Dealing with COVID-19’s impact on current HVAC systems and equipment

As we collect and assess empirical data on the impact of the COVID-19 pandemic on our everyday lives here in the United States, all of our resources are focused on how we might minimize – and possibly prevent – the considerable damage caused by COVID-19 or another super-contagious virus in the future. There are no easily identifiable catch-all solutions. However, if we approach this task from a preventive, proactive vantage point, separately deal with each mechanism of virus spread and how to combat them, we stand a far better chance of containing the spread of the virus and limiting the collateral damage it can cause.

Some of the facts gleaned from watching the White House’s daily briefings during the first two months of the COVID-19 pandemic in the U.S. are that crucial to slowing the forward progress of any pandemic is identifying and understanding how the virus spreads. Per information shared by the Coronavirus Task Force, the primary assumptions given concerning the spread of the virus are that the main transmission routes are:

1. Water vapor droplets from sneezing, coughing or talking individuals infected with the virus.
2. Touching individuals infected with the virus.
3. Touching surfaces carrying the infection such as clothes, utensils, furniture, etc., and then touching one’s face.

The following information was published by the Centers for Disease Control and Prevention concerning the spread of COVID-19:

• The virus is thought to spread mainly from person-to-person.
• Between people who are in close contact with one another (within about 6 ft.).
• Through respiratory droplets produced when an infected person coughs, sneezes, or talks.
• These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled.
• COVID-19 may be spread by people who are not showing symptoms.

It could be concluded from the above discussion that the heating ventilation & air conditioning (HVAC) systems in commercial buildings for employment, socializing, and even in individual homes and residences, could be possible pandemic transmission routes and promote the ongoing spread of the virus. This is somewhat intuitive because it is generally understood that airborne respiratory droplets from virus-infected individuals can be carried by a facility’s HVAC/air handling system. Such conclusions have been reached by reputable organizations like the National Institutes of Health, which recently stated: “Scientists found that Severe Acute Respiratory Syndrome Coronavirus 2 was detectable in aerosols for up to three hours.”

Other research and studies, undertaken in the U.S. and abroad, have concluded that aerosol transmission of a virus is possible since a virus can remain active in aerosols for several hours. If the airborne virus happens to be present in a location that has poor, or minimal ventilation, then secondary infections from the virus are certainly possible.

In addition, there have been reports and recommendations that return air (i.e. air recirculated by a facility’s HVAC system) should not be utilized in any building’s mechanical ventilation system, that virus particles present in an HVAC system’s return air ducts can re-enter and be re-introduced into the building’s supply air distribution system, and that all return air and/or recirculation air dampers should therefore be manually closed at the central HVAC equipment so that this equipment operates on 100% outdoor air (OA).

While utilizing increased amounts of OA does help dilute the concentration of any airborne contaminants, increasing the percentage of OA can also result in major HVAC system problems if the HVAC equipment is not designed to handle the increased percentage of OA. It is critical, then, that building/facility owners and individuals in the HVAC industry must apply common sense, logic and reasoning to some of the recommendations to ensure that they accomplish what they intend to, without creating other problems in the process.

This author agrees that increasing the amount of outdoor air (OA) ventilation provided in a building’s HVAC system, and reducing the amount of recirculated (or return air) utilized, is a viable means of restricting the spread of the COVID-19 virus, but this cannot be done indiscriminately with any or all HVAC air handling units serving a given facility. Neither can this recommendation be carried out without carefully analyzing the facility as an integrated whole without producing catastrophic results.

Unless HVAC air handling equipment has been specifically designed and sized for delivering 100% conditioned outdoor airflow, unit cooling coils and unit heating coils will not be able to handle the increased cooling and/or heating loads associated with large increases in the amount of OA handled by the HVAC air handling equipment either during summertime operation (large increase in hot/humid outdoor airflow) or during wintertime operation (large increase in colder/dryer outdoor airflow). Arbitrarily increasing the OA airflow on typical, commercially available, HVAC air handling equipment will almost certainly result in cooling coil capacity failure (summertime) or heating coil capacity failure (wintertime).

Such failures might include, but are not necessarily limited to, the following:

Summertime operation

1. Significantly higher HVAC air handling equipment discharge air temperatures and higher space relative humidity levels in the areas served by this equipment.
2. Increased complaints from facility occupants concerning high space temperatures and high relative humidity levels.
3. Significant amounts of surface condensation generated at supply air grilles and portions of uninsulated supply air ducts (especially when the supply air ductwork is located in the above ceiling plenum space).
4. Greatly increased potential for developing indoor air quality issues due to the development of ideal conditions for organic growth on interior facility surfaces (i.e. presence of moisture due to surface condensation, higher HVAC air handling equipment discharge air temperatures, and higher space temperature and relative humidity levels).
5. Potential flooding of spaces located adjacent to, or below, HVAC air handling equipment, due to overflowing of equipment’s cooling coil condensate drain pan from excess condensate (equipment condensate drain pan was not sized to handle the amount of condensation encountered by utilizing 100% OA). It should also be noted that the flooding of HVAC air handling equipment, cooling coil condensate drain pans can lead to stopped-up condensate drains, which in turn leads to the pooling of water within the equipment. Pooled condensate drain water will become stagnant and ultimately result in ideal petri dish conditions within the equipment for cultivating potentially dangerous organic growth such as Legionella. ¹
6. Greatly increased potential for organic growth in HVAC air handling equipment cooling coil sections, as well as all sections located downstream of the cooling coil such as the supply air fan section and the discharge air plenum section.

