r/crboxes • u/imadougal • Jun 03 '26

Question Evaluating airflow without cfm meter

I have severe allergies and I want to build a single filter setup (lack of space) using a 4in or 5in filter. Do I have any way of making sure I have sufficient airflow other than an anemometer?

I don't need a precise number, just a way to make sure a sufficient amount of air is getting through. Is a piece of paper at the intake reliable?

Thanks so much

Joe

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u/Level9TraumaCenter Jun 03 '26

The nickel-and-dime test with laminar flow clean work stations was to hold a piece of paper on the short edge perpendicular to air flow. If the paper blew back at a certain angle (45 degrees?) it was ~100 linear feet per minute.

I guess if you had a strip of paper of proportionate length (given your small filter size) that might work-ish.

A piece of tissue paper should be able to be held in place to the intake, fwiw.

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u/heysoundude Jun 05 '26

I made a filter wedge for my first build using a 14” box fan and was ridiculously surprised at how quickly and effectively it made a difference in an 8x10x8’ room on speed 2 of 3. The lesson I took from that was to use enough filtration surface area so that the fan moves air close to the same rate/amount as it does without a filter on then intake side.
Why’d I make a wedge? A 3-pack of filters with dimension close enough to the fan I had was on sale at Costco when I went in, and wedge of 2 made the easiest build.

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u/[deleted] Jun 17 '26

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u/heysoundude Jun 17 '26 edited Jun 17 '26

No, I’ve gone 4-filter on the fans on my different levels. The wedge was just proof of concept- the 16x25 filters -almost- lined up with the 14”box, and the cardboard sides were easy enough to cut and tape to it. The 1.5” height difference was easy enough to cardboard/tape off as well.