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HVAC System Considerations in the COVID-19 Era – Part 2: Increasing Outdoor Air Ventilation

May 7, 2020

Discussing the CDC and ASHRAE Recommendations for HVAC Systems in Non-Healthcare Buildings

In part one of this three-part series, we discussed the Centers for Disease Control (CDC) and American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) guidelines with regards to design and operation of heating, ventilation and air conditioning (HVAC) systems to cope with the COVID-19 pandemic.

In part two, we will focus on the topic of increased ventilation which is one of the main recommendations from both the CDC and ASHRAE guidelines with regards to HVAC systems handling of COVID-19. Although both the CDC and ASHRAE agree that increased ventilation can reduce the airborne concentration and associated transmission through the air via HVAC systems serving all types of buildings, this article will focus on non-healthcare type buildings since healthcare buildings tend to have more sophisticated types of systems and filtration in general and therefore are typically better equipped to mitigate airborne transmission of infectious diseases.

Increasing Outdoor Air Ventilation

While studies have shown that increasing ventilation within a building can be an effective way of reducing the potential for airborne transmission, [1] simply doing so may have some unintended consequences. Depending on the climate, increasing ventilation into a building, either through mechanical means or simply opening a window, increases moisture infiltration into the building as well. The effects of moisture within a building depends on a variety of factors, which include but not limited to, current HVAC design and operation, exterior climate conditions, construction type and the types of building materials used. Before one can fully understand the effects of moisture, one needs to understand that moisture will still infiltrate into a building, even if an HVAC system is off.

Vapor Pressure and Relative Humidity

Vapor pressure is essentially the portion of atmospheric air pressure attributable to water vapor. In short, air is primarily made up of nitrogen and oxygen; however, several other small amounts of gases (usually less than 1%) also exist. In addition, the air molecules also include a portion of water vapor, the amount of which depends on the atmospheric conditions (i.e. temperature) in which it resides.

As air temperature increases, so does its ability to hold more moisture. The specific amount of moisture air can hold at a particular environmental condition is typically referred to as specific humidity. However, most people usually refer to humidity levels in a term called “relative humidity” or %RH. Relative humidity is the amount of moisture air can hold compared to the maximum amount of moisture the air can hold at a specific temperature. For example, air at 75 degrees F and 50% RH means the air is holding 50% of the total capacity of moisture it has the ability to hold while at 75 degrees F. Just as the ability to hold more moisture when air temperature is increased, if you decrease air temperature, the ability of the air at the cooler temperatures to hold moisture decreases also, however, the relative humidity actually goes up while the specific humidity remains the same. Graphical representation on a psychrometric chart is shown below.

Since most people may not know how to say “psychrometric”, yet alone understand it, a better representation that many of us who are self-quarantining can relate to involves, shall we say, a favorite beverage of choice. As shown in the diagram below, if we assume the “beverage of choice” is water vapor in air at a specific temperature. Note the smaller 8 ounce glass, which for our purposes here, we will assume is at 60 degrees F (dry-bulb), the glass is full or about 100% capacity (or about 100% relative humidity) but the specific quantity of liquid (or moisture) is 8 ounces. In this condition, we can say that the dry-bulb temperature is about equal to the dew point temperature. If we were to add any more liquid/moisture or slightly lower the temperature (by making the glass smaller), we will cause the liquid to overflow. Therefore, in terms of air, when relative humidity reaches 100%, the air would be saturated and condensation will form. The air temperature at this point is typically referred to as the “dewpoint” temperature. Dewpoint temperature is the temperature at which water vapor condenses in the air.

Now to simulate increasing temperature without increasing specific moisture, we look at the larger 16 ounce glass and assume this represents an air molecule at 90 degrees F (dry-bulb). If we simply pour the liquid (which represents moisture) from the 60 degree F air molecule (small glass) into the 90 degree F air molecule (larger glass), we see that the specific quantity of moisture or humidity stays the same (8 ounces) but the larger glass is only 50% full. Therefore, the relative humidity of the 90 degree F air molecule (or larger glass) lowers to 50% since the air molecule now has more room to hold water vapor and therefore will not condense at that specific condition.

In addition, warmer air also has a higher vapor pressure than cooler air. The higher pressure of warmer air tends to move toward areas which have lower vapor pressures. This process is called “diffusion.” This is essentially the natural tendency of air to want to move from warm to cold. This is why one typically insulates buildings, in south Florida, to keep out the heat and moisture from the exterior and the indoor conditioned air within the interior spaces. Insulation and air conditioning will never stop the occurrence of vapor pressure and its tendency to want to infiltrate from the exterior as this occurs naturally in nature. However, the intent of proper HVAC design depends on the understanding of both the HVAC system and the building envelope in which it resides.