¹ The Occupational Safety and Health Administration website Legionnaires’ Disease page indicates that “improperly draining condensate pans may produce the ideal tepid conditions that can encourage microbial and fungal growth.”

Wintertime operation

1. Lower HVAC air handling equipment discharge air conditions, and lower (i.e. dryer) space relative humidity levels in the areas served by this equipment.
2. Increased complaints from facility occupants concerning cold space temperatures and very dry air resulting in space static electricity issues.
3. Greatly increased potential for freezing the HVAC air handling equipment cooling and/or heating coils due to the significantly lower mixed air temperatures entering the equipment.

Typical scenario example

By way of example, a typical commercial 20-ton HVAC air conditioning unit (floor-mounted, ceiling-suspended or rooftop-mounted equipment) sized for a typical minimum outdoor airflow of 20% of the equipment’s total airflow, has the following typical operating parameters:

1. Unit cooling capacity = 20 tons or 240,000 British thermal units per hour (BTUH) (Note: 1000 BTUH = 1 MBH).
2. Unit heating capacity = 521 MBH.
3. Unit nominal rated airflow = 8,000 CFM.
4. Unit outdoor airflow = 1,600 CFM (i.e. 20%).
5. Typical HVAC equipment OA entering temperature & humidity conditions during the cooling season: 80°F dry bulb (DB)/67°F wet bulb (WB)/50% relative humidity. (Note: temperature units = °F = degrees Fahrenheit, humidity units = percent).
6. Typical HVAC equipment OA entering conditions during the heating season: 10°F DB.
7. Typical indoor space conditions in an area served by HVAC equipment: 75°F DB/50% relative humidity.
8. Typical cooling coil discharge air conditions during the cooling season: 55°F DB/53.75°F WB.
9. Typical heating coil discharge air conditions during the heating season: 122°F DB.

The percent of this HVAC unit’s 20-ton cooling capacity represented by the minimum OA airflow load component is approximately 5.42 tons, or approximately 27.1% of the unit’s total cooling capacity. Every incremental increase of 100 CFM in the amount of OA handled by this unit adds approximately 4,000 BTUH of additional cooling load or roughly a third of a ton.

If service personnel were to simply increase the amount of OA ventilation airflow on this typical HVAC air conditioning unit to say, 50% of total unit airflow (i.e. increasing OA airflow from 1,600 CFM to 4,000 CFM), without making any other adjustments, the immediate impact on the unit cooling coil would be the new total amount of cooling required, approximately 22.7 tons, or 113.6% of the unit’s original cooling coil capacity (note that a 2,400 CFM increase in OA airflow added an additional 2.7 tons of cooling load to the HVAC unit).

If this unit has been sized with minimal or no cooling load safety factor, then it is likely that it will perform very poorly in terms of providing sufficient cooling capacity for the space that it is serving. The new HVAC unit’s discharge air temperature will be increased by 1.5 to 2.0°F and the space is relative humidity will increase significantly as well.

Increasing the amount of OA ventilation airflow on this same unit to 75% of total unit airflow (i.e. from 1,600 CFM to 6,000 CFM), without making any other adjustments, means the unit cooling load now equals 24.9 tons or 124.5% of the unit’s original cooling coil capacity (note that a 4,400 CFM increase in OA airflow added an additional 4.9 tons of cooling load to the HVAC unit).

It should be noted that the tonnage now required by just the outside air (OA) airflow equals the original 20-ton capacity of the unit, which begs the question: “What about the space’s original internal appliance cooling loads and solar cooling loads? What capacity is left in the HVAC unit’s cooling coil to handle the load not already absorbed by the load requirement of the new OA quantity cooling load?”

Increasing the amount of OA ventilation on this same unit to 100% of total unit airflow (i.e. from 1,600 CFM to 8,000 CFM), without making any other adjustments, means the unit cooling load now equals 27.1 tons or 135.5% of the unit’s original cooling coil capacity (note that this adds an additional 7.1 tons of cooling load to the HVAC unit). Under this scenario, this HVAC unit would simply not be able to keep up with the cooling demands of the space that it is serving or be able to provide any significant amount of dehumidification. Interior space conditions would be extremely hot and humid; organic growth on space interior surfaces would likely flourish; and the occupants would likely be very, very uncomfortable.

Though the details will not be covered in this article, a different set of problems would exist for the example 20-ton HVAC air conditioning unit cited above during a wintertime scenario with increasing increments of percent OA.

