How does a MERV 13 filter compare to a HEPA filter?
Did I make the right choice using a MERV 13 filter? Everyone is saying you need a HEPA filter. A HEPA filter has an efficiency of 99.97% and so it must be better right?
Well lets see what others have found.
No Statistic Difference Between MERV 16 and HEPA
Comparison of MERV 16 and HEPA filters for cab filtration of underground mining equipment found no statistic difference in air quality when using a MERV 16 (95% efficiency) and a HEPA filter (99.97% efficiency), and in the end suggested a MERV 16 filter to be used.
In contrast to the HEPA filters, the MERV 16 filters showed improved filtering efficiency over time and use as the filters loaded with dust. Because the MERV 16 filters were less restrictive and provided greater cab pressure, they did not have to be replaced as often as the HEPA filters. This testing also showed the benefits of using a mechanical filtering media, which becomes more efficient with dust loading and the creation of a filter cake. For both the face drill and the roof bolter used in this comparative study, the MERV 16 mechanical filter design was the optimal choice, not only for performance but also for cost. As MERV 16 filters are less expensive than HEPA filters and do not need to be changed as often, which significantly lowers maintenance labor costs, this equates to significant cost savings.
HVAC Simulation Research
A simulation study found diminishing returns going from MERV 13 to MERV 16 as shown below. It found a MERV 13 removed 70% of outdoor PM2.5, while a MERV 16 removed 76%, so 10% better. |
| Source |
Air Flow
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| Airflow testing a HEPA filter |
A HEPA filter typically has twice the static pressure of a MERV filter. I assume this means it will have half the airflow for the same fan. A quick comparison I did using my MERV 13 and a HEPA filter of the same m2 size (slightly different dimensions) on the same fan showed a reduction in airflow of 49.3%. That is surprisingly close to my expected value of 50%. However a followup test I found it dropped by 35%, see HEPA Test 1 below.
As I outline in Why air flow matters more than filter rating the reduction in airflow is not great.
Air Changes per Hour
A filter in the house will process the air multiple times per hour. The general advice for HEPA is 5 times, while I was running my DIY at around 11. If on each pass the filter is removing 76% of PM2.5, then after 2 passes there is only 5.76% of PM2.5 left.If we apply this to the ratings we get some interesting results:
CADR
When looking at purifiers many mention their CADR (Clean Air Delivery Rate). This is typically 60% of the actual air flow. SmartAir explain CADR well in their post What’s CADR and How is it Different From Airflow? As the purifier may process the same air multiple times even though its delivering say 100m3 of airflow, its only cleaning 60m3 as 40m3 of that was cleaned multiple times. As a HEPA filter leaves little after 1 pass the additional passes don't assist.
The DIY Purifier actually needs to clean the air multiple times, so it doesn't lose out as much. I am not about to light cigirettes to find out what its CADR is, however in order to get my simulation to match the actual results I needed to use 70%-95%.
My Estimated Difference
Based on a 10% increase in PM2.5 reduction, and a 50% reduction in airflow, my guess is a HEPA filter will be 0.5 * 1.1 = 55% as effective as a MERV 13. i.e. It will not work as well.But only HEPA can do small particles, right?
Another concern I had was around filtering small particles, and if a MERV 13 can. It turns out it can, which I explain in Does the DIY purifier capture small particles?Spreadsheet Simulation: MERV 13 vs HEPA
HEPA Test 1
I have run a test using my HEPA filter I brought for $25 off eBay. It is the same overall size as the MERV 13. The test was not ideal as inside level was 29 vs 52 for MERV 13 test, and outside was 54 vs 88 MERV 13 test. |
| PM2.5 Count vs Time |
To try and normalise the conditions here is how they went using Inside PM2.5 / Outside PM2.5.
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| Inside PM2.5 / Outside PM2.5 vs Time (minutes) |
HEPA Test 2
This test is similar to HEPA Test 1 however the outside level was higher being 74. Not quite as high as the 88 for the MERV 13 test.
They both closely match until around the 30 minute mark where HEPA goes a little lower. In this test HEPA reduced from 51 to 3 after 60 minutes while MERV 13 reduced from 52 to 6.
HEPA Test 3: Office
I have tried running MERV 13 and HEPA for awhile in my home office to see how they go. This is more a test of keeping the PM2.4 low than getting it down from a high reading.
- HEPA: 5 PM2.5 @ 10:00
- MERV 13: 6 PM2.5 @ 12:30
- MERV 13: 6 PM2.5 @ 16:00 - 84 outside
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