Plan and are ready to shut down, operate, and re-open their building. This can be done in either mode, ECiP or P-ECiP.

The following guide is to provide practical guidance for the Mechanical Systems for those scenarios.

Keep in mind, that for the P-ECiP mode, there are really two phases to consider; Prior to Occupying, and Operational Considerations once Occupied.


Building Readiness Team | Return to Top

The Building Readiness Team could include licensed and certified professionals and companies that can perform the analysis, testing, design, construction, control programming, balancing, commissioning, maintenance and operation services required to make the adjustments and achieve the performance included in these recommendations. The following are the typical service providers that may be required:

  • Commissioning Provider (CxP) – engage a CxP that has a recognized certification from ASHRAE (BCxP), ACG (CxA), BCA (CCP), NEBB (BSC and RCx), or others. They should also have completed several Retro-Commissioning or New Building Commissioning projects in the building type in question.
  • Test and Balance Company (TAB) – engage a TAB that has recognized certification from Associated Air Balance Council (AABC), National Balancing Council (NBC), National Environmental Balancing Bureau (NEBB) and Testing Adjusting and Balancing Bureau (TABB) or another certifying body. The TAB agent or service provider should have experience with the building type and systems being evaluated. These certifying bodies require a TAB company operator to have been trained and certified and requires the use of calibrated instruments.
  • Building Automation Systems (BAS) Company – the Owner should engage the company currently providing service and support for the control system(s) that are installed in the building. If a new service provider is required, finding a local company that has experience working with and operating the building's existing control systems and preferably certified by the manufacturer to provide services for their equipment.
  • Contractors – the Owner should engage, if necessary, the appropriate contractors to install or repair equipment or systems identified by the CxP, TAB, or BAS. This could include the following:
    • General Contractors (GC)
    • Mechanical Contractor (MC)
    • Electrical Contractor (EC)
    • Specialty contractors for fire alarm and smoke control systems and interfaces.
  • Architect and Engineer (AE) – the Owner should engage a design team for any issues that might require permit drawings. It is preferred that the original Engineer or Architect of Record that was involved with the original construction or the latest renovation or addition to the facility be engaged if possible. Those professionals should be most familiar with the building's current operation.
  • Owner's Facility Staff – the Owner should make sure that their facility staff are involved in the process. This allows for the information transfer on how systems might be altered to operate.

Building Readiness Plan

This is a document that should be created to document the mitigation strategies that the facility is going to utilize, whether temporary or permanent modifications, for the facility operators and occupants to understand the plan.

 This should include the non-HVAC strategies as well as the HVAC mitigation strategies that are discussed in this document.  Non-HVAC strategies could include, but not be limited to, the following items:

  • Building Occupancy Levels Allowed
  • Face mask requirement or recommendation
  • Social distancing between desks, breakrooms, conference rooms, elevator, etc.
  • Directional flow for office space
  • Personal hygiene
  • Cleaning requirements

 HVAC strategies could include, but not limited to, the following items:

  • Increased Ventilation
  • Improved Filtration
  • Air cleaning devices (such as UVGI and other newer technologies)

 It is crucial to note, that each HVAC system needs to be analyzed for the appropriate engineering controls to utilize to improve its potential to reduce virus transmission in the building. 


Epidemic Conditions in Place (ECiP) | Return to Top

Systems Evaluation:

The Owner should consider evaluating their building systems to check that it is operating in proper order (per design conditions or current operational strategies), is capable of being modified to align with HVAC mitigation strategies, and to identify deficiencies that should be repaired. This could be viewed as tactical commissioning of the systems to determine risk areas for the building operating in epidemic conditions.

Systems evaluation should include the following steps:

1. Gather and review building and systems documentation, including but not limited to:

  1. Most recent design documents, specifically the HVAC and Plumbing Water systems construction documents
  2. Record documents (as-built, marked up drawings and specifications received from the Contractor at the conclusion of construction)
  3. Original, approved equipment and system submittal documents
  4. Systems manuals or turnover package
  5. Controls and Building Automation System (BAS) drawings and sequences of operation and initial system parameters
  6. Equipment control wiring diagrams and troubleshooting guidelines
  7. Service contracts and maintenance logs
  8. BAS Trend reports and alerts and notifications reports
  9. Most recent Testing, Adjusting and Balancing (TAB) reports
  10. Most recent Commissioning Reports (if available)

2. Inspect equipment, systems and controls to determine where existing problems may exist. Start with components, then move to systems, finally move to the BAS and integrated, whole building operations. Look for energy savings opportunities available based on poor optimization from all systems.

