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Strategies to optimize your HVAC system during the pandemic

A guide for employers, public entities, schools and more

There is a great deal of uncertainty today regarding our health and the health of those around us. This concern remains true for occupants of our buildings, whether they be students we are entrusted to care for, employees or the general public. Our buildings and their mechanical systems can greatly increase the amount of protection provided against COVID-19 and other illnesses – or they can impede it. An engineering and environmental survey can help to maintain expected levels of safety. Consider the following areas of focus.


There are many unknowns regarding how COVID-19 transmission is impacted by the performance of a heating, ventilation and air conditioning (HVAC) system within a space. But professional bodies such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) have indicated transmission of the coronavirus through the air is sufficiently likely and changes to building operations, including the operation of HVAC systems, can reduce airborne exposures.

Historically, studies showing the mechanics of airborne droplets and aerosols have been well documented, demonstrating that these particles can remain aloft in a still room for many days. However, studies have also shown that increased ventilation and air changes can reduce the risk of spreading the virus within indoor environments (AIHA 2020).

Air changes vs. ventilation

Typically, when people talk about “air changes,” they’re referring to a number of air changes per hour provided by the ventilation system. The air change per hour (ACH) rate is a simple calculation that uses the supply air provided by the HVAC system and the room volume. It should be noted that for most ventilation systems, the supply air that comes from an air handling unit (AHU) is a mix of return air (RA) and outside air. This means that for such a system, the ACH is based on a mix of outside air and return/recycled air from the building. Depending on how the HVAC system is configured, that return air can come from a number of spaces. For a typical AHU, the outside air is 20-25% of the total supply air flow. This means that if you have an ACH rate of 4 and 25% outside air, you effectively replace the air in that space once per hour with outside air.

It should be noted that newer systems often consist of in-room recirculation devices, such as chilled beams (CBs) or fan coil units (FCUs), combined with dedicated outside air systems. The purpose of these combined systems is to increase energy efficiency. They are often combined with a heat recovery system and tend to reduce fan energy usage.

AHU filtration

The typical AHU for school and office applications is designed to use filters with MERV 8 ratings. The higher the MERV rating, the higher the filter effectiveness. ASHRAE recently released a one-page document “Guidance for the Re-Opening of Schools”– although the guidance is pertinent to most buildings. In that document, they recommend increasing the filtration to MERV 13 rated filters. The big caveat in that document is, “…if it does not adversely impact system operation.” This is important because MERV 13 filters will increase the pressure drop through the filters by reducing the amount of air that is able to pass through the membrane. In addition, all filter racks may not be able to accommodate the dimensions of higher rated filters.

If someone decides to proceed with a filter rating alteration, they should investigate whether the system has enough excess capacity to overcome the increased pressure drop and that they can fit the higher rated filters. This should involve an engineering analysis of the system.

Local HEPA filtration units

An option that may be worth considering is the use of local standalone high-efficiency particulate air (HEPA) filter units that use 115V power and can be plugged into any wall outlet. These units are equipped with HEPA filters that can be replaced and recirculate the room air while filtering it through a HEPA filter. High-performance HEPA filters can clear out very small particles such as viruses and are often used in biological containment applications. Smaller capacity HEPA units will not normally impact the performance of building HVAC systems. These units are often referred to as HEPA purification units and are readily available for purchase from many vendors on the internet.

Even with correctly functioning HVAC systems, many buildings may experience poor mixing, end of branch isolation, or poorly regulated areas which may require a local filtration unit. It is also important to consider your at-risk building occupants, as these locations may require local filtration units as well for additional protection.

