Thank you! I suspect that what’s happening is that the bottleneck in the case of highly-pleated 4″ filters isn’t pushing air thru the large filter surface, but pushing air into the wedge-shaped “waiting area”, if you know what I mean. So when the static pressure drop across your 4″ filter appears to be .06 iwc, that isn’t actually the drop across the filter; that’s the drop when squeezing into the high-pressure volume upstream of the filter (but “in” the filter if it’s considered as a 20″ x 20″ x 4″ box).

Phil: For more filter area to correlate to lower pressure drop is the logical assumption. At least in one study, though, that’s not always the case. I wrote about this and other research on air flow and pressure drop in a 2019 article:

Do High-MERV Filters Always Reduce Air Flow?
https://www.energyvanguard.com/blog/do-high-merv-filters-always-reduce-air-flow

About study #2, I wrote:

“The final result I’ll mention here is that they didn’t see as much difference as they expected for filters of different depths. For example, ‘the 4-inch Filtrete 1550 (MERV 12) was only marginally better than the 1-inch Filtrete 1700 (also MERV 12) and the two other [1-inch] MERV 11 filters of the same brand (1000 and 1085).'”

It seems to me that when you relate filter area and air velocity to pressure drop, that’s meaningless unless you measure the actual filter area, not the cross-sectional area of the filter. I would expect the pressure drop across a 4″ thick 20″x20″ filter to be roughly one-fourth that across a 1″ thick 20×20 filter. But then again, maybe not, because the air going thru that 4″ thick filter is at the same pressure, and maybe crossing the filter at the same air velocity? Do we measure velocity along the vector perpendicular to the filter surface, or in the direction the air is moving? What direction IS the air moving when it goes thru a heavily pleated filter?

I’d really like to know the answers, because if a 4″ thick give the same pressure drop as a 1″ thick with the same cross-sectional area, then I can save a lot of money on filters. My expectation is that the 4″ thick will have somewhere between 1/1 and 1/4 as much pressure drop. But that range is so high that any talk about numbers without knowing that is pointless.

Thomas: Sorry, I should have mentioned that. It’s a snippet from a photo of the ASHRAE duct calculator. Here’s the page for it in the ASHRAE online store:

https://store.accuristech.com/ashrae/standards/duct-size-calculator-i-p-si?product_id=1935218

Roy: We’ve done a little commissioning in the Atlanta area but haven’t done much. There are people in a lot of the metro areas in North America who can do it, and we may start doing more some day. We’ve certainly talked about it.