For example:
a. Components

  1. Boilers
  2. Chillers
  3. Air Handling Units (including filter rating, filter installation for leakage, coil conditions and accessories used to control outdoor air flow)
  4. Control Dampers (check they are functioning and that the positions seem correct)
  5. Control Valves
  6. Control sensors
  7. Airflow measuring stations (AFMS)
  8. Fan Coil Units
  9. Grilles, registers and diffusers
  10. Variable speed drives
  11. Variable Air Volume terminal units,
  12. Water-to-water heat exchangers
  13. Water-to-refrigerant heat exchangers
  14. Water to air heat exchangers
  15. Steam-to-water heat exchangers

b. Systems

  1. Chilled water systems
  2. Hot water systems
  3. Condenser water systems
  4. Air handling systems (Air handling equipment and air distribution networks: supply ducts, return ducts, exhaust ducts)
  5. Steam distribution systems
  6. Refrigerant systems

c. Building Automation Systems (BAS) and Integrated Systems

  1. Graphic user interfaces
  2. Set Points (Temperature, Humidity, Airflow, CO2, etc)
  3. Schedules (Occupied and Unoccupied)
  4. Trend reports
  5. Alarm, alert and notification logs
  6. Remote access capabilities
  7. Life safety system interfaces and interlocks
  8. Access control interfaces
  9. Smoke control system interfaces
  10. Lighting control interfaces
  11. Electronic security system interfaces

3. The investigators should be considering the HVAC mitigation strategies to reduce the potential bio-burden in the building that could be implemented on the systems.

4. When checking calibration, use the guidance in ASHRAE Guideline 11-2018, Field Testing of HVAC Control Components 

5. Prepare a deficiency log and issue work orders to in-house maintenance personnel and purchase orders to qualified service providers to correct any critical issue identified in steps 1 and 2 that would prevent the system(s) from functioning in accordance with the systems' original design intent or the building's current use, occupancy and activity.

6. Prepare a Building Readiness Plan that identifies the HVAC mitigation strategies for the systems. This should include a brief work order description for the in-house maintenance personnel and qualified service providers. This should detail modifications or additions to components, systems and controls necessary so that the recommendations included in this document may be implemented.


Building Automation Systems (BAS) | Return to Top

Evaluate your BAS:

It is crucial for the owner to understand the type of BAS they have in their building. HVAC controls range from simple single zone thermostats controlling a single HVAC unit's heating and cooling modes of operation, to complex BAS that integrate the controls from large building owners and owners' with multiple large buildings in their portfolios, such as school districts, university campuses and large government installations, and everything thing in between.

In addition, there are legacy HVAC systems and BAS that still use electric and pneumatic controls and time clocks that do not have modern, digital communications interfaces and, therefore, do not allow building operators any insight into how their buildings are performing without being physically in the building or at the piece of HVAC equipment.

Remote Access:

If the building is equipped with a Building Automation System (BAS), it should have an existing method for remote access.

If the BAS does not have a method for remote access, the owner should coordinate with the buildings IT provider and BAS provider for secure remote access for the required users.

  • Cybersecurity must be put at the forefront of this endeavor as to not open the BAS and other building networks to unauthorized access.
  • If the BAS is not on its own Virtual Local Area Network (VLAN), consider segregating the building systems (BAS, Fire Alarm, Card Access, Cameras, etc.) into a VLAN to limit remote exposure to the buildings internal networks.
  • Consider two step authentication as mandatory for remote access.
  • Care should be taken in granting editing access to the BAS to knowledgeable, trained operators only.
  • Set up user logging such that a virtual log of all changes are documented.

These remote systems range from the simple to complex communication capabilities.

  • The simple could be dial up modems transmitting alerts and notifications to cell phones and/or email addresses.
  • The more complex is a BAS system that is connected to local area networks that can be accessed via VPN connections.

