When you think of indoor air quality (IAQ) monitoring, you probably picture wall-mounted air quality monitors that are measuring a variety of parameters in a conference room, lobby, etc. This is the primary application of IAQ monitors: to understand the quality of the air in a given room or space.
However, more buildings are expanding their focus not only to measure air quality in a specific room but also the air that’s being delivered to and extracted from that room. This requires a different strategy: in-duct monitoring, where sensors are placed in the ducts of the HVAC system.
In this article, we’ll cover the basics of in-duct air quality monitoring, why more buildings have started using it, how it differs from standard air quality monitoring, and how to use in-duct monitoring data to make a difference for your occupants.
In-duct monitoring involves placing a specialized air quality monitor in one of three locations in a building’s HVAC system, each of which provides a different type of data:
The air intake is the entry point where outdoor air is drawn into the HVAC system to be filtered, conditioned, and distributed around the building. Any HVAC system with an air handling unit (AHU) will have an air intake system.
Because the air intake handles air that enters from outside, installing monitors here is essentially the same as monitoring outdoor air quality. If you’re unable to measure the outdoor air that is directly feeding into your HVAC system (if you’re on the 40th floor of a glass skyscraper, for example), then monitoring at the air intake is a great way to get the same information.
Once outdoor air enters the HVAC system via the air intake, it goes through a series of filters and is then either heated or cooled, depending on the temperature needs of the building. This conditioned air goes to the supply duct, which delivers the air from the HVAC system to the interior spaces of a building.
Placing an air quality monitor here will give you a picture of the air that you are delivering to your occupants, which can be incredibly useful for landlords and building owners.
The return duct pulls used air from the interior spaces of the building back into the HVAC system for reconditioning. The return air is mixed with fresh outdoor air, re-filtered, and either re-heated or re-cooled to be distributed around the building again.
Return-duct monitoring tells you how the air has changed since circulating the room or space; however, because the return air mixes with the outdoor air in the duct, it can be difficult to understand which air quality changes were the result of occupant activity.
If you’re considering monitoring air in ducts, you should ideally install sensors in all three locations. This will give you a 360º view of the entire mechanical process and help you immediately pinpoint where your systems are going wrong and impacting your IAQ.
There are two main reasons why you should consider in-duct monitoring:
The air that we breathe inside a building is impacted by three factors:
Conditions and activity outside the building
Conditions and activity inside the building
The building’s mechanical systems
Ideally, buildings will measure and track each of these factors to gain a holistic view of their air quality, which is crucial for effectively and quickly diagnosing any issues that surface. For example, if you only monitor air quality inside a room and find that PM2.5 levels are high, then you’ll automatically go looking for the PM2.5 source inside your building.
But if the true source of the PM2.5 is coming from outside, then you’ll likely waste a significant amount of time and effort before connecting the dots and realizing that outdoor air is the source of the PM2.5 (and the solution might be something as simple as closing a window!).
By adding in-duct monitors to your deployment, you can expand your database to include measurements from all three factors. Standard IAQ monitors track what’s happening inside, while in-duct monitors measure the air that’s entering and leaving the space. Together, these two solutions provide a complete picture of your building’s air quality.
This is a common challenge that landlords face. Landlords are free to monitor IAQ in common spaces of the building, but not on floors or suites that tenants are renting out. This can be a problem because landlords are responsible (some via direct terms in their lease agreements) to deliver clean air to their tenants, but have no way of directly monitoring air quality inside tenants’ spaces. We’ve seen many cases where tenants will bring IAQ readings from their space to the landlord, who has no idea that the building wasn’t doing what it was supposed to do.
In-duct monitoring offers a strategic workaround to this challenge. By placing monitors in the supply duct, landlords can measure the quality of air they’re delivering to each tenant’s space and use this data to either proactively correct mechanical issues or to prove that they’re upholding their commitment to providing optimal IAQ.
Due to the structure and complexity of ductwork, you cannot use wall-mounted monitors to measure air quality in ducts. You have to have specialized equipment for this type of monitoring because:
In most cases, you can’t install a regular IAQ monitor in the place that you want to measure inside the duct because of the monitor’s size and shape. You’ll need a specialized monitor that’s designed to fit into these spaces.
Compared to regular indoor spaces, ducts are considered an “extreme” environment for air quality monitors. There are constant changes in the speed and direction of airflow that can dramatically alter readings for many parameters.
PM2.5 sensors, for example, rely on a steady airflow rate to accurately count the number of particulates in the air. Inside a duct, airflow rates can change drastically as the system pushes and pulls air through the building. This can artificially increase or decrease the sensor’s PM2.5 count and cause you to take the wrong action based on the data.
Specialized in-duct air quality monitors are intentionally designed to calibrate for these conditions and provide consistently accurate readings, despite any changes in airflow.
The right monitor will be different for every project, but at Kaiterra, we recommend looking for an in-duct monitoring solution that:
To learn more about choosing an air quality monitor, read this article for an in-depth explanation of how to evaluate your goals, preferences, and limitations to find the best solution for your project.
The value of in-duct monitoring lies in its ability to directly trace how air quality changes throughout a building. If you have monitors installed in all three types of locations (air intake, supply duct, and return duct), then you’ll be able to diagnose HVAC issues much more quickly. Here are a few examples:
If PM2.5 levels are high at both the air intake and supply duct, then your filtration system may not be working properly.
If you detect pollutant spikes in the supply duct, but not the air intake, then the HVAC system itself might have a problem, like a dirty duct, degraded filter, or malfunctioning component.
If return air shows a spike in CO2 that wasn’t present in the supply air, the likely source is occupant activity, like an overcrowded conference room.
Combining in-duct air quality data with standard IAQ data delivers even better insights that can help you troubleshoot issues even faster. For example, picture a large space with two AHUs. The IAQ sensor detects high PM2.5 levels, but it’s not immediately clear if an AHU is responsible or if the source is inside the space itself. Here’s how you could go about troubleshooting:
In total, this problem would likely take just a handful of minutes to diagnose if you had all of this data available. If you only had the room-based IAQ monitor, however, then you would probably need at least a day or two to rule out occupant sources, identify the malfunctioning AHU, and figure out that the filter needs to be replaced.
Of course, having the room-based IAQ monitor alone would still help you fix the problem much faster than if you had no data at all, but this example goes to show just how quickly you can troubleshoot IAQ issues when you have more data at your disposal. And the faster you can troubleshoot air quality issues, the better your building will perform, which will ultimately benefit your occupants’ health, well-being, comfort, and productivity.
Interested in learning more about in-duct IAQ monitoring for your building? Feel free to reach out to a member of our team!