The diffusion process depends on many factors. Some of which include the construction of the building envelope, insulation properties and quality of the construction itself. In addition, the exterior environmental conditions and interior space conditions play an important role as well. As previously indicated, the higher the temperature and humidity, the higher the vapor pressure is at those conditions. The vapor pressure differential between the exterior and interior environments affects the rate and ability for warmer air to infiltrate into cooler spaces. The higher the pressure differential, the easier it is for warmer air and subsequent higher moisture quantities, to infiltrate a space. It is possible to elevate the vapor pressure differential by increasing the outdoor environmental conditions and maintaining a stable indoor environment, however, the opposite is also true. The vapor pressure differential may also be increased by decreasing the interior space temperature conditions while maintaining a stable exterior environment. Many people do not typically think about the latter, however, many indoor environmental problems occur for spaces that are potentially too cool just as they are when they are too warm.

An example of this is having a space located in an area where the exterior conditions are typical of South Florida in which they are approximately 91 degrees F dry bulb (DB) and 78 degrees F wet bulb (WB). Per air psychrometric charts, the vapor pressure at these conditions is approximately 0.83 inches of mercury. Typical indoor space conditions of 75 degrees F and 50% RH, translate to a vapor pressure of approximately 0.45 inches of mercury. This correlates to a vapor pressure differential of approximately 0.38 inches of mercury. If one was to decrease the indoor space conditions to 70 degrees F and 50% RH (vapor pressure – 0.34 inches of mercury, the vapor pressure differential would increase to 0.49 inches of mercury (0.83-0.34). See Figures 1 and 2 below.

Figure 1:
Vapor Pressure between 91°F DB/78°F WB and 75°F DB/50% RH = 0.38 in. Hg

Figure 2:
Vapor Pressure between 91°F DB/78°F WB and 70°F DB/50% RH = 0.47 in. Hg

The proper balancing of outdoor and exhaust airflows within a space is essential in maintaining a proper indoor environmental condition for both comfort and health/safety of indoor occupants. It is important to realize that if one were to increase ventilation within a space, this has the potential for also increasing relative humidity. According to ASHRAE standards [2], occupied-spaces should be controlled to limit the indoor humidity to a maximum dew point temperature of 60 degrees Fahrenheit (°F) which, assuming an ASHRAE recommended comfort level dry-bulb temperature between 72°F and 78°F [3], corresponds to a maximum relative humidity level of between 55 and 65% relative humidity (%RH). Therefore, the operation of HVAC systems should be controlled to achieve levels below the recommended dew point temperature at all times, regardless of COVID-19 or other potential similar threats, to prevent the potential for other adverse effects relating to moisture and condensation to occur. Spaces considered to be under “negative” pressure may introduce excess unconditioned outdoor air and associated moisture into the interior spaces thus increasing humidity levels causing discomfort to occupants. Most occupants unfamiliar with air properties would tend to lower the space temperatures via a thermostat or have their air conditioning systems run longer to remove the excess moisture. However, this sometimes can create an adverse effect as it may end up pulling more moisture into an interior space due to diffusion as previously explained. This may lead to the same or additional problems with regards to maintaining proper interior temperature and humidity levels and controlling interior moisture levels.

ASHRAE’s PD “does not make a definitive recommendation on indoor temperature and humidity set points for the purpose of controlling infectious aerosol transmission. Practitioners may use the information herein to make building design and operation decisions on a case-by-case basis.” [4]

Consider Your Environment and Building Systems

Regarding ventilation, ASHRAE’s PD states “General dilution ventilation and pressure differentials do not significantly influence short-range transmission….generally speaking, designs that achieve higher ventilation rates will reduce risk. However, such buildings will be more affected by local outdoor air quality, including the level of allergens and pollutants within the outdoor air, varying temperature and humidity conditions, and flying insects.” [5] Therefore, if you are adding non-conditioned ventilation into your building, you are adding moisture, the quantities of which depend on where your building is located and the exterior conditions. Therefore, the key is to control the ventilation and its associated moisture so it doesn’t have an adverse effect on the building materials and personnel. Before simply running around and opening every outdoor damper 100%, consider the location of your building. Buildings located in warmer climates will typically have greater quantities of moisture in the outdoor air and therefore will have more moisture to control once it is inside the respective building. That’s not to say that buildings in cooler climates will not have similar issues as they will need to deal with moisture in warmer months and heating the extra outdoor ventilation in cooler months.