Depending on that connection, there are variations to the amount of data access which can range from limited data to a fully web based, graphic user interfaces connected to a host of mobile devices such as smartphones, tablets and stationary PC workstations.

Prior to making any changes:

  • Perform a full backup of all BAS software, databases, programs, graphics, trends, schedules, etc. and store off site either physically or in the cloud.
  • Consider printing them physically or to a pdf so that values can easily be returned when the epidemic is over.
  • Inspect or replace any batteries in building controllers such that databases are not lost during any extended power interruptions.
  • If your building is not on a scheduled BAS inspection (either by third parties or self-performed) consider performing a preventative maintenance inspection of all systems to ensure proper operation prior to any changes being made. Consider retaining the services of an independent 3rd party commissioning service provider (CxP) to help you review the scope of work for any control system modifications and who can verify the systems are functioning as intended.
  • Review the access requirements with all parties that the owner wants to have remote access during the unoccupied or modified mode of operation period.
  • Determine the level of access and permissions each person with access should have such as full access to make changes in set points, schedules and system programming, schedule overrides only, alerts and notification access only and view only access.
  • Confirm with company IT departments what requirements may be in place to qualify, screen and approve people for remote access to control systems and company IT networks.
  • Set each person up as a unique user having unique usernames and password and permission levels so that access and changes to the system can be monitored and documented.
  • Have a trained and experienced operator go over the existing systems remote access features of the system and its interface with anyone who will be given remote access to the system.
  • Review all alerts, notifications, event logs and system and control point trend reports prior to making any modifications and download those reports to create a baseline for comparing the effects of any changes that may be made in the future.
  • If possible, walk the facility or facilities being controlled and managed by the BAS to become familiar with the location, size and scale of the control network.
  • As minimum, review system graphics for all system types and buildings to become familiar with the system(s).
  • Make note of any communication issues with components, sensors, controllers, buildings, etc. and develop a list of repairs that may need to be made before the system is placed in extended shut down, unoccupied or partial occupancy modes of operation.
  • Review system graphics or text-based reports to determine if temperatures, humidity, CO2, airflows (supply, return, outside air, exhaust), damper positions, control valve positions, motor speeds and status are returning or reporting reasonable values.
  • Use test instruments to verify any questionable information and to spot check a representative quantity of points. Start with verifying critical sensors, such as CO2 or airflow measuring stations.
  • Collaborate with the building owner, building users and building operators and create a plan for modifications to sequences, set points and system operations.
  • Note who was in attendance, what was discussed, and any decisions made and implemented.
  • Obtain buy-in and approval from key stakeholders before making any changes.
  • Repairs to systems involved in this response should be considered mandatory as any new sequences may not be able to be implemented via the BAS.

Making changes to accommodate epidemic responses:

  • After determining what sequence of operation changes are appropriate, make small changes to the system at a time and monitor for a few days or through some varying weather conditions to make sure the system and building(s) is responding to the changes as expected.
    • Have the CxP or Control Contractor verify and document the effect of the changes through key trend reports and physical measurements or standalone data loggers.
  • Keep good records and document all meetings, agreed to repairs, maintenance and changes with written communication.
  • The team should consider making the changes to include an automated response such that you may return to the original sequences (or pre and post pandemic sequences) at the push of "virtual" button.
    • Care should be taken to limit access to the initiation of these automated sequences as they may have a large energy and comfort impact on your facility.
  • Existing alarm parameters may need to be adjusted during these new sequences as the original "normal" conditions may not be able to be met.
  • Ensure that this team follows the guidance for the facilities Systems Manual later in this document.

Ventilation per Code/ Design: | Return to Top

ASHRAE is indicating that the building systems need to be evaluated to confirm that the building’s HVAC systems are capable and operating to provide the code required or design levels of outdoor air when the building is occupied.

The intent is to operate the systems in this manner when the building is occupied.

Increasing outdoor air above code / design is considered a mitigation strategy to be evaluated.


Increased Ventilation above Code | Return to Top

There is potential that building operators could increase their systems outdoor air ventilation to reduce the recirculation air back to the space. The guidance indicates that this should be done, if it is the selected mitigation strategy for this system, as much as the system and or space conditions will allow. It is very important that these overall building systems are evaluated by a qualified TAB firm, Cx provider or design professional to ensure that the modifications for pandemic safety do not create additional issues.

