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Filtration and Airflow in Whole House HVAC Systems: Filter Cabinets and MERV Options

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Filters remove dust and other particles, including biological contaminants, from the air.  This basic but important function plays a critical role in maintaining indoor air quality, and yet most of us are just starting to catch up with its importance and also how we can improve filtration in support of a healthier indoor environment.  It’s widely reported that we spend up to 90% of our time indoors and that indoor air can be 2 to 5 times as toxic as outdoor air.  At a high level, the main strategies for keeping indoor air healthy are:
  • Prevent pollutants from entering homes
  • Add fresh air to dilute pollutants
  • Remove or exhaust pollutants from our homes 
  • Maintain a temperature within the ideal range for human health and materials (which also affect human health)
  • Control humidity with the ideal range for human health and materials (which also affect human health)
  • Properly maintain all equipment related to indoor air and environmental quality 
Filtration is primarily related to preventing pollutants from entering our indoor space, and also to removing pollutants when they have already entered or have been created within our indoor spaces.  Filtration also helps protect our HVAC equipment overall, but keeping contaminants away from system parts.  Even with a top whole-house ventilation system, we still open our doors and windows at times.  Even if we also happen to rarely allow air to enter our homes in bulk, human activity also generates pollutants, from simple activities like breathing to actions like cooking and cleaning, and also due to the toxic substances we might bring in through furnishings and other materials.  Ironically, even the act of cleaning can lower our air quality.  
While it might be tempting to just increase filtration to the top levels, it’s worth noting that higher filtration can mean higher air flow resistance, which can result in a condition known as pressure drop.  Technically, pressure drop is related to the difference of pressure between two define areas.  In filters, this difference means greater resistance to airflow.  The risk of pressure drop is that we can make our system motors work too hard and shorten their life, or even burn them out and cause failure in extreme circumstances. Resistance also means a loss of energy efficiency.  it’s worth noting that any obstruction or restitution in the HAVC system can cause pressure drop… including dirty filters.  So it’s not enough to just purchase great equipment.  Proper maintenance is a must.   
So what we need to achieve is the greatest or desired air filtration level possible without causing undue stress to an HVAC system, and these calculations require that our building and HVAC professionals must design for each structure for its design and ideally how it will be used, to the best of our ability to predict.  Corbett Lunsford made this point well when he ran into Matt Hoots at IBS 2022 on the show floor. Corbett also reminds us in this video that if we’re not testing, we’re just guessing…    Corbett actually did the testing on the 1920sMakeoverATL Showhouse, and it was the leakiest home he’s ever tested! 


As Matt and Corbett agree, not all filters are created equally.  In this video, Matt discusses different filters and also the idea of making sure that all the air actually flows through the filter.  If air is able to escape around or “blow by” the filter for any reason, then they value or performance of the filter is decreased or even lost.  Matt points out that all filters are not created equally. The majority of filters in comes are 1-inch or maybe 2-inch filters that fit into an open slot in the ventilation system.  One challenge is that these thin and simple filters often shrink or collapse when installed or as they get clogged up, from the pressure, and pockets are created where air can pass through.  Another point is that a sealed cabinet designed for a specific filter size and type can better ensure that air won’t escape unfiltered.  
Aprilaire makes a filter cabinet that can accommodate 4” MERV 11 to 16 filters.  MERV stands for minimum energy efficiency rating, or how effective a filter is at trapping particles.  The higher the MERV rating, the more effective, meaning the smaller the particles are the filter can trap.  For example a MERV 11 filter can trap larger particles like mold and dust, where a MERV 16 can trap 99% of asthma triggers, such as dust mites, as well as odors, pollution, and even viruses.  
All of these higher level 4” filters are pleated and surprisingly have 28 feet of surface area for trapping more contaminants.  Particles get trapped deep in the filters, allowing for the maintenance of airflow and a longer life expectancy of the filter.  Matt observes a minimum of MERV 13 for Sawhorse, Inc. projects.  As he says, MERV 8 or 11 might beat thin fiberglass filters at best, but MERV 13 better protects humans and the equipment.   


Matt also discussed filters and filter cabinets at IBS with Aprilaire’s Chris Howells.  Check out this video for a look at Aprilaire’s filter cabinet and a “live” (live then!) demo on how it works. 



Chris runs though the different MERV levels the company offers and shows how the cabinet really keeps air from escaping where it shouldn’t, especially in a damp and musty basement where we want to filter out contaminants.  Two goals of this equipment are to maintain airflow and operate the cleanest HVAC system. For some people, MERV 11 might be sufficient to have a cleaner, healthier home for maybe people.  But if someone in a home has allergies and asthma, you might prefer a a MERV 16 filter.  If your needs change, or you want to upgrade your filter, all of the available filters are interchangeable in the Aprilaire system.        
Chris also reviews surface loading, where a filter is thicker and can trap particulates but still maintain efficiency and airflow.  It’s key to put in the right size filter cabinet to maintain the recommended level of pressure for the equipment.  
Corbett Lunsford presented a video at IBS in partnership with Aprilaire where he reviews 6 HVAC case studies.  Corbett reviews that as our homes become increasingly energy efficient and “tighter” to reduce air and energy leakage, the key indoor air quality measures become even more important.  We have to properly dilute and remove the pollutants we’re bringing into our homes and that we generate living in our homes and buildings.  Otherwise, we’re risking trapping ourselves in with air that can be increasingly toxic.  In his words, “What’s getting in will stay in" if we don’t design, operate, and maintain our HVAC systems properly.   


For more information about Indoor Air Quality (IAQ): 
To learn more about Filtration:


Matt and Chris Howells on the IBS Show floor:





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