The percent of this HVAC unit’s 521 MBH heating capacity represented by the minimum OA airflow load component (i.e. 1,600 CFM) is approximately 194 MBH, or 37.4% of the unit’s total heating capacity. If the HVAC unit is changed from handling 20 to 50% OA, then the heating capacity required to temper the OA to the leaving air temperature of the HVAC unit is 401.4 MBH or 77.2% of the unit’s total heating capacity. It should also be pointed out that under this scenario (i.e. 50% OA airflow), the mixed air temperature of return air and OA has dropped to 42.5ºF, which is approaching the temperature that could freeze the HVAC unit’s cooling coil.

It is evident that if this unit’s OA is increased to 65% OA, then the mixed air temperature would be in the range of 32ºF and freeze the cooling coil. If the HVAC unit is changed from handling 50 to 75% OA, then the heating capacity required to temper the OA to the leaving air temperature of the HVAC unit is 496.7 MBH or 95.5% of the unit’s total heating capacity.

Under this scenario (i.e. 75% OA airflow) the mixed air temperature of return air and OA would drop to 26.3ºF, and definitely freeze the cooling coil. Additionally, if most, or all the unit’s heating capacity is consumed warming the OA airflow to an acceptable coil-leaving air temperature, then the interior space temperatures might not be suitable for keeping occupants warm enough to be able to be productive during wintertime operation.

As can be readily seen in the above scenario, just because utilizing 100% OA on your existing HVAC equipment can be helpful in inhibiting the spread of COVID-19 in your facility, it does not mean that you can actually implement this change on your existing equipment without being significantly penalized.

If you take this approach without doing anything else, it is guaranteed that you will be worse-off for implementing this change.

Not only will you have exceeded the cooling capacity of your existing HVAC equipment, and no longer be able to cool the facility, but you will have laid the groundwork for experiencing future indoor air quality problems and organic growth on interior facility and HVAC unit surfaces, including the possibility of culturing Legionella if a flooded or clogged coil condensate drain pan is now causing condensate to puddle on the floor of the HVAC unit cooling coil, supply air fan, or discharge air plenum sections.

These problems would be in addition to COVID-19. Increasing the amount of OA ventilation in your facility can help minimize the spread of the COVID-19 virus by diluting the concentration of any airborne virus in the space with clean and filtered OA. However, do not attempt to increase the OA airflow percentage on your existing HVAC equipment unless you are certain that the equipment has sufficient cooling capacity to handle the increased cooling load caused by increased OA utilization.

If you do not have sufficient cooling capacity to convert your existing HVAC equipment to 100% OA ventilation, then don’t even attempt to do so, because you will only be creating additional long-term problems. The best and most expedient way to increase your facility’s OA ventilation capacity is to add a separate dedicated OA supply air handling unit, properly sized to fully condition 100% OA airflow at summertime conditions down to the required discharge air temperature of your existing HVAC equipment. Other suggestions for helping contain the spread of any airborne COVID-19 virus include the following:

1. Have a professional engineer confirm the actual cooling capacity available on your existing HVAC equipment, and ascertain whether or not any of the HVAC air handling units can have their specified design minimum OA airflows increased by some nominal percentage – say 10 to 15% – without exceeding the units rated cooling capacity.
2. If your HVAC equipment controls currently include a demand control ventilation mode, disable it. The goal is to dilute the air supplied to the facility’s interior spaces with as much outdoor airflow as possible.
3. Increase the amount of OA airflow provided by the existing HVAC equipment during facility unoccupied operating hours, when it would be acceptable to allow interior space temperatures to be somewhat higher than normal, as a means of purging the facility with additional OA ventilation during these times.
4. If you have cooling coil discharge ultraviolet lights, make sure that they are operating properly, and utilize them continuously because the Environmental Protection Agency and others have demonstrated that UV germicidal irradiation cleaners can destroy pollutants, bacteria, and viruses.
5. Make every effort to ensure that cooling coil condensate drain pans remain clean, unflooded and unclogged, in order to prevent the occurrence of standing water which could stagnate, and lead to the formation of Legionella.
6. Ensure that filters on existing HVAC air handling units are replaced on a very regular basis. In addition, have a professional engineer confirm the static pressure capabilities of your existing HVAC air handling units and determine what maximum MERV (Minimum Efficiency Reporting Value) level of filtration can be tolerated without exceeding the nameplate horsepower rating of the HVAC unit supply air fans. If it is demonstrated that your existing HVAC equipment can tolerate a higher MERV level filter, then change out your existing filters to the higher MERV level rating, but change them on a more frequent time interval to ensure that HVAC equipment nameplate horsepower ratings are not exceeded.
7. Wipe down your HVAC equipment with a manufacturer’s recommended disinfectant, both inside and outside, on a regular periodic basis.
8. Ensure that your HVAC equipment is operating as efficiently as possible by having all equipment periodically undergoing a testing and balancing recheck by an independent AABC certified TAB firm, and all HVAC control systems periodically undergoing a re-commissioning by an independent ACG certified commissioning firm.

This article originally appeared on the Associated Air Balance Council (AABC) TAB Journal. AABC is a CFE Media content partner. 

Original content can be found at www.nxtbook.com.

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