ABSTRACT

The coronavirus disease 2019 (COVID-19) pandemic, brought about by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is putting a new focus on the approach to ventilation design and operation of buildings. This is because control of ventilation above normal rates is necessary to create a safe indoor environment while protecting the building infrastructure. A safe and healthy building is dependent on dilution ventilation and maintaining proper pressurization. A building’s ventilation system during normal operation strives to achieve acceptable indoor air quality (IAQ) based on building codes. The setup and operation of ventilation systems have variances and limitations. When a ventilation system fails to provide adequate ventilation, the effects of this are not immediately known. Unless the rate of outdoor airflow is actively measured, determining under-ventilation may only come after it is too late to prevent the outcome. A building is a complex system, and it and the HVAC systems are subjected to dynamic changes. Degradation, over time, impedes the HVAC system’s capabilities. Weather and seasonal changes impact the ability to provide consistent ventilation. Proxy solutions, such as CO2 control, can provide an indication of ventilation but no definitive value. Controlling ventilation and exhaust is necessary to set building pressurization and to control indoor temperature and humidity. There are health, productivity, and increased property value benefits when creating a healthy indoor environment. In the case of combating a health crisis, monitoring and controlling ventilation are paramount. SARS-CoV-2 is recognized as an airborne virus through aerosolization. This fact has changed the dynamic from providing ventilation rates to minimizing dissatisfaction with IAQ to providing ventilation rates that dilute the concentration of virus particles in the air (1, 2, 3).

This dilution through mechanical ventilation is recognized as the primary means of virus removal for all indoor environments (1, 3, 4, 5). Monitoring and controlling actual flow rates in and out of the building is essential in ensuring dilution and pressurization, maximizing efficient operation of HVAC equipment, and creating a safe and healthy building.

BACKGROUND

Building codes and standards specify minimum ventilation rates to provide acceptable IAQ. Achieving acceptable IAQ is when 80% or more of the occupants do not express dissatisfaction and there are no significant known contaminant concentrations or an unusual source that would create a contaminant of concern (6). Acceptable IAQ is designed in accordance with the Ventilation Rate Procedure to determine the minimum outdoor air for the occupants per space use and building type (6). The rates are intended to dilute dissatisfying odors from individuals and off-gassing from furniture, components, and building materials. Some spaces within buildings have exhaust requirements (e.g. toilet rooms, kitchens, janitor closets), as they have higher concentrations of odors. Buildings that are designed for laboratory or patient care use do not fall under this procedure, as they have a higher level of ventilation and exhaust design requirements and operational controls to limit the migration of infectious diseases or harmful contaminants. When buildings outdoor air ventilation systems are designed for commercial buildings, they are typically engineered in accordance with the mechanical code prevailing for local jurisdiction (e.g. International Mechanical Code (IMC), Uniform Mechanical Code (UMC), California Title 24 part 6 (T24), ANSI/ASHRAE Standard 62.1 (62.1)). These codes and standards are under continuous maintenance and constantly evolving, with a new full publication every 3 years. Throughout North America, there are several published versions in effect by state, province, county, or city. The prescriptive ventilation rates within these codes are minimum requirements. Most professionals design only to these minimum rates as there is always a trade-off between ventilation and energy, although some owners choose a design that enhances ventilation by following WELL (7), LEED (8), and IGCC (9).

Ventilation rates are typically set by a Testing Adjusting Balance (TAB) professional. If a project does not enlist these services, then it typically falls on the installing contractor or, in the case of T24, an Acceptance Test Technician. If there is a Commissioning (Cx) professional on the project, they may perform functional performance tests to verify outdoor air ventilation and exhaust systems are operational. The conventional thinking is that a designed ventilation rate will be set up during construction and that this one-time setting will forever maintain the correct ventilation requirements of the building. However, certification that airflow rates meet specifications is the most difficult field measurement that a TAB engineer has to perform (10). The amount of outdoor air ventilation in an indoor environment is difficult to perceive with our senses. Unless it is directly measured, a person cannot determine if the amount of ventilation air is the correct amount. There are delayed indications of inadequate ventilation that affect humans, such as headaches, fatigue, and upper respiratory irritations. When this happens to a large percentage of building occupants, it is termed sick building syndrome (SBS) (11).

