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The COVID-19 pandemic has significantly increased indoor air quality (IAQ) awareness for commercial building owners. Before the pandemic, IAQ was mainly focused on occupant comfort and maintaining minimum outdoor air requirements at best.
Now, various organizations such as the U.S. EPA, WHO and ASHRAE are strongly recommending increased HVAC system ventilation as a key building control initiative in minimizing the spread and recirculation within an occupied space.
Even before the pandemic began, a ventilation system could be used to bring more fresh outdoor air into a building to dilute the indoor air and reduce the occupant risk of exposure to potentially harmful particles, gases and odors. The same principle works for airborne viruses like SARS-CoV-2, the virus that causes COVID-19. Opening windows will help bring in fresh outdoor air, too, but not all commercial buildings have operable windows, and it may not be practical during the winter and summer months.
Since we spend the majority of our time indoors, we are highly dependent on the mechanical HVAC systems to provide occupant comfort and safety. ANSI/ASHRAE Standard 62.1-2019 - Ventilation for Acceptable Indoor Air Quality provides minimum outdoor air requirements for various spaces based on flow per person (ft3/min or l/s per person).
But, before taking the steps to increase the amount of outside air entering a building or occupied space, how do you determine what is coming in right now? The only way to truly know how much air is being delivered into the building or to a given space is to measure the percentage of outdoor air coming in.
Step 1: Calculating the percent of outdoor air (OA)
You need three measurements—outdoor air, supply air, and return air to determine the percentage of outdoor air entering a system and there are two methods you can use to calculate it.
The first method is to conduct a duct traverse of the outdoor air duct intake. However, this is not always feasible because there may not be sufficient straight ducting to do a good duct traverse of either the return air or the outdoor air. An alternate method uses air temperature or CO2 measurements.*
To calculate the %OA, use the following equation for CO2 or temperature:
Where:
- XR = Return air CO2 concentration OR temperature
- XS = Supply air CO2 concentration OR temperature
- XO = Outdoor air CO2 concentration OR temperature
Also, rather than hand calculating the % of outdoor air, TSI’s IAQ monitors models 7545 and 7575 or the brand-new expandable Q-Trak™ XP Indoor Air Quality Monitor (PM and gas sensing in a single handheld instrument) have a built-in percent of outdoor air calculation workflow. Calculations from TSI’s IAQ monitors make it extremely easy to determine the percentage of outdoor air entering the HVAC system.
Watch this 5-minute video on calculating the percent of outside air:
Step 2: Calculating the amount of outdoor air per person
Once the percentage of outdoor air is known, the air volume of outdoor air per person to a given space can be determined. Simply follow this equation:
The total air volume being delivered to a particular space X The percentage of outdoor air entering the space = The amount of outdoor air (ft3/min or l/s)
Next, divide the amount of outdoor air by the number of occupants.
The amount of outdoor air (ft3/min or l/s)
The number of occupants in the space
The final calculation can be used to determine if the minimum outdoor air recommendations per occupant density is sufficient or not.
What’s Next?
Increasing the amount of outdoor air entering a building must also coincide with increasing the amount of air exhausted from the building. If only the outdoor air intake rate is increased, building pressures will increase and create issues such as difficulty opening and closing doors, additional noise at entrance doors and between spaces, possibly affect the pressure dependent spaces.
An issue that will need more investigation by the facility and their consultants is the ability of the HVAC system to maintain space conditions when increasing the amount of outdoor air. More outdoor air requires more conditioning of the air before being delivered throughout the space. This requires evaluating the heating and cooling coil efficiencies to determine if they can handle the increased heating and cooling demands which coincides with increasing pump flow thru the hydronic system to maintain indoor comfort levels.
Any major adjustments made to the HVAC system, especially those that were not part of the original design intent, requires professionals to inspect the system and to make recommendations to the building owners. This may involve TAB, Commissioning, Building Automation System firms and others to thoroughly review the HVAC components, building plans, mechanical schedule, sequence of operations and automation controls to effectively implement the desired change requests.
By itself, ventilation cannot prevent the spread of coronavirus (COVID-19), but improving ventilation can reduce occupant exposure to particles suspended in the air, including SARS-CoV-2 and other contaminants.
*Important Note – When to Use CO2 or Temperature to Calculate % Outdoor Air
Using CO2 is generally much better for determining the ventilation effectiveness of the HVAC system. However, there are benefits to both approaches, and it’s important to understand when to use one over the other.
CO2 should be used when the building that you are interested in has a lot of occupants. This ensures that the CO2 concentration values of outdoor air and return air are far apart. If there are a sufficient number of occupants, then outdoor air should always be the lowest value and return air should always be the highest. Determining the %OA using CO2 can be performed at the air handler or in individual rooms or spaces.
Temperature can be used when the building or room of interest has a big difference in temperature from outdoor air temperatures. This means when it is significantly hotter or colder outdoors, then it is a good time to use temperature as a means to calculate %OA. Temperature calculated %OA value is made at the air handling unit and indicates the %OA for an entire area of the building or areas the air handling unit(s) serves.