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Indoor Air Quality Part 2—Filtration  

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In the last blog, we identified how big a problem IAQ is for many homeowners in Villa Grove, IL & the benefits of annual maintenance. We also discussed where the problem is coming from—just like most horror movies—the problem is inside the house!

In this blog—we’ll discuss our first line of defense. Just like a football team uses lineman, our first, and one of our most effective lines of defense is Filtration!   Nerd-alert—here comes some jargon—but I hope to break it down in a way that makes sense. Any filtration discussion has to start with an explanation of MERV’s & Microns. (No I’m not referring to the French President—that would be President Macron!)

Let’s start with a micron

That’s a tiny particle.  Some pollutants you can see, while many are invisible to the naked eye (anything below 40 microns cannot be seen).  How’s this for a mental picture: take a strand of a human hair—it’s about 20 microns.  Cut it in half—and take that half and cut it in half again. Take that tiny sliver of a hair and cut in half it one more time—and you are down to a “fine particle” size of 2.5 microns.  This size of micron is significant because particles smaller than 2.5 represent the greatest health risk—according to the EPA.

One reason why small particles are the most harmful?  They are so small they get past your body’s first and second line of natural defenses:  your skin, the hairs, and mucus in your nose or even the finger-like cilia in your respiratory system.  Talk about an awesome filtration system—the human body is amazing!

Now let’s move on to MERV

That stands for Minimum Efficiency Reporting Value and ranges from MERV 1 to MERV 16 (see figures 1.3 below).   When it comes to MERV, it shows two things:  1) Particle Size containment & 2) Efficiency Rating. The higher the MERV value—the more efficient it is at catching smaller particles.

(MERV chart showing the efficiency of particle capture at various micron sizes)

Be careful though—while some 1” filters like the very popular Filtrete filters actually do what they advertise—catch many large and small particles—they can also reduce airflow. Reduction of airflow increases static pressure, decreases efficiency, and can have significant consequences like taking out blower motors!  Picture your furnace as a runner in the middle of a marathon. Now put a straw in the runners’ mouth—it would not take long for the runner to stop running!   IAQ Tip — change your filters regularly and make sure they don’t “clog” your system’s airflow.  Changing your filters (about every 1-2 months on 1” filters and every 4-6 months on 4-5” filters) regularly can save you 5-15% on energy costs

MERV 16 filters, which are usually 4-5 inches thick and have many pleats can capture 95% of particles down to .3 microns.   Think of the pleats like the cilia we mentioned earlier.  These pleats increase the surface area of the filter, so we are able to catch the tiniest particles—without decreasing the airflow.


2016 ASHRAE HVAC Systems and Equipment Handbook
EPA Residential Air Cleaners A Technical Summary
2018 EPA “Guide to Air Cleaners in the Home”:
EPA EnergyStar Guide to Efficient Heating & Cooling:
US Department of Energy:
Indoor Environmental Quality:  CDC
AHRI:  Humidity & Occupants:
“Optimize Occupant Health, Building Energy Performance,” Stephanie Taylor, M.D. Harvard Medical School
Aprilaire Presentation on Ventilation
Honeywell Presentation on Humidity Control

Written by Bill Yeakel


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