One major concern is the ability to maintain space conditions. Hot and humid climates could struggle to keep the space below acceptable temperature and relative humidity for comfort. Cold climates could struggle to keep the space above acceptable space temperature and relative humidity for comfort. It is important to note that research indicates that maintaining the space relative humidity between 40% and 60% decreases the bio-burden of infectious particles in the space and decreases the infectivity of many viruses in the air. The team should consider adjusting the space comfort setpoints to increase the system's ability to use more outside air.

The ability for a cooling coil to provide additional capacity was evaluated using a typical cooling coil at various percent of
outside air. This evaluation shows the additional required cooling capacity and gpm required[1] if the same exact coil
experiences the different entering air conditions while achieving constant leaving air conditions. The following shows the
impact of increasing the percent of outside air:

The ability for a heating coil using heating hot water was also evaluated for its ability to handle the added load for various percent of outside air. This evaluation shows the additional required heating capacity and gpm required if the same exact coil experiences the different entering air conditions while achieving constant leaving air conditions. This case was to use a pre-heat coil that conditions the outdoor air prior to it connecting to an air handling unit, so it is in essence a 100% outdoor air coil. This assumes the same 10,000 CFM total supply air from the AHU to understand the percent OA.

The following shows the impact of increasing the percent of outside air:

This coil is based on sizing it at 2,000 CFM for 20% OA of a 10,000 CFM AHU. The two-row coil was held at 14” long by 44” wide and 29” high. Fins are spaced at 80 per foot. The entering air was assumed to be near the ASHRAE 99.6% Heating conditions for Springfield, Illinois which is 4.3°F dry bulb and 2.2°F wet bulb. The selection use 4°F dry bulb with a target discharge of 55°F air to the AHU.

Increased Ventilation Continued:

Another way to potentially increase the quantity of outside air is to clean the cooling coil to recapture lost heat transfer from fouling.

  • Studies indicate that dirty coils reduce the capability for heat transfer.
  • Please follow the appropriate maintenance procedures for coils.


Increased Ventilation Control | Return to Top

The assessment team determining how much more a coil can handle can see that increasing from 20% outside air to 90% outside air doubles the required chilled water, triples the coil pressure drop and requires just over twice the amount of cooling source from the chiller plant. And even with a large capacity heating plant where heating capacity might not be an issue the coil may be the limiting factor. The capacity of the plant and coils need to be evaluated.
 
There are other options to increase the outside air in an AHU as much as the building automation system (BAS) will allow based on space conditions. There are at least two approaches to modify a system to optimize the outside air without ignoring space comfort that is a twist on the dynamic supply air temperature reset strategy. This is assuming a typical variable air volume AHU serving multiple VAV boxes or as a single zone VAV unit. The outside air damper and return dampers could be linked or separate, but they work in opposite directions in any option presented.

Option 1: Increased OA based on Cooling Coil

  • If the cooling coil control valve is less than 90% AND the discharge air temperature (or space temperatures) are satisfied, OPEN the OAD [CLOSE the RAD] 3% every 15 minutes.
  •   If the cooling coil control valve is greater than 90% OR the discharge air temperature (or space temperatures) is exceeded by 1 degree F, CLOSE the OAD [OPEN the RAD] 6% every 5 minutes.

Option 2: Increased OA based on Space Conditions

  • This option assumes that a coil leaving air temperature controls the CHW valve to maintain a constant setpoint.
  •   If the space temperatures are satisfied and the relative humidity is less than 55%, OPEN the OAD [CLOSE the RAD] 3% every 15 minutes.
  •   If the space temperatures are exceeded by 1 degree F OR the relative humidity is greater than 60%, CLOSE the OAD [OPEN the RAD] 6% every 5 minutes.

These options require different sensors to be installed in the unit to work properly. Either sequence would allow the unit to increase the outdoor air without exceeding the space comfort conditions. It is also important to note that demand controlled ventilation, static pressure reset strategies and the typical supply air temperature reset strategies should be disabled.