Increase ventilation

To many building operators, it is tempting to increase the outside air rate when confronted with occupant concerns; however, this should only be done with caution after a careful engineering review. As mentioned above, a typical AHU is designed for 20-25% outside air. The design team that designed the HVAC system based the sizing of the heating coil on the mix of RA and outside air that the unit was designed for, not 100% outside air. This means that if the operator increases the amount of outside air in a system without carefully considering the limits of the system, they will effectively have an undersized heating coil and increase the risk for freezing the coil with potential disastrous results. Making alterations to any engineered system can be a high-risk decision. To avoid extensive damages, equipment failure or reduction of equipment life expectancy, ensure an engineering evaluation is conducted before altering the operation of a system.

It may also be tempting to leave windows open to increase the amount of outside air in a space. This solution can only be applied as a temporary option, as it impacts the performance of the building HVAC system. Additionally, when it is cold or hot outside, it will negatively impact thermal comfort in a space. There is also the additional risk that windows will be left open accidently, thus increasing the potential for freezing a space and increasing security risks.

If an HVAC system is operating on a schedule, the operators may opt to increase the operating hours so that the system operates for additional hours and thus “flush out” the building before and after occupancy. The operators should also review the condition of the ventilation equipment. Debris on intake screens, coils and heat recovery components can reduce the airflow significantly. Making sure everything is clean and free flowing will help ensure better airflows. Increasing operating time on a system with known deficiencies can cause those deficiencies to be exacerbated and can negatively impact the system.

Local units

Many spaces are equipped with smaller local units that either work alone or with other building HVAC systems. Examples of such units are the CBs and FCUs mentioned previously, as well as cabinet unit heaters. These units do not bring in any outside air and simply recirculate the room air. Some units have filters; however, when they do, they are typically low-performing filters. These types of units are not intended to be used with high-performing filters.

There are local units that do bring in some outside air. These units are often referred to as unit ventilators and they are typically mounted on the inside of an exterior wall and have a ducted connection to the outdoors through the wall. They are very similar to FCUs and are fairly common in school classrooms. They are designed to bring in small volumes of outdoor air that is mixed with room return air. These units are typically equipped with a heating coil and sometimes a cooling coil that is used to maintain their service zone temperature at setpoint. While these units are typically equipped with filters, they are often low-performing filters. Similar to larger AHUs, these units are not intended to bring in 100% outdoor air when it is cold outside and altering them to do this will likely result in frozen coils and water damage.

HVAC system assessment

Over longer periods of time (years), the operation/performance of a commercial HVAC system can often vary considerably from its originally intended design characteristics. If a municipality/owner has concerns about the operation of its HVAC systems and is unsure of what they can and cannot do, they should contact a firm with expertise in evaluating existing systems and the environment before proceeding with any alterations. The firm should investigate the condition and operation of the existing systems and evaluate their original designs to determine which enhancements might be implemented to increase outdoor air delivery or increase filtration capabilities. The firm should also inspect the HVAC systems and evaluate how those systems are working in concert with one another. For instance, it may be possible to increase heating coil outputs if the heating water temperature is raised, but one must first determine if the coil is configured correctly for this operation to prevent freezing. A consulting firm may also be able to identify deficiencies in existing HVAC systems that can be corrected and thereby increase system performance and, potentially, efficiency as well. Examples of such deficiencies can be malfunctioning dampers, plugged-up intake screens, fans not working at full capacity and failed heat recovery equipment.

An additional scope item to consider is the measurement of ACH rates for critical spaces. The consulting firm that evaluates HVAC systems in this way will need to be capable of measuring supply air flows and calculating ACH rates.

EFI Global has experience in performing these kinds of assessments and would be happy to assist you with your environmental and engineering evaluations to ensure the continued longevity and health of your building occupants.

Here are some resources that may help answer additional questions, or you can always reach out to us for further support.

> Read this blog about the trending themes in building safety and repair

> Visit Sedgwick's COVID-19 update center

> Explore Sedgwick's back to business toolkit


> American Industrial Hygiene Association, Reducing the Risk of COVID-19 Using Engineering Controls, 2020

> ASHRAE, Guidance for Re-Opening Buildings, 2020

> ASHRAE, Position Document on Infectious Aerosols, 2020

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