Central Ventilation vs. In-Room Air Cleaners
Which method is more effective at providing clean air: upgrading filters in air handling units to MERV-13 or adding in portable HEPA filters? One is located in the central HVAC system and one is located in the room. The effectiveness and practicality vary significantly depending on the chosen location. This post will delve into the different types of equipment available, where they can be placed, and assess their effectiveness in creating healthier indoor environments.
Central Ventilation
Description
Central ventilation systems are standard in most buildings. They are designed to provide minimum ventilation as required in the building code and to heat or cool the air. Typically, more air is needed to heat or cool a space than the minimum required for outdoor ventilation. Therefore, extra air is recirculated instead of introducing additional outdoor air.
In equation form:
Total air (for heating & cooling)=Outdoor air (ventilation)+Recirculated air
This recirculated air is generally uncleaned, so by using air cleaning strategies, it provides an opportunity to supply a large volume of cleaned air to the space without requiring to increase airflow or any structural changes.
For instance, consider a standard classroom with a 1000 cubic feet per minute (CFM) airflow, comprising 25% (250 CFM) outdoor air and 75% (750 CFM) recirculated air. Upgrading to MERV-13 filters can introduce an extra 578 CFM of clean air (750 CFM x 77% efficiency). This more than triples the clean air delivery rate. It is difficult to achieve this with in-room air filters without compromising on noise levels. This highlights the cost-effectiveness of such an improvement.
Air from the central ventilation can be cleaned by multiple methods including:
- Increasing the amount of outdoor air which reduces the amount of recirculated air
- Upgrading the filters so they are effective against fine particulate matter, which includes infectious aerosols.
- Adding UV to the duct to disinfect any air that passes through it.
- Adding alternative air cleaning methods, including ionization, hydrogen peroxide or photocatalytic oxidation.
Addressing the advantages and disadvantages of different technologies is beyond the scope of this post. I address them here.
How it works
In general, these systems can be thought of as passing air through a series of stages where each stage can clean a certain percentage of the air.
For example, let’s say there are 3 stages of air cleaning:
- Ventilation: 25% of the air is exhausted and replaced by outdoor air, 75% remains as uncleaned recirculated air
- Filtration: Air is passed through a MERV-13 filter that is 77% effective against respiratory aerosols. 23% of that air remains uncleaned
- UV: An in-duct UV system is placed in the supply duct. It disinfects 90% of the air that passes by it. 10% remains uncleaned
It would look like this:
At each stage, the system treats both the already cleaned air and the air that has not yet been cleaned. For example, the MERV-13 filter will filter pathogens out of the 75% return air, but will also filter the 25% outdoor air which is already pathogen free. Consequently, as the amount of stages increase, the stages will clean more and more air that has already been cleaned leading to diminishing returns.
Another option is to use additive technologies where reactive compounds or ions are supplied to the air. Some examples include ionization, photocatalytic oxidation or gaseous hydrogen peroxide. These compounds then travel into the space and react with the air. Although they can be located inside central ventilation systems, they are not limited to cleaning the air in the central ventilation systems, so they can be viewed as being similar to in-room air cleaning equipment. Currently, none of these technologies comply with ASHRAE 241, so there is insufficient evidence to claim they are both safe and cost-effective.
Advantages and Disadvantages
Effectiveness
It is clear that ventilation plays a significant role in mitigation of airborne disease transmission. However, the risk of infection from uncleaned recirculated air is still unclear. For example, refer to FAQ #1 from the CDC Ventilation in Buildings.
A significant issue is that proving any mode of transmission can be difficult, which was largely responsible for the denial of airborne transmission of COVID-19 at the beginning of the pandemic. Even if infection did occur through contaminated recirculated air, it is very difficult to prove it didn’t happen in some other setting or through an air current that didn’t travel through the HVAC system.
From a physics perspective, supplying cleaned air to the space dilutes pollutant concentrations and mitigates harm, regardless of the origin. Furthermore, there is no evidence to the contrary — that supplying uncleaned recirculated air through ventilation systems helps reduce risk.
Diminishing Returns
As more air cleaning occurs, the supply air approaches 100% clean and new stages do not assist anymore. Consequently, the cost-effectiveness of adding a new stage will continually decrease. To improve the air cleaning from the central ventilation system further, a capital project involving increasing the system’s capability might be required. Conversely, in-room air cleaning is additive and equipment can continually be added to increase the clean air delivery rate.
Ease of Installation
Major projects are not required to add equipment to existing air handling units. These systems are often fairly simple to install, like changing the type of filter or adding UV lighting into the ducts. This often makes it a more attractive option than in-room air cleaning equipment.
No Occupant Issues
In-room air cleaning equipment can affect occupants through noise, take up space and have more difficult installations. Equipment installed in ducts is not noticeable by the occupants and does not affect them in any way.
