Coronavirus Prevention: ASHRAE Guidelines for Building Ventilation

Topics: HVAC, Ventilation, mechanical ventilation, ASHRAE, coronavirus

Chelsey Bipat
Author : Chelsey Bipat on May 18, 2020

In modern HVAC engineering, ventilation systems are often designed to reduce their outdoor air supply when the full capacity is not needed. Outdoor air is not conditioned, and an increased airflow also increases the workload on space heating or cooling systems, depending on the time of the year. The energy cost of using more outdoor air is especially high on the hottest summer days and the coldest winter days.

However, ASHRAE recommends a different approach to prevent coronavirus infections in your building during the emergency and after. The outdoor air supply should be increased as much as possible, to avoid spreading the virus with indoor air recirculation. If possible, a building should sustain maximum outdoor airflow 24/7.

  • There is a limit to how much outdoor air can be supplied when the outdoor temperature reaches extreme values, since the capacity of heating and cooling units may be exceeded.
  • The same applies when the outdoor air is too dry or too humid, since the relative humidity may fall outside the range of 40% to 60% recommended by ASHRAE.

The recommendation is providing as much outdoor air as possible, subject to these constraints. An increased outdoor airflow helps prevent the spread of viruses and bacteria, by removing their concentration in indoor air, and by removing them without recirculation to other areas. Increased outdoor ventilation can be complemented with a filter upgrade to at least MERV 13, and the use of Ultraviolet Germicidal Irradiation. We will discuss these methods in the following blog posts.


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ASHRAE is aware that more outdoor air comes with an extra energy cost. This can be mitigated by adding an airside economizer, if the local climate allows it. Another option is using an energy recovery ventilator to exchange heat and humidity between the supply air and the exhaust air. Both options help compensate for the increased energy burden of a higher outdoor air supply.

Disabling Demand Controlled Ventilation for COVID-19 Prevention

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Modern ventilation systems have automatic controls that reduce the outdoor air supply in response to occupancy. In other words, the maximum outdoor airflow is only used when the building is at full occupancy. This concept is called demand controlled ventilation, or DCV. However, ASHRAE recommends disabling DCV systems in response to the coronavirus emergency. This allows a high outdoor airflow regardless of occupancy.

DCV is very effective as an energy efficiency measure, since it optimizes the workload on all HVAC components - air handlers, air conditioning units, and space heating systems. However, during the coronavirus emergency, preventing infections in building interiors has a higher priority than saving energy.

Buildings with a Building Automation System (BAS) or Building Management System (BMS) can be reconfigured more easily to following ASHRAE guidelines for coronavirus prevention. The DCV control can be paused temporarily, and replaced with a program that maximizes the outdoor air supplied to the building, regardless of occupancy.

Inspecting Ventilation Systems Before Reopening a Building

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Since ventilation plays an important role in coronavirus prevention, you should make sure the system is in optimal operating conditions. If your building has been on pause due to the lockdown orders, checking the ventilation system is very important before letting occupants in. There are two main issues that must be checked before reopening a building:

  • Technical issues: Are all HVAC components working correctly?
  • Health hazards: Are all HVAC components free of pathogens and fungi?

The 2019 coronavirus (SARS-CoV-2) is a new threat, but other air pollutants have existed for long. In particular, mold and Legionella bacteria can proliferate in HVAC components when no prevention measures are taken. Mold spores can irritate the respiratory system, and patients with allergic rhinitis and asthma are especially vulnerable. On the other hand, Legionella bacteria can cause a severe type of pneumonia called Legionnaires’ disease.

Mold is a very resilient organism, and it can grow almost everywhere when relative humidity is above 60%. It can be controlled in most building areas by keeping humidity within the ASHRAE recommended range of 40-60%. However, humidity is unavoidable in some components, such as cooling coils and condensation pans. In these cases, mold can be inactivated with Ultraviolet Germicidal Irradiation, or UVGI. 

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Mold growth in HVAC components is an issue that requires quick attention, since spores can be easily spread to other building areas using air ducts. Some components that are susceptible to mold growth include air filters, insulation, cooling coils and ducts.

Legionella bacteria tends to grow in stagnant water at warm temperatures. Some likely places to find this bacteria are hot water piping, condensation pans and cooling towers. Since Legionella can produce severe pneumonia, building owners must make sure it is detected and eliminated before returning to normal occupancy. Water droplets contaminated with Legionella can cause an infection if they are inhaled.

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