The COVID-19 disease caused by the SARS-CoV-2 which is declared as a global pandemic is changing the way buildings are designed to minimize the risk of spreading. Currently, the World Health Organization (WHO) and other authorities identify inhalation or direct contact with virus droplets, and indirect contact through droplets with contaminated surfaces as the two primary causes for the rapid spread of the virus. Below are a range of strategies and their impact on a building's EUI that have been proposed by leading firms to minimize the risk of COVID-19.
// 1. Adjusting the Occupant Density
It is believed that the virus droplets are relatively heavy particles which drop out of the air after traveling a short distance of 3 to 6 feet. Due to this fact the practice of 'social distancing' i.e. staying 6 feet apart from others to avoid the transmission came into place. You can determine the correct occupancy using our COVID Occupancy Score tool.
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From design perspective, it is recommended to imagine a 6ft circle allocated for each occupant. Therefore, the minimum area per person essential to practice social distancing would be Pi*(radius)*(radius), which comes down to 113.04 ft2. Any building type which has more ft2/person value than 113.04 will be able to maintain social distancing inside the space. According the AIA Re-Occupancy Assessment framework a occupant density of 1 person per 100 sf is the current best practice.
According to the ASHRAE user's manual, the office building type should have 275 ft/person and the education building type should have 75 ft2/person. Therefore, if the social distancing rules are applied for the building space then the area per person in education building type may need to be increased to 100 ft2/person. Many "open office" concept plans have less square footage per person and will need to be reconfigured to work.
The model in consideration is a secondary school building prototype from Pacific Northwest National Laboratory (PNNL) located in Denver, Colorado and the energy code selected is ASHRAE 90.1 2016. The following chart shows the difference in EUI when the people density is changed from 75 ft2/person to 100 ft2/person. Since the number of people reduced, the heating demand went up.
// 2. Adjusting the CFM Rate
Based on the adjusted occupant density, total CFM value should be modified. The CFM/person rates based on the energy codes like ASHRAE 62.1 and California Title 24 should be referred.
100% outside air seems unnecessary for building types such as offices, schools and retail since ASHRAE 170, which is the standard for hospital ventilation, allows for recirculated air in most zones.
Also, according to the research paper by Pantelic and Tham, the increase in air changes per hour (ACH) will not lead to improvement of disease propagation. Therefore, it is safe to conclude that minimum ventilation rates from standards like ASHRAE 62.1 and Title 24 are still favorable considering the energy consumption.
// 3. System Operation Time
There has been a lot of discussion about running the ventilation system 24/7. It is important to keep in mind that the energy impact of running a system 24/7, roughly double the normal schedule for non-residential buildings, is significant, especially with constant volume systems where fan energy would really be double. The following chart compares the EUI when the system is running 24/7:
The better solution would be to keep the ventilation on 24/7 but at lowered capacity when the building is not occupied to remove the particles outside the building. Another strategy would be to stagger work hours to have lower occupancy for a longer. Also, it is highly recommended to switch on the ventilation systems at full capacity an hour or couple hours before the occupancy of the building. Please refer to this article for converting the fractional schedule to cove.tool's format.
If the natural ventilation system is used for the building then it is recommended to actively use operable windows for 15-30 mins after entering the room especially if the space was occupied beforehand.
Here are some recommended practices aimed towards curbing the spread of Covid-19:
· Install filters with a Minimum Efficiency Reporting Value (MERV) of 13 or higher.
· Disable the Demand Control Ventilation (DCV).
· Disable the use of ceiling fans.
· Disable the occupancy sensor standby status i.e. set it to ‘occupied’.
· Prioritize on having a higher relative humidity within designed space.