Building and Space Pressure | Return to Top

Building and space pressurization is another important consideration.

Care should be taken when increasing outside air but keeping exhaust and relief air systems as designed. New problems can be created such as:

  • Doors that will not close,
  • Excessive noise at entrance doors and between adjacent spaces,
  • Access / egress issues at common hallways or egress points (in extreme conditions).
  • Reverse of the intended pressure required for a space.

Excessive building pressurization can also affect vertical transportation systems and areas that are intended to be negatively pressurized, such as commercial kitchens, bathrooms, process areas, and custodial areas.

It is very important that these overall building systems are evaluated by a qualified TAB firm, Cx provider or design professional to ensure that the modifications for pandemic safety do not create additional issues.


Pre- or Post-Occupancy Flushing Strategy: | Return to Top

The intent is to ensure that while the building is occupied, your ventilation schedule should assist in removing bioburden during, pre-, or post- occupancy of the spaces. Flush the spaces for a duration sufficient to reduce concentration of airborne infectious particles by 95%. For a well-mixed space, this would require 3 changes of space volume using outdoor air (or equivalent outdoor air including the effect of filtration and air cleaners) as detailed in the calculation methodology.

In lieu of calculating the air change rate, pre- or post-occupancy flushing periods of 2 hours may be used since this should be sufficient for most systems meeting minimum ventilation standards.

So for each mode, the control would be as follows:

  • Occupied: bring in the code / design outdoor air per system.
  • Pre- or Post- Space Occupancy: The general method is to operate the systems in Occupied Mode for “x” hours prior to, or after, occupancy. Use the calculation to determine “x”.
Flushing Air Changes Calculations for Well-Mixed Spaces
  • The flushing process is intended to be for removing gases in a well-mixed space.
  • It is good for removing VOC's, CO2, and anything contaminant that approximate gases.
  • While all gases can be assumed with good science to be evenly distributed in the space, particles may not. Airborne particles cannot be treated as gases and these particles follow relaxation theory and move on air currents, some unknown percentage of particles may go back to the return and likely do.
  • The entire principles that this section is based on is delivering viral load particulate reduction to the space diluting the clean air breathing zone.


Equivalent Outdoor Air: | Return to Top

The equivalent outdoor air calculation indicates that the outdoor air can be calculated by using the combination of the actual outdoor air, impact of filtration or air cleaning technologies on recirculated air, and the impact of air cleaning technologies in the space.
 
This is using the principal of filters in series and the effectiveness at reducing particles. For items in series, the initial item would see the recirculated airflow to clean. The second item in the series would see the “cleaned” air from Item 1 and so the impact of Item 1 must be accounted for in Item 2.

As an example, if the unit had a filter at the return and a UVC single pass inactivation system after the cooling coil. This makes the filter Item 1 and the UVC Item 2, and so the air that is treated by UVC has already been treated by the filter.
 
Let’s look at it via equations.

Assumptions:
 
Q_R is the recirculated air
ACH_e is the air changes of equivalent outdoor air
ACH_oa is the air changes of outdoor air
ACH_f is the equivalent outdoor air changes due to filtration
ACH _e,c is the equivalent outdoor air changes due to the air cleaner after the effect of the filter
 
Q_R is the recirculated air
Q-e is the equivalent outdoor air flow rate
Q-e,f is the equivalent outdoor air flow rate from the filter
Q-e,f+c is the equivalent outdoor air flow rate from the air cleaner (after it went across the filter)
Ez = zone air distribution effectiveness (From Std 62.1 Table 6-4)
eta_f is the efficiency of filter
eta_c is the efficiency of the air cleaner
eta_T is the total efficiency of the series of devices

So, eta_T = 1 – (1-eta_f) * (1-eta_c), which allows us to state that
 
Q-e,f+c = Q-e,f + Q_e,c Q_R* [1-(1-eta_f)
 
OR
 
Q-R * [1-(1-eta_f)*(1-eta_c)] = Q_R * eta_f + Q_e,c
 
Re-arranged it results in the following equation:
 
Q_e,c = Q_R * (1-eta_f) * eta_c
 
The goal is to determine equivalent air changes, so revise the equation to be:
 
ACH_e = ACH_oa + ACH_f + ACH_e,c
Converting from Q to ACH is based on the fact that ACH = (CFM * 60) ÷ Volume
 
Then, you want to apply the impact of the Zone Air Distribution Effectiveness.

