By Jim Rosenthal, CAFS
Users of air filters – whether they are residential or commercial – are not interested in filter efficiency. Efficiency is just a number. What they are interested in is whether or not an air filter will protect their HVAC system and provide better indoor air quality in their home or building. How well is the filter doing the job of removing particles? In other words, their concern is with filter effectiveness.
The most common method for classifying filters in the US and Canada is the ANSI/ASHRAE 52.2-2017 Standard. This is the method that determines the MERV (Minimum Efficiency Reporting Value) of a filter. For a more complete explanation of this Standard and how MERV is determined see – https://www.texairfilters.com/what-is-a-merv-air-filter-testing-explained/ Simply put – the higher the MERV – the more efficient the filter.
The MERV is determined by efficiency by particle size in three ranges: E1 for particles 0.3 (um) microns to 1 um, E2 for particles 1um to 3 um and E3 for particles 3-10 microns. Here are representative E1-E3 percentages for 4 types of filters – MERV 8, MERV 8 Electret, MERV 11 Electret and MERV 14 Electret.
One would think that indoor particle counts would reflect these test results. They do not. In fact, studies have shown that there are significant differences. Actual indoor counts vary significantly from what we would expect based on the filter tests.
For example, in the study – “In-situ effectiveness of residential HVAC filters” (Indoor Air, January 2020, Volume 30 / No. 1, p.156-166) by Alavy and Siegel, twenty one homes were tested over a one year period to determine how air filters performed in daily use. Here is one summary of the results:
Filtration system effectiveness is expressed as actual indoor particle counts in relationship to particle counts anticipated by filter efficiency tests. For comparison purposes, we are assuming that PM1, PM2.5 and PM10 roughly relate to E1, E2 and E3 from the 52.2 tests. All four filters showed low filter effectiveness. In other words, the ASHRAE 52.2 tests (and the resulting MERV’s) were a poor predictor of indoor particle counts.
The question is – why?
There are several reasons:
- The nature of particle generation – Tests assume particle generation is static ie. remove a known quantity of particles. Particle generation in a space is dynamic. It is increasing (or decreasing) all the time based on the activity in the space. Think about it. Does the Covid-19 patient stop breathing to wait for the particles to be picked up by the filter?
- The nature of particles – When you look at the two charts above, the biggest difference is in the PM10 range. You would expect the highest effectiveness since these larger particles are the easiest to capture by the filter. In reality, they are pulled out of the air by another filter – gravity – before they have a chance to make it to the filter.
- Most importantly, the air containing the particles does not make it through the filter. This can be caused by:
- Filter Bypass – The air follows the path of least resistance and flows around the filter. Just a 10mm gap in a filter installation can make a MERV 15 filter into a MERV 8.
- Low airflow through the HVAC system – Either because of poor system design or the age of the HVAC system, the air flowing through the filter is going too slowly to enable the filter to perform properly. Also, using a high efficiency filter (like a MERV 13) with a higher resistance can cause damage to the HVAC system.
- System run time – Most residential and small commercial HVAC systems only operate 20-30% of the time – even in times of high or low outdoor temperatures where the air conditioning or heating system is used frequently to obtain acceptable indoor temperatures. The air can’t go through the filter when the HVAC system is not running.
That is the biggest problem with the effectiveness of air filters – the air must pass through the filter for it to work properly. Seems logical.
This got me thinking – what would happen if we had LOTS of air going through a filter? How would that affect indoor air quality?
To answer this question I decided to have a competition. One of the finest HEPA air purifiers on the market – the IQ Air Healthpro versus the “hanky janky” DIY “Corsi Box Fan and Filters” air cleaner. (Directions for making one of these are here: https://www.texairfilters.com/a-variation-on-the-box-fan-with-merv-13-filter-air-cleaner/ )
To make the competition “fairer” I used MERV 11 filters instead of the original MERV 13. I used two Dylos particle counters giving a total of 4 particle channels – 0.5um, 1um, 2.5um and 5um. The readings were taken in a small conference room with open doors on two sides – one going to an office and the other going into the hallway.
(The idea for the addition of the supports courtesy of Dr. Clark Vangilder)
Here are the results:
| Particle Counts | HEPA 12:40 | MERV 11 – 1:15PM | MERV 11 – 2:10 | HEPA 3:05PM | MERV 11 – 4:30 | HEPA- 5PM | MERV 11 – 5:30PM |
| 0.5um | 8000 | 4200 | 1500 | 6000 | 3700 | 8500 | 5000 |
| 1um | 3600 | 1800 | 600 | 3300 | 1000 | 5100 | 2300 |
| 2.5um | 900 | 400 | 0 | 700 | 300 | 1200 | 400 |
| 5um | 200 | 100 | 0 | 200 | 0 | 300 | 100 |
Whoa! How did that happen? Not only did the $70 Box Fan with MERV 11 filters outperform the $900 HEPA air cleaner – it did it on all 4 particle sizes including the 0.5um. Remember MERV 11 filters are only around 25-30% efficient on E1 particles – 0.3um to 1um. The HEPA is close to 100%!
The only answer is air flow through the filters. The airflow for the IQAir was set at the maximum 300cfm. The airflow on medium speed for the Box Fan with MERV 11 filters is about 700cfm. It looks like multiple passes of the room air through the filters results in an additive effect. At least in this room environment the “Corsi Box with MERV 11 filters” is the clear winner!
Let’s move to a different room. This one is 20 feet by 30 feet with 8 foot ceilings. The room was not occupied. We were able to start our test with a similar baseline particle count for both air cleaners by opening the door to the outside. Particle counts were taken with a Lighthouse Handheld 3016-IAQ with 6 particle size channels – 0.3um, 0.5um, 1um, 2.5um, 5um and 10um. Here are the results:
| Particle Sizes | Start HEPA 11:30 | HEPA – 12:30PM | Particle Reduction | Effectiveness |
| 0.3um | 509100 | 142600 | 366500 | 72% |
| 0.5um | 95300 | 22400 | 72900 | 76% |
| 1um | 26300 | 3800 | 22500 | 86% |
| 2.5um | 6100 | 200 | 5900 | 97% |
| 5um | 1400 | 100 | 1300 | 93% |
| 10um | 800 | 100 | 700 | 88% |
| Particle Sizes | Start MERV 11 – 2:30PM | MERV 11 – 3:30PM | Particle Reduction | Effectiveness |
| 0.3um | 657500 | 135400 | 522100 | 79% |
| 0.5um | 69500 | 10100 | 59400 | 85% |
| 1um | 23800 | 2400 | 21400 | 90% |
| 2.5um | 9700 | 800 | 8900 | 92% |
| 5um | 4100 | 100 | 4000 | 98% |
| 10um | 2700 | 100 | 2600 | 96% |
It’s pretty close. The HEPA Air Purifier and the Corsi Box with MERV 11 filters performed about the same. But the MERV 11 Corsi-Rosenthal Box outperformed the high quality HEPA air cleaner on all particle sizes.
Again, the amazing thing is that the Corsi Box with MERV 11 filters was so effective at removing particles at 1um and below. This is unexpected and contrary to everything I have seen on air filter efficiency and effectiveness. As far as I can tell, it is not an anomaly. These results are repeatable. One thing is for sure – it is a very effective air cleaner at a very low price.
From an air filtration standpoint, the major takeaway is that efficiency is only half of the effectiveness equation. The other half is air going through the filter. We have seen numerous ways that a lack of airflow through the filter has decreased filter performance. This test shows there may be some benefits of higher airflow through the filter. It should be an avenue for fruitful further research.