# Would you choose a DIY box fan air filter over a manufactured air purifier?



## Druidia (Oct 9, 2011)

I was intending to buy a couple of Holmes HAP9422 HEPA-type tower air purifier for my home office and bedroom but then I came across reports, forum posts, and blogs about DIY 20" box fan filters. I'm leaning towards the Holmes air purifier because the specs and reviews seem good and also because it looks aesthetically pleasing but thought that given the size of a 20" box fan, wouldn't the box fan with a MERV 11 filter be more efficient as it would be able to suck in a greater volume of air for filtering? I'm not doing aseptic technique so a HEPA filter to remove 99% of 0.3 micrometer or larger particles is not important to me. A MERV 11 filter would be satisfactory.

What do you guys think? I don't have an airborne particle counter or an air flow meter so I can only compare based on information I've read.

The Holmes HAP9422 is recommended for 170 sq ft, comes with HEPA and carbon filters (the carbon filter is not that important to me as "odor" isn't an issue), 3-speed, and smoke CADR 105.

This guy (Director of the Uni of Michigan Health System's Michigan Sinus Center) demonstrated that a MERV 13 filter placed in front of a box fan reduced airborn particles by 90%. 
http://www.uofmhealth.org/News/sinus-hepa-0630

A bunch of graduate students did quantitative measurements and showed that a HEPA filter in front of a box fan reduced 2.5 uM particulates (and larger) by 92% in 8 h and 0.5 uM (and larger) by 84% in 8 h. They have a comparison to two brands of manufactured air purifiers, IQ Air and Blue Air.
http://smartairfilters.com/#data

The American Allergy Supply used to sell a box fan with a back grill meant to hold a 20 x 20 x 1" filter. Using a MERV 11 filter in a 1800 cu ft room, their data show significant reduction in particulates in 3 h.
http://www.americanallergysupply.com/box-fan-air-cleaner.htm

There are also several woodworking/woodshop/workshop blogs where DIY box fan filters were used to augment dust removal/collection.
http://lumberjocks.com/woodspar/blog/1553
http://www.woodworkingtips.com/etips/etip040130sn.html
http://www.dzplanet.com/three-stage-double-box-fan-air-filter/

This approach to making a DIY box fan air filter looks like it would make it look less Frankenstein-like compared to attaching the filter using duct tape and pieces of cardboard. They used vinyl J trims as filter holder.
http://woodworkingtips.com/etips/etip020705ws.html


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## dftc (Nov 12, 2012)

I've used the box fan filter for woodworking and it works fairly well for that purpose. I also built a huge ceiling-mounted dust filter using a big attic fan and 5 furnace filters. It worked really well but was super loud. It only cost about 50 bucks to make though.

One thing to consider is that adding a dense filter at the intake on a regular box fan will make it work a lot harder, so it won't be moving as high a volume of air as it's spec'd for in its original configuration.

A box fan may also be louder than a purpose built air filter and you need to think about where it's going to be placed for best effect. A purpose built air filter may be easier to place or mount in an ideal location. A box fan is pretty much limited to sitting on the floor.


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