When the ventilation setpoint is made, it is typically adjusted to deliver the ventilation rate designed for full occupancy at full flow with all zones proportionally balanced (10, 12, 13). Sections 5.1 and 5.3 of 62.1 require the minimum ventilation airflow to be maintained and delivered under any load condition or dynamic reset condition and provided with controls that maintain no less than the outdoor air intake flow as required by the prescriptive ventilation procedure (6). The 62.1 User’s Manual 14 clarifies this by stating, “To comply, most variable-air-volume (VAV) systems will need special design considerations and often features such as outdoor airflow sensors, modulating dampers, and injection fans.” It further states, “A major consideration with VAV systems is that the negative pressure behind the outdoor air intake in the mixed air plenum will typically vary with supply air volume. At low supply airflow rates, this negative pressure will decrease, and sufficient outdoor airflow may not be maintained with a fixed outdoor air intake damper position—or even if a dedicated fixed minimum air intake is used.”

With the ongoing effort to save energy in the operation of buildings, the energy codes and standards (e.g. International Energy Conservation Code (IECC), California Title 24 part 6 (T24), ANSI/ ASHRAE Standard 90.1 (90.1)) have incorporated fan airflow control on constant volume units that reduce the supply fan speed as a function of load. The effect of this change in airflow is no different than VAV. Therefore, newer constant volume systems need to address outdoor ventilation control in the same manner as VAV. Furthermore, the 62.1 User’s Manual14 addresses another problem with constant volume systems: “If the thermostat in such a system has a fan on/off/auto switch and the switch is left in the auto position, the fan will cycle off when the temperature set point is reached. If the fan system also supplies outdoor ventilation air, the cycling fan causes the outdoor air supply and ventilation to be discontinuous. Since many untrained people do not understand this, the switch is often placed in the auto position, resulting in inadequate ventilation.”

Outdoor damper curves are different for system operation at design and turndown. The amount of damper opening to maintain constant minimum is greater at turndown than design (15). Additionally, dampers and actuators have combined hysteresis, the tendency to arrive at different positions for the same signal input when moving in different directions (15). It makes the position of the actuators very inaccurate; this is especially serious concerning the minimum position of the outside air damper (16). Hysteresis is caused by internal damper linkages and low leak seals, large damper sections, actuator to damper linkages, multiple sections jack-shafted, and higher torque applications requiring “piggy backed” actuators. Actuators have signal hysteresis to protect motors and mechanical hysteresis due to internal gear trains and springs. An actuator’s feedback is believed to be a closed-loop control. However, it only provides information on the actuator position, not the actual damper position or the ventilation flow rate.

Changes in weather and seasons impact ventilation setpoints. Wind can hinder intakes or exhausts; the ventilation rate can be reduced by wind pressure (17). Wind creates pressures that affect the mixed air plenum pressure in a similar way as varying the fan speed. On systems without mixing plenums (e.g. exhaust, makeup air, Dedicated Outdoor Air Systems (DOAS)), the wind can add a pressure source in series with the fan that impacts operation on the fan curve (17). Fixed damper setpoints are affected by wind and stack pressure (18). Stack pressure, known as Stack Effect, is due to density differences in the air. This density difference is caused by temperature differences between indoor and outdoor and creates a pressure difference that changes with seasons. This pressure can act on HVAC openings and extend through return or exhaust ducts that extend multiple floors. The pressure differential is directly proportional to building height and the difference between indoor and outdoor temperatures (19). Although it is often considered a phenomenon in tall buildings, locations that have large temperature differences between summer and winter can experience a stack effect on smaller buildings as well. Thereby an effort to balance intakes and exhausts in the summer can result in reduced flow rates in the winter. This pressure also varies across the building envelope and floor to floor, making maintaining building pressurization control to prevent infiltration a dynamic problem.

