Your Building Doesn’t Have Enough Ventilation – And What to do About it!
Imagine living in a submarine or space station. The air would get very stale very quickly. Fortunately, buildings can get rid of stale air just by exchanging it with outdoor air, which can be accomplished by doing something as simple as opening a window. As straightforward as this sounds, many questions abound including “how should I bring in outdoor air?”, “how much is enough?”, and “how do I monitor ventilation?” These questions are especially poignant following the CDC’s emphasis on the airborne transmission of COVID-19.
How should I bring in outdoor air?
One way to bring in outdoor air is to open up windows. But with large commercial buildings and high-rise residential towers, many properties weren’t designed with operable windows. These buildings are “mechanically ventilated” rather than “naturally ventilated”. Mechanically ventilated means that fans are used to drive the flow of outdoor air into buildings.
Bringing in outdoor air mechanically through the heating, ventilating, and air conditioning (HVAC) system has several advantages over relying on natural ventilation and open windows. When a building brings in outdoor air through the HVAC system it is mixed with recirculated indoor air and conditioned before being delivered to the space. Conditioning typically entails filtration, dehumidification, and temperature control. This conditioning helps mitigate occupant concerns with humidity and outdoor pollution (e.g. ozone, PM2.5), which can cause indoor air quality problems.
Conditioning outdoor air can be expensive, so building operators often look for ways to minimize ventilation. Well-meaning energy-saving measures can result in insufficient ventilation. Our experience is that most commercial buildings have some spaces that are regularly under-ventilated, for one reason or another. This often happens when the thermostat is satisfied and the HVAC shuts down completely or only supplies a trickle of air.
The best way to bring in outdoor air is to have a dedicated system that supplies just the right amount of ventilation. This is typically achieved by disconnecting the V (ventilation) from the HAC (heating and air conditioning). Importantly, dedicated outdoor air systems (DOAS) don’t slow down the outdoor air when the thermostat is satisfied, which happens with variable air volume systems.
How much is enough?
Building codes dictate the minimum amount of outdoor air ventilation in buildings. Ventilation standards go beyond code minimums to reflect the consensus of HVAC engineers and researchers. ASHRAE is an organization that has published ventilation standards since 1973. Their standard 62.1, titled “Ventilation for Acceptable Indoor Air Quality”, is the most widely cited ventilation standard in the United States and is often the basis of litigation.
Compliance with ASHRAE 62.1 is more complicated than one would think. Typically, building engineers think in terms of “percent outdoor air”. Ask a building engineer how much outdoor air ventilation a building is bringing in and you might get an answer like “20%”. This percentage refers to the amount of total supply air that is attributed to outdoor air versus recirculated (“return”) air. The problem is that 20% does not indicate an actual quantity of outdoor air. To illustrate, I can offer you 20% of my pizza for dinner, but if I don’t specify the size of the pizza, you won’t know what you are getting. Many buildings’ HVAC systems supply 20% of a very small amount and therefore don’t meet standards. Industry standards require calculating the actual amount of outdoor air ventilation delivered to each person as measured in cubic feet per minute (CFM).
How do I Monitor Ventilation?
The amount of outdoor air ventilation is constantly changing. This variability is caused, in part, by controls and equipment such as economizers and variable air volume boxes. The building’s air exchange rate is further impacted by outdoor temperature and wind speed. So how does a building measure something that is constantly changing? The best way is to monitor ventilation on a minute-by-minute basis.
One way to monitor ventilation is through a building automation system (BAS), which is typically found in most modern, large buildings. Older facilities and smaller buildings typically have more basic controls. The BAS, at best, will give you a quantity of outdoor air, but it will not calculate the outdoor air per person, which is a key component of ventilation standards. Furthermore, it doesn’t calculate the natural air exchanges that buildings get through small openings, which can be significant.
Another way to monitor ventilation is by measuring indoor levels of carbon dioxide (CO2), which comes from the breath of occupants, as an indicator. In a fully-occupied space, CO2 levels will go down as ventilation increases and CO2 levels will go up as ventilation decreases. By using something called “mass balance equations”, you can approximate the amount of outdoor air ventilation just by monitoring indoor CO2 concentrations.
Accurately monitoring CO2 and other indoor air quality parameters is complex and requires experts to strategically place devices and interpret the results. Indoor Science would like the opportunity to share with you how indoor air quality monitoring can fit into your overall risk mitigation and response plans for COVID-19.
Indoor Science is now part of Aftermath Environmental Quality Services and provides nationwide coverage. Please contact us at 877-695-7054 to discuss how we can partner with you and your organization.