The types of HVAC systems are important as some systems are simply better than others with regards to controlling moisture. However, each system within a building was likely designed to handle a specific quantity of outdoor air ventilation per ASHRAE standards. Increasing the outdoor air ventilation for a specific air handling unit (AHU) system increases the mixed air temperature being delivered to the cooling coil, which in turn, increases the supply air temperature leaving the AHU and delivered into the space along with any excess moisture the AHU system was not able to remove. This excess moisture can not only make occupants feel uncomfortable but can lead to condensation/moisture issues which can settle on various building materials leading to microbial growth if not corrected. Specific AHU systems may have some ability to be adjusted in order to handle some excess outdoor air but this should be evaluated by a licensed mechanical engineer or vendor familiar with such systems before any changes to the systems are made. In addition, should additional outdoor air ventilation be able to be introduced into a building and controlled properly by the existing HVAC system, this excess outdoor air ventilation will need to go someplace. Therefore, exhaust system capabilities to handle this additional ventilation should be evaluated as well.

Example of excessive non-conditioned outdoor air effects into an outdoor air fan/duct system causing deterioration and corrosion of surrounding equipment and materials.

It is important to remember that excess moisture from a warm environment into a cooler environment also brings the added potential for condensation to form. This is particularly apparent around areas where, for example, duct leakage may be occurring. Therefore, it is imperative to verify that insulation around ductwork, air devices, piping and air handling equipment is properly installed. In addition, insulation surrounding building envelope areas including attic spaces should be checked for proper consistent installation throughout and repaired/reinstalled as necessary. This would help assure that warmer moist air would not be able to settle on cooler surfaces thus allowing condensation to form leading to the potential for microbial growth in select areas under the right conditions.

Photo of moisture/condensation stains around supply diffuser due to lack of properly installed insulation.

If a building utilizes a plenum system [6], consideration should be given to providing a fully ducted system, even temporarily, to avoid the potential for the spread of airborne particles due to the difficulty to fully control the air movement within a plenum based system. However, it is understood existing building configurations may not allow for such a system to be constructed cost effectively. Outdoor air ventilation directly into a plenum space, above a ceiling, for example, should be avoided, for many of the reasons noted above. If at all possible, outdoor air should be ducted directly to the return side of its respective AHU system, and therefore, the return/outdoor air mixture can occur directly at the AHU and not in the plenum space. This way, the outdoor air ventilation, and its associated moisture will have a better chance to be conditioned and filtered before it enters the occupied spaces.

Photo of microbial growth along interior wall of a closet primarily due to vapor pressure diffusion from attic into plenum space above.
Photo of outdoor air duct stubbed into apartment AHU plenum closet (not ideal). Duct outdoor air directly to return opening of AHU and provide volume damper for control.

Its quite easy to play Monday morning quarterback and survey systems once they fail to determine what should or should not have been done. This reality is, if we want the world and our businesses to reopen, no one has the time and finances to make changes more than once. It is much more valuable to have a licensed professional analyze any proposed changes ahead of time before spending the extra time and money to implement them and finding that the work performed to achieve a betterment in the short term may be detrimental in the long term to the facility.

In part three of this series, we will focus on the topic of increased filtration which is another recommendation from both the CDC and ASHRAE guidelines with regards to HVAC systems handling of COVID-19.

To learn more about VERTEX’s COVID-19 Services and Industrial Hygiene & Building Sciences services or to speak with an Environmental Expert, call 888.298.5162 or submit an inquiry.

Series: HVAC System Considerations in the COVID-19 Era

HVAC System Considerations in the COVID-19 Era – Part 1: CDC and ASHRAE GuidelinesPart 1
HVAC System Considerations in the COVID-19 Era – Part 2: Increasing Outdoor Air VentilationPart 2
HVAC System Considerations in the COVID-19 Era – Part 3: Increasing FiltrationPart 3


[2] ASHRAE Standard 62.1-2019

[3] ASHRAE Standard 55-2017

[5] ASHRAE Position Document on Infectious Aerosols, Sections 2.1 and 3.1

[6] A plenum system is a space within a building utilized for air circulation for HVAC systems. Typically plenum spaces are located above ceiling spaces, below floors, or within mechanical rooms and are used as mixing chambers typically for return, outdoor air, or both.

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