The December issue of the ASHRAE Journal included an article entitled “How Do Particle Filters Work?” The article has graphs of “Efficiencies over time for two filters.” They come from ASHRAE Research Project (RP) 1190 — that was used to develop Loading Dust and Dust Loading Procedures for the ANSI/ASHRAE Filter Test Standards 52.1 and 52.2. The result was the optional Appendix J to the current ANSI/ASHRAE Standard 52.2. One of the filters is a MERV 5 – representing the Non-electret Media Filters. The second is of a MERV 11 – representing Electret Media Filters. Both show the effects of “time in use” from a clean filter to the filter at 12 weeks.
This graph represents the RP-1190 data for a new MERV 11 filter and for MERV 11 filters in use after 2 weeks, 6 weeks and 12 weeks.
The Appendix J conditioning step using fine KCL particles was based on the above data. The Appendix J curve is close to the used filter test results. However, many people have found that the Appendix J conditioning step consistently overstates the loss in electret filter efficiency. In addition, it implies that the loss will continue at a rapid rate to end at very low efficiency levels. We have found that the efficiency loss is less than predicted and that the performance of electret filters will continue at a high level for an extended period of time. We have done some additional research to help us understand why this might be the case.
We decided to test MERV 11 filters to see how they performed in real life. The four filters studied in this project were all produced in the same factory, using the same methods and the same materials. One filter was new. The other three filters were removed from commercial buildings where they had been used under normal operating procedures. The filters had been in use from one month to 4 months and were exposed to varying levels of outdoor and recirculated air. The new filter was tested according to ANSI/ASHRAE 52.2-2012. The “used” or in-situ filters were tested using the “initial” test only since they had been “loaded” through actual exposure in the field. All filters were tested by an independent laboratory (Blue Heaven Technologies – Test No. 17-322-1, No. 17-332-2 and No. 17-332-3). Here are the results:
There was very little drop in efficiency at one month (4 weeks). The efficiency levels at 1.5 months (6 weeks) and 4 months (14 weeks) were virtually the same. In all cases the efficiency levels exceeded the Appendix J levels at every particle size. Most importantly, the largest difference was in particles below 1 micron in size. One of the biggest benefits of filters using electret media is their superior performance on small particles. These particles have been shown to be the most dangerous to human health. Their removal is very important for the protection of building occupants from everything from infectious aerosols to wildfire smoke. The fact that the used MERV 11 filters outperform predicted levels from Appendix J is an important consideration in filter usage and filter requirements.
Let’s take a closer look at the efficiency numbers for E1 (0.3-1.0 microns), E2 (1.0-3.0 microns) and E3 (3.0-10 microns) from our test results and the Appendix J results. As stated above, the biggest difference is in the E1 range. It is important to understand that the used MERV 11 filters are not just 9-17% more efficient than the Appendix J numbers, they are close to 2 to 3 times as efficient. In other words, they will pick up 2 to 3 times more particles in this size range. When you multiply this by numerous passes through the filters as you would see in a well-designed HVAC system, the difference can be very substantial.
| MERV 11 – New | MERV 11 – one month | MERV 11- 1.5 months | MERV 11 – 4 months | MERV 11 – Appendix J | |
| E1 | 27 | 28 | 21 | 20 | 11 |
| E2 | 67 | 63 | 58 | 57 | 55 |
| E3 | 85 | 82 | 80 | 82 | 75 |
Fortunately, we have some examples where we can see this “efficiency multiplication” in action. My favorite is still the face-off between the $1,000 HEPA air cleaner and the Corsi-Rosenthal Box using MERV 11 filters. The HEPA air cleaner is tested at 99.97% efficiency. The CR Box with MERV 11 filters had been running in my office for about a month. I ran the “test” in a small office with one door open to another office and the other open to a hallway in a suite of offices.
While it is true that the “media velocity” in the CR box is low because of the large filter surface area of the 4 filters, this only accounts for a small percentage of the improved performance. The biggest gain is created by the much higher air flow through the box fan. The interesting thing is that the MERV 11 CR Box beats the HEPA filter at every particle size – 0.5um – 5 um. This probably would not have been attempted if one relied on the test results from Appendix J.