Then, you want to apply the impact of the Zone Air Distribution Effectiveness to include the impact that a space is not always well-mixed. ASRHAE Std 62.1 Table 6-4 [7] provides the Ez value for multiple systems that will be used to adjust the time to flush. The starting point is 1.0 and can be adjusted in the calculation tool.

Select the appropriate Ez for your system from this table from Standard 62.1:

Equivalent Outdoor Air: EXAMPLE

The following is an example on applying these concepts to a real system and space.

Example:

Keep in mind that this is to account for them in series and the equation would have to be modified to align with the arrangement of the systems engineering controls being applied to the recirculated air.

Also, only the recirculated airflow should receive capture or inactivation of virus by the filter and air cleaning technology. Outdoor should typically not be included as it is typically accepted to be clean of virus.

Now, for an actual example that has MERV-10 filters (the equipment could not handle the MERV-13 filter target) and UVC after the cooling coil with an inactivation of 95%. You also need to use the filter’s Droplet Nuclei Efficiency as described in Filter Droplet Nuclei Efficiency.

The school system AHU was a recirculating system supply air to 900 SF with 9-foot ceiling space from a variable air volume boxes with electric reheat. The VAV was set-up to supply 1,350 CFM Supply Air (SA) to the spaces resulting in 10.0 effective supply air (SA) air changes per hour (ACH). The unit is balanced to provide 400 CFM of outdoor air (OA), resulting in 2.96 effective ACH of OA. The unit would look something like this with resulting calculations:

ACH_oa = 2.96 ACH

ACH_f = (10-2.96) * 63.53% (MERV 10 Droplet Nuclei Efficiency) resulting in 4.47 ACH

ACH_e.c = (1 – eta_f) * eta_c * (ACH_SA – ACH_OA) = (1 - .63) * 0.95 * (10.0-2.96) = 2.444 ACH

ACH_e = 2.96 + 4.47 + 2.44 = 9.87 ACH

Then the Zone Air Distribution Effectives will be applied when calculating the time. In this case, we have and overhead supply and return whose system is controlled to keep the heating air no more than 15 F (-9.44 C) above room temperature setpoint. This results in an Ex of 1.0.

Therefore, the time required to complete the 3 building volume changes with outdoor air equals:

Time = 3 / 9.8716 / 1 = 0.3 hours or 18.23 minutes.

This means that by doing the calculation, the pre- or post-occupancy flush would only be set up for approximately 20 minutes instead of 120 minutes.

There is a calculator created to help use this method to determine your system’s time to flush. This is set-up for the specific scenario of the filter and then UVC or In Room HEPA Fan Filter Unit in the space. If your arrangement is different, you would have to revise the calculation method to make sure the impact of Item 1 is incorporated before the treatment of Item 2.

Equivalent Outdoor Air Calculator

Some key items to note about this spreadsheet:

  • This calculator allows you to adjust the virus distribution you anticipate. The starting point is based on the information presented for Influenza A but could change as more research is made available for SARS-CoV-2.
  • This calculator allows you to enter in your specific filter that is in the facility. There is information for filters that are used to determine the filter performance in the Filter Droplet Nuclei Efficiency section.
  • This calculator allows you to select your Zone Air Distribution Effectiveness value based on the HVAC system installed. This helps adjust for spaces that may not be well mixed.
  • The following impacts are not being addressed here that result in a more conservative time:
    • Deposition of particulate (PM2.5 is typically assumed to be about ½ air change of particle reduction)
    • Temperature of the space and or change of temperature.
    • Relative humidity of the space and or change of RH.

The following is a chart [8] that shows the time required to flush the building to three different contaminant removal goals (90%, 95%, or 99%) if you know your equivalent outdoor air changes per hour of your system.