No Transparency
When equipment is not visible to the occupants, there is no transparency about its operation. A less honorable building manager can swap MERV-13 with lower efficiency filters and no one will notice. UV lights can burn out and not be replaced. The effectiveness of these systems is essentially left to the occupant to trust the building operators to ensure the equipment is continually monitored and operating properly. From my experience, this trust would be unfounded and we currently have no mechanisms to ensure maintenance and compliance.
Conclusion
It is beneficial to clean recirculated air and can be an easy measure to take. Given the risk of recirculated air is still unclear and there are diminishing returns by using multiple methods, using only a single method is a reasonable choice. Upgrading filters to MERV-13 appears to be an optimal solution as it is cost-effective and also protects against particulate matter. ASHRAE has recommended upgrades to MERV-13 in their Enhanced Indoor Air Quality Guideline, Design Guidance for Educational Facilities and Epidemic Task Force. The CDC also recommends use of MERV-13 filters.
While theoretically, in-duct UV can be employed, its effectiveness in reducing the risk of airborne transmission is low when MERV-13 filters are already in use. Furthermore, in-duct UV needs to supply a significant amount of UV light to disinfect the air because of a short contact time and often installations are not sufficiently powerful. Consequently, in my opinion, in-duct UV can be deprioritized with the sole method to clean recirculated air being MERV-13 filters.
In-Room Air Cleaning
Description
Improving airflow for central ventilation is often challenging and could involve significant capital costs for a new ventilation system in the building. Air cleaners can alternatively be placed inside rooms without significant capital costs. By increasing the size or number of air cleaners, the clean air delivery rate can continually be increased.
Natural ventilation involves outdoor air entering the building through openings and windows. Some buildings are designed to be naturally ventilated, but most new buildings are mechanically ventilated.
In-room air cleaners can take many different forms, but usually entail an enclosure with a fan. The fan draws air into the enclosure and then the device treats the air to remove infectious pathogens and supplies the air back to the space. The most common tool is using high efficiency filters. Other options exist including UV or alternative technologies.
A combination of in-room air cleaning technologies would look like this:
Advantages and Disadvantages
UV Lighting Systems
Upper room UV and far-UV, which are in-room UV lighting systems, do not use a fan and generally ensure disinfected air is better distributed throughout the space than other air cleaners. They can be used in a safe and effective manner, but it requires much more caution than air cleaners with an enclosure and a fan. The systems can often supply equivalent clean air delivery rates far higher than what is capable through central ventilations systems or in-room air cleaners using fans.
Noise
In-room air cleaners that use a fan can run into multiple issues. The primary problem is noise. It is often difficult to provide a high clean air delivery rate at acceptable noise levels. There are methods of addressing noise, especially using air cleaners made with PC fans.
Air Distribution
Another significant issue with in-room air cleaners is air distribution. This equipment can often short-circuit and just clean the air that was supplied to the room instead of having that air properly mix with the space. This can reduce their effectiveness by as much as 50%. Having good airflow throughout the space can mitigate air distribution problems.
Installation
Portable equipment requires no installation at all. However, it requires occupants to run the equipment, which often does not work. Equipment can also be wall mounted or ceiling mounted and controlled remotely which is not too difficult, but might require skilled trades for installation. It is preferable to ensure these systems do not obstruct the space and are not controlled by the occupants.
Effectiveness
Adding in-room air cleaners continually increases the clean air delivery rate, making them effective choices to provide clean air.
Transparency
Occupants can see these systems in place and whether or not they are operating, providing better transparency about air quality in the space.
Conclusion
Here’s a table summarizing the advantages and disadvantages of the different types of air cleaning equipment:
No air cleaning solution is perfect. Each method has its own advantages and disadvantages. When deciding on which systems to use, various factors such as cost, effectiveness, ease of installation, transparency, energy consumption, safety, overall health benefits, and more can be considered. It’s also important to remember that these methods are not mutually exclusive. They can be used together to achieve acceptable clean air delivery rates.
In my view, the optimal approach is to provide a good amount of ventilation (minimum requirements + 30%), use MERV-13 filtration and supplement these measures with in-room air cleaning, such as filtration units or upper-room UV, to meet the required clean airflow rates from ASHRAE 241.
For new construction, increased airflow rates with high efficiency filtration can generally be included with low additional cost ensuring the system is compliant with ASHRAE 241 without requiring in-room air cleaning. In high-density spaces such as hospital waiting rooms, where elevated equivalent clean airflow rates are necessary, the practical solution may require the use of upper-room UV or far-UV systems to meet ASHRAE 241 clean airflow requirements.
Further Reading on Air Cleaning Technologies
Intro to Ventilation
Intro to Filtration
Intro to UV Light Against Diseases
Intro to Upper Room UV
Intro to Far-UV
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