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. Part two discussed the topic of increased ventilation in HVAC systems as it relates to the CDC and ASHRAE guidelines and what to research beforehand.
In the final article of this series, we will discuss what the CDC and ASHRAE are saying about Filtration as well as some of the other HVAC strategies noted per their recent published guidelines. Similar to part two, 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.
ASHRAE’s PD states “The use of highly efficient particle filtration in centralized HVAC systems reduces the airborne load of infectious particles.”  The PD also goes on to state to “Improve central air and other HVAC filtration to MERV-13 (ASHRAE 2017b) or the highest level achievable.” 
Most would agree that upgrades to filter efficiency would be an effective way to assist in filtering out unwanted airborne particles. However, careful consideration needs to be made when selecting the types of filters used for existing AHU systems. In general, filters with higher MERV  ratings have smaller openings in them which allow the higher MERV filters to capture more and smaller airborne particles; however, in doing so, it can restrict the airflow passing through them. Increasing filter efficiency means increased resistance or static pressure  within a system that relates to a drop in airflow velocity across the filter itself. This can lead to an overall drop in total airflow being delivered to a space. In addition, if the velocity, particularly across a cooling coil, is too low, this can lead to coils freezing up. However, how the airflow behaves depends upon the type of HVAC system and specifically the fan serving it. Therefore, in order to counteract this effect, the system must have a way to make up the difference between the greater pressure drop caused by the increased filter efficiency. In some larger AHU systems, simply increasing the speed of the fan or the even the horsepower (hp) of the fan motor may make up for the added pressure drop within the system. However, for smaller AHU systems, such as those found in most residential, multi-family, and light commercial buildings, the installed AHU systems likely do not have these capabilities due to the size and hp limitations of the supply fans themselves.
All filters have specific pressure drops associated with them which varies depending upon the type of the filter itself as well as the quantity of airflow across it. Pressure drops also increase as the filters become dirty. Installing a higher efficiency filter may work fine initially while the filter is clean but may not when the filter gets dirty because the AHU fan may then be outside of its performance range resulting in a decrease in total airflow.
Therefore, it is important to have each system evaluated by a licensed mechanical engineer or professional familiar with such systems before any changes to the systems are made. This may include verifying the AHU system performance by a licensed test and balance or mechanical contractor to assure the system is performing in accordance with its intended design standards and also to verify what, if any, excess capacity the respective systems have before making temporary or permanent changes to their current operation.
In addition to selecting the proper filter and respective filter efficiency, careful consideration and protocols should be employed to removing and disposing of used filters. This includes, but not limited to, wearing the proper personal protective equipment (PPE) in accordance with CDC recommendations; protecting the surrounding work environment with proper drop cloths or similar enclosures to collect any particles which fall during filter extraction; cleaning and/or disinfecting/sanitizing the surrounding surfaces before new filters are installed, and properly bagging/disposing of all used filter media per CDC and related guidelines. Consulting with a licensed engineering firm for the development of such protocols and standards can be an option for those facility managers wishing to improve their standard operating procedures.
ASHRAE’s PD recommendation to “Keep systems running longer hours (24/7 if possible)” may be warranted but not without its own concerns, namely the increased usage of energy and the potential for overcooling the space. Overcooling a building space can bring detrimental effects for many of the reasons noted above. Therefore, controlling the space temperatures by adjusting control settings, namely night setback settings, may assist with maintaining a proper indoor environment. Controlling energy usage, and the costs associated with it, has always been important to any building owner or facility manager. Many, if not all, would agree that the safety of the public is on everyone’s mind these days much more than energy savings. Nevertheless, if additional measures are implemented, it would be in the facility manager’s and owner’s best interest to provide the additional measures in the most cost-effective way possible without jeopardizing the health and safety of the building’s occupants as well as their business continuity plan.
Portable room air cleaners with HEPA filters as well as UVGI systems may provide added protection for indoor occupants. As indicated in ASHRAE’s PD, “while UVGI is well researched and validated, many new technologies are not.”  ASHRAE’s PD itself does not make a recommendation for or against the use of UV energy in air systems for minimizing the risks from infectious aerosols; however, the CDC has approved UVGI as an adjunct to filtration for reduction of tuberculosis risk and has published a guideline on its application.  Therefore, like any new system, the incorporation of these types of systems should be evaluated prior to installation to determine how and if operating these new systems will negatively affect the performance of the existing building’s HVAC systems.
The reality is that there is always a way to do something better and the articles in this series are not, in any way, recommending that additional measures, like those proposed by ASHRAE and the CDC, not be implemented. It is important to remember that there are many factors related to HVAC performance which are not commonly thought of when trying to implement general recommendations, including building location, construction type, building envelope, and how well the building was constructed in the first place. Therefore, it is always in the best interest of any facility/building owner, manager, and tenant, to determine the pros and cons of any recommendation and how they will work specifically for their building and respective HVAC systems.
-  ASHRAE Position Document on Infectious Aerosols, Section 3.1.
-  ASHRAE Position Document on Infectious Aerosols, Section 4.1.
-  Minimum Efficiency Reporting Value (MERV) is a rating (on a scale from 1 to 20) developed by ASHRAE on the efficiency of the filter to trap airborne particles. For reference, clean room HEPA and ULPA filters are rated at between MERV 17 and 20.
-  Static pressure or pressure drop is essentially air resistance within a fan or duct system which grows proportionally to the airflow. The amount of static pressure in a system can be affected by a number of factors which include, but are not limited to, friction losses due to filters, ductwork configuration, valves, dampers, etc.
-  ASHRAE Position Document on Infectious Aerosols, Section 3.2.
-  ASHRAE Position Document on Infectious Aerosols, Section 3.2.