{"id":6702,"date":"2024-02-28T17:27:51","date_gmt":"2024-02-28T09:27:51","guid":{"rendered":"https:\/\/smartairfilters.com\/?p=6702"},"modified":"2024-02-28T17:27:53","modified_gmt":"2024-02-28T09:27:53","slug":"air-purifier-filter-on-the-front-or-back","status":"publish","type":"post","link":"https:\/\/smartairfilters.com\/en\/blog\/air-purifier-filter-on-the-front-or-back\/","title":{"rendered":"HEPA Filter Fan Placement: Should Air Purifiers Have the HEPA Filter on the Front or Back?"},"content":{"rendered":"

Should air purifiers have the HEPA filter on the front or the back–is it better to push or pull air? The Smart Air lab in Beijing ran tests of the same fan with the HEPA filter on either side, and the results contradict most people’s intuition.<\/p>\n\n\n\n

When I made my first DIY purifier<\/a>, I put the filter on the front:<\/p>\n\n\n

\n
\"DIY<\/figure><\/div>\n\n\n

Within a few weeks, several people told me I was an idiot. Why’d you put the filter on the front? It should go on the back!<\/p>\n\n\n\n

Many people have a strong intuition the filter should go in the back<\/h2>\n\n\n\n

Here\u2019s a more-respectful-than-usual suggestion on my <\/span>live test video<\/a><\/span>:<\/p>\n\n\n

\n
\"\"<\/figure><\/div>\n\n\n

I\u2019m fascinated by how many people have this intuition. For example, this commenter on the website Quora said it’s like trying to push a rope.<\/p>\n\n\n

\n
\"Purifier<\/figure><\/div>\n\n\n

But when I push further, most people struggle to come up with a good reason. For some people, I think it\u2019s purely an intuition about having air come from a pure source. But here are the two most coherent reasons I\u2019ve heard:<\/p>\n\n\n\n

Claim 1: The HEPA Filter on the Front \u201cProtects\u201d The Fan.<\/h3>\n\n\n\n

This is probably the best reason I\u2019ve heard. Yes, putting the filter behind the fan will prevent dust from hitting the motor and collecting on it. I think there\u2019s a small, non-zero benefit of that, although I haven\u2019t seen anyone actually test it.<\/p>\n\n\n\n

But here’s a gut check: people run \u201cunprotected\u201d fans (we just call them \u201cfans\u201d) all the time, and it doesn\u2019t seem to cause much harm.<\/p>\n\n\n

\n
\"\"<\/figure><\/div>\n\n\n

Claim 2: The HEPA Filter on Front Improves Performance.<\/h3>\n\n\n\n

This is an odd idea to me. I\u2019ve asked several physics PhDs, and they\u2019ve said it shouldn\u2019t make a difference. The amount of air coming in is equal to the amount of air coming out.<\/p>\n\n\n\n

But who knows? Maybe there\u2019s something about the air flow process that I don\u2019t understand. My pet peeve is when people point to a theory and assume that proves the matter, without actually testing it. So I went out and tested it!<\/p>\n\n\n\n

Air Purifier Filter Front vs. Back Test<\/h3>\n\n\n\n

It\u2019s hard to test with that first fan I used (above) because putting the filter on the back would require me to tape around the back screen. That would alter the fan, so we can\u2019t compare apples to apples.<\/p>\n\n\n\n

Fortunately, I tested lots of different fans.<\/p>\n\n\n\n

<\/div>\n\n\n
\n
\"Different<\/figure><\/div>\n\n\n
<\/div>\n\n\n\n
And eventually I settled on this tube-shaped fan I call the <\/span>Cannon<\/a><\/span>.<\/span><\/div>\n\n\n
\n
\"canon<\/figure><\/div>\n\n\n
<\/div>\n\n\n\n
<\/div>\n\n\n\n
It\u2019s perfect for this test because I can easily put the HEPA filter on the front or back. So that\u2019s what I did! I ran 20 tests of 0.5 micron particulate in my apartment in Beijing.<\/span><\/div>\n\n\n\n
<\/div>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
<\/div>\n\n\n\n
<\/div>\n\n\n\n
I also had a third-party lab test the two versions in a CADR<\/a><\/span> test.<\/span><\/div>\n\n\n\n
\n