Upgrading & Improving Filtration | Return to Top

Building owners are encouraged to improve the efficiency of the filters serving their HVAC systems within the guidance provided for most of the building types listed on the ASHRAE COVID-19 Preparedness Resources website. Mechanical filters are the most common types of filters found in HVAC systems. According to the ASHRAE Position Document on Filtration and Air Cleaning, the term used to describe mechanical filter efficiency is MERV, which is an acronym for Minimum Efficiency Reporting Value(mechanical filter definition also include filters that have a static electrical charge applied to media prior to use). The MERV rating of a mechanical filter is determined by filter manufacturers in accordance with ASHRAE Standard 52.2 - Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size . Standard 52.2, table 12-1 lists filter MERV rating parameters for MERV 1 through MERV 16. The higher the MERV number the better the ability of a filter to remove particles from the air ranging in sizes from 0.3 micron diameter up to 10 microns in diameter at standard airflow conditions and face velocities specified in the test standard. A more detailed discussion of the various air filtration and disinfection technologies available may be found on the ASHRAE COVID-19 Preparedness Resources main page under the Filtration/Disinfection tab.

ASHRAE recommends that mechanical filter efficiency be at least MERV 13 and preferable MERV 14 or better to help mitigate the transmission of infectious aerosols. Many existing HVAC systems were designed and installed to operate using MERV 6 to MERV 8 filters. While MERV 13 and greater filters are better at removing particles in the 0.3 micron to 1 micron diameter size (the size of many virus particles) the higher efficiency does not come without a penalty. Higher efficiency filters may require greater air pressures to drive or force air through the filter. Care must be taken when increasing the filter efficiency in an HVAC system to verify that the capacity of the HVAC system is sufficient to accommodate the better filters without adversely affecting the system's ability to maintain the owner's required indoor temperature and humidity conditions and space pressure relationships.


Filter Droplet Nuclei Efficiency / Particle Size Expectations: | Return to Top

TARGET for the SARS-CoV-2 Virus is based on Influenza A Studies

The virus particle itself is very small, less than 0.14 micron. However, we know that it is sticky due to the lipid envelope as well as the sputum / saliva and therefor clumps with other particles making them larger. This is shown in the diagram below from the:

Azimi, Parham, and Brent Stephens. “HVAC filtration for controlling infectious airborne disease transmission in indoor environments: Predicting risk reductions and operational costs.” Building and environment vol. 70 (2013): 150-160. doi:10.1016/j.buildenv.2013.08.025

Also, filters are rated at capture efficiency in three different ranges of particles [E1 is 0.3 to 1 um (micron), E2 is 1 to 3 um (micron), and E3 is 3 to 10 um (micron)].

Therefore, we must understand the range of virus sizes in the airstream to evaluate a filters overall performance in capturing the virus.

Several research studies used quantitative polymerase chain reaction, or q-PCR to identify the presence of viruses or bacteria in expelled droplets and droplet and offered insights not only into what size aerosols exist after expulsion from the human body, but in what size-fractions are viruses or bacteria actually present and are thus of most concern for infectious disease transmission.

A study published by Dr. Brent Stephens on March 1, 2012 for The National Air Filtration Association (NAFA) Foundation titled “HVAC Filtration and the Wells-Riley approach to assessing risks for infectious airborne diseases”. The following is an excerpt from that study:

These previous studies all confirm that aerosols generated during coughing by influenza patients and subsequently remaining suspended in indoor environments indeed contain the influenza virus and that much of that viral RNA is contained within particles in the respirable size range (i.e., <4 μm). However, whereas ~100% of the number of particles emitted during the aforementioned coughing and breathing studies were smaller than 4 μm in size, only 40-70% of the influenza virus RNA is typically detected on particles in this size range (Blachere et al., 2009; Lindsley et al., 2010; Lindsley et al., 2010a), suggesting that the virus content of aerosols may actually be skewed toward larger particles. A bias toward the presence of virus particles contained in larger particle size fractions also suggests that particle surface area or volume may be more appropriate for characterizing infectious aerosols, which is reasonable considering that a 5 μm particle would have a volume approximately 100 times greater than a 1 μm particle and thus likely store a greater amount of virus particles inside droplet nuclei. Although there is considerable uncertainty in all of these measurements and sample sizes remain limited, these studies provide important insight into what size of expelled and suspended particles actually contain virus particles.

Below is a summary Table published by Azimi and Stephens in 2013: