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Discussing COVID-19 CDC Guidelines for HVAC Systems

By Scott M. Katzer, P.E., CFEI

COVID-19 CDC guidelines relating to modifying HVAC systems state as follows1:

Consider improving the engineering controls using the building ventilation system. This may include some or all of the following activities:

  • Increase ventilation rates.
  • Increase the percentage of outdoor air that circulates into the system.

These recommendations do not appear to consider the building’s climate location as outdoor moisture quantities are different, particularly in warmer climates. Increasing building ventilation can be an effective way of reducing airborne transmission2, but doing so may have unintended consequences. Depending on the climate, increasing building ventilation also increases moisture infiltration. The effects of moisture depends on a variety of factors, including HVAC operation, climate conditions and building construction.

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

Moisture infiltrates into a building, even if an HVAC system is off, due to vapor pressure diffusion. Therefore, the key is to control the moisture to avoid an adverse effect. Before opening every outdoor damper 100%, consider the location of your building. Buildings located in warmer climates will typically have greater quantities of outdoor air moisture and therefore have more moisture to control inside the building. Buildings in cooler climates have similar issues dealing with moisture in warmer months and heating outdoor ventilation in cooler months.

A building’s air handling unit (AHU) should be designed to handle a specific quantity of outdoor air ventilation per ASHRAE standards. Increasing outdoor air ventilation increases the mixed air temperature being delivered to the cooling coil. This increases the supply air temperature being delivered along with excess moisture the coil was not able to remove. This excess moisture makes occupants feel uncomfortable leading to condensation/moisture issues resulting in microbial growth if not corrected.

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

Excess moisture from a warm environment into a cooler environment brings potential for condensation to form. This is apparent around areas where, for example, duct leakage may be occurring. It is imperative to verify that insulation around ductwork, air devices, etc. is properly installed as well as building envelope and attic space insulation.

For plenum systems3, providing a fully ducted system should be considered to avoid the potential for the spread of airborne particles. However, finding cost effective solutions due to existing building configurations may be difficult. Non-conditioned outdoor air directly into a plenum space should be avoided.

Facility managers should understand the implications these recommendations may have before implementing them and have their systems evaluated by licensed professionals to verify existing system performance and if any excess system capacity exists.

Scott M. Katzer, P.E., CFEI
Scott M. Katzer, P.E., CFEI

About the Author

Mr. Katzer is a Senior Forensic Engineer and Florida Division Manager with The VERTEX Companies, Inc. He earned a Bachelor of Science in Mechanical Engineering from Northeastern University and is a licensed professional mechanical engineer in 12 states. He is an experienced mechanical engineer in the evaluation and design of healthcare, institutional, commercial, residential and industrial technically complex projects, involving HVAC, electrical, plumbing, fire protection, building envelope and indoor air quality issues.


1 https://www.cdc.gov/coronavirus/2019-ncov/community/guidance-business-response.html?CDC_AA_refVal=https%3A%2F%2Fwww.cdc.gov%2Fcoronavirus%2F2019-ncov%2Fspecific-groups%2Fguidance-business-response.html
2 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6072925/
3 A plenum system is a space utilized for air circulation for HVAC systems. Typically plenum spaces are located above ceiling spaces, below floors or within mechanical rooms and used as airflow mixing chambers.

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