Not all velocity and airflow measurement systems are equal. Some have limitations in ranges and application, are more affected by installed conditions, are dependent upon field measurements, or have wide ranges in precision (17). Since all systems have some degree of turndown, it is important to look at the complete range of operations. It is important to understand all the components that make up the flow measurement device, such as measurement limitations and potential drift, and any maintenance or calibration requirements associated with them (e.g. pressure transducers). It is best to use a measurement device that has been factory calibrated for the application and is traceable to a national airflow measurement standard such as NIST. Products that have a low-density measurement or require setup based on field measurement carry additional errors.

Demand control ventilation (DCV) is an energy saving strategy that is required by energy codes, for some high occupancy and variable load spaces. It is permitted by ventilation codes and standards as a means of Dynamic Reset. One of the more popular ways to reset the zone ventilation and/or the outdoor air ventilation rate is by sensing CO2 in the space as a proxy for occupancy count. This is because the VRP has a ventilation rate per person component. CO2 DCV has many challenges in implementation; among them is determining the correct generation rate by knowing the population makeup and activity level (20). It is important to note that CO2 is a lagging indicator, and steady-state concentrations will only be achieved if conditions, including occupancy, are constant for long periods of time. Sources of error for this strategy are the accuracy of CO2 sensors, their placement, and requiring that the air in the space is well-mixed. Too often, control strategies and some standards make incorrect assumptions in the parameters used. There is a codependent relationship between CO2 levels and ventilation rates, and it will differ for occupancy type. It is also common for controls to not increase ventilation off-floor area base rates until desired steady-state measurements (e.g., 1000 ppm) are reached. This approach saves energy; however, it causes under-ventilation by not providing the ventilation rate for the actual current population. There are improved approaches to incorporate CO2 DCV (21).

DISCUSSION

Viruses have been identified as the most common cause of infectious disease acquired within the indoor environment (22). Airborne transmission can occur via direct contact with droplets, skin flakes, and fungal spores. Aerosols can be generated (atomized) during breathing, speech, sneezing, singing, vomiting, feces evacuation, and toilet flushing (22, 23). Aerosols can be as large as 100 µm (microns) in diameter and can exist in liquid or solid form with the majority of respiratory aerosols being less than 10 microns (22, 23, 24). As a reference, the human hair is between 50 to 70 microns. A study has shown that individuals coughing while infected with influenza can release 75,000 respiratory droplets (25). The size distinction is important because larger droplets settle (fall to the floor, furniture, clothing), and smaller ones remain suspended in the air from minutes to hours (4, 22, 26).

The room environment impacts the survival of the virus and its ability to remain airborne (26). When a respiratory droplet is expelled, it is at 100% relative humidity (RH), and it decreases rapidly in size by evaporation into a room with a lower RH (27,28). This reduced size droplet has the potential to survive longer in the air and to reach different parts of the respiratory tract. Those that remain in the air may eventually become inactivated, settle out, or be diluted out by ventilation and exhaust (29). It is important to note that the virus itself is smaller in size than the respiratory droplet, so there can be more than one virus particle per respiratory droplet. As an example, SARS-CoV-2 is 0.12 microns and influenza is 0.10 microns. It is also possible for a larger droplet to settle out and then to be resuspended back into the air as a smaller droplet from clothing, the floor, or other surface that is disturbed4,6. Other viruses that can be transmitted via airborne aerosols are tuberculosis, measles, chickenpox, legionella, SARS CoV-1, and MERS-CoV (4, 5, 30).

Characteristics of viruses are unique to a virus and require significant research. Typically, a virus crisis ends before much is known about it. We have a version of Influenza on an annual basis; however, scientists still do not know all there is to know about its ability to transport and infect people (31). What we know is that the infectious concentration in the air is dependent on the generation rate (Quanta) and the removal of the virus by settling inactivation (decay rate), and ventilation (29). The Quanta is influenced by how it is atomized, droplet size, temperature, and humidity. Indoor environments are perfect for the transmission of viruses. However, we can increase the decay rate and strengthen the human response if we keep room RH between 40-60%. Viruses have more vitality or less chance of inactivation when the RH is > 90% or <40% (27, 29, 32, 33). It is also known that the amount of virus RNA copies atomized, increases with metabolic rate and vocal level (34). It is recognized that coughing and sneezing produce significant levels of atomized particles. However, in some cases, more particles are produced by singing and shouting; also, breathing and general speaking will also release virus particles (23, 35, 36).