The results? In both tests, putting the HEPA on the intake side actually did slightly worse! Here are the more-controlled CADR test results.<\/p>\n

<\/p>\n

\"Filter<\/figure>

<\/p>\n

<\/p>\n

I was surprised. I thought they\u2019d perform the same.<\/p>\n

Now this next part is hard for me to understand, but the Smart Air team is fortunate to have an aeronautical engineer. Here\u2019s an important detail about the Cannon fan I didn\u2019t mention:<\/p>\n

<\/p>\n<\/div>\n\n\n

\n
\"DIY<\/figure><\/div>\n\n\n

The fan isn\u2019t in the middle. Instead, it\u2019s about 2\/3rds of the way down the tube. According to our engineer, it helps to have more distance between the fan and the filter so that a column of pressure can build up.<\/p>\n\n\n\n

Again, I still haven\u2019t grasped the physics, so I invite anyone with more expertise to add their explanation here. However, two independent datasets show that putting the HEPA on the front leads to higher reductions in particulate.<\/p>\n\n\n\n

If the fan were in the exact middle, we hypothesize that performance would be equal with the HEPA on the front or the back.<\/p>\n\n\n

Bottom Line<\/div>
Bottom Line:<\/div>
Controlled tests found no benefit of putting the air filter on the back (pulling) compared to the front (pushing). Instead, results showed distance from the filter to the fan affected purifier performance more.<\/div>
Smart Air<\/div><\/div>\n\n\n\n

Input-Output Testing<\/b><\/h2>\n\n\n\n

I think this is a fascinating demonstration of how I bumbled into a good design. I didn\u2019t know much about airflow dynamics, but I did lots of input-output tests before I chose this setup. In other words, I was agnostic about what was going on inside the fan, but I tested different fans and measured the output I cared about (reduction in particulate), and that led me to a good design\u2014even though I couldn\u2019t explain why at the time.<\/p>\n\n\n\n

Speculation: Does turbulent airflow explain why \u201cfilter first\u201d is worst?<\/h2>\n\n\n\n

Now we\u2019re not 100% sure on this but based on research on <\/span>guide vanes<\/a><\/span>, putting a filter before the fan will make the air going into the fan turbulent.<\/p>\n\n\n

\n
\"Purifier<\/figure><\/div>\n\n\n

Making the fan operate in turbulent airflow (as opposed to laminar flow) has two consequences:<\/p>\n\n\n\n

A. The efficiency of the fan is reduced.<\/p>\n\n\n\n

B. The fan will be noisier.<\/p>\n\n\n\n

However, we don\u2019t know whether this effect would be large enough to be meaningful.<\/p>\n\n\n\n

Putting the filter after the fan means turbulent air (from the fan) is going into the filter. We\u2019re unsure what effect that has on the efficiency of the filter.<\/p>","protected":false},"excerpt":{"rendered":"

Should air purifiers have the HEPA filter on the front or the back–is it better to push or pull air? The Smart Air lab in Beijing ran tests of the same fan with the HEPA filter on either side, and the results contradict most people’s intuition. When I made my … <\/p>\n

Read more<\/a><\/p>\n","protected":false},"author":1035,"featured_media":29191,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_relevanssi_hide_post":"","_relevanssi_hide_content":"","_relevanssi_pin_for_all":"","_relevanssi_pin_keywords":"","_relevanssi_unpin_keywords":"","_relevanssi_related_keywords":"","_relevanssi_related_include_ids":"","_relevanssi_related_exclude_ids":"","_relevanssi_related_no_append":"","_relevanssi_related_not_related":"","_relevanssi_related_posts":"7830,11976,20657,12577,22029,13492","_relevanssi_noindex_reason":"","footnotes":""},"categories":[190,259,264],"tags":[283,326,256,514,544],"class_list":["post-6702","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-air-purifiers","category-filters","category-indoor-air","tag-air-purifier","tag-design","tag-diy","tag-filter","tag-tests","resize-featured-image"],"acf":[],"_links":{"self":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts\/6702","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/users\/1035"}],"replies":[{"embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/comments?post=6702"}],"version-history":[{"count":30,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts\/6702\/revisions"}],"predecessor-version":[{"id":29452,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts\/6702\/revisions\/29452"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/media\/29191"}],"wp:attachment":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/media?parent=6702"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/categories?post=6702"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/tags?post=6702"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}