Most of that is wrong without knowing all the parts used. You cant just say 1/3 restriction from filters. Then comes the size and CFM of the fans and not a clue if the case in question even has filters.
1. If you read the whole post, you will see that I took pains to state:
" On a well designed case..."
The words "well designed case" and "case w/o filters" are mutually exclusive. So no, by the stated definition, it's not wrong.
2. After having personally conducted days of testing, please explain why I can't say that ?
a) Again, if you read what I actually wrote ...
"air filters cause a restriction which can reduce air flow by
as much as a third ". Is it your position that we would normally see more than a third, because everything from 0 to 33% in fact, fits that definition ? Now with a clean filter it's certainly less, probably on the order of 10% but that has no significance here. When we design a building, we design for the largest load and that's what I have done here. You can object all ya want saying "it's all wrong because the snow and wind loads aren't there all the time".... but the fact remains, when it is there, if I don't design for the worse case condition, then the occupants of the building die when the roof collapses.
Since doing those tests three years back we have always tested every completed build. Don't always break out the fog machine, cigarette will do but it is a simple test to undertake. Our last build that came in, the user brought in a box of parts, a Corsair Spec 01 and new red LRD fans for every fan mount
Front: (x2) 120mm
Top: (x2) 120mm
Rear: (x1) 120mm
He asked for all 5 fans to be installed and we did it with the top fans as exhausts as requested. That gave him 3 out and 2 in. So I gave him the above speech and it just didn't seem to make sense in his mind and we did the siggie test.... that smoke went right into the case. Reversed one of the top fans and did it again.... granted wouldn't actually do this for a working build because of obvious short circuiting issues but it's valid for the experiment)
Now we had 3 in and 2 out. Lit the ciggie again and now air was clearly coming out the rear grilles and a decent clip.... no doubt as filters clogged up a bit, this situation would deteriorate somewhat. Just for "schitz and giggles" we did it with 2 / 2 and again, smoke went into the case. He went home with 4 in and 1 out simply because after testing all potential possibilities, 3 in 2 out resulted in negative pressure.
b) I can say because we have tested this on our test bench and have physically measured the impact using various fan combinations using 6 thermal probes, a infrared thermometer, a fog machine and various fan combinations at varying speeds. We took out the fan filters, laid them out on newspaper, misted them with water and then took some dust from a shop vac on them (was recently used to clean whole house filter and heat exhangers for house forced air system) and let them dry. Then we used the vacuum to suck from the "clean side" at a reasonable distance and finally gave them a shake to get off the loose stuff and pulling off any dust bunnies by hand till we got a "look" similar to what we normally see in a typically user's build when brought over to get something fixed.... certainly much less than i typically see in my youngest sons build and those get cleaned every 3 months.
These were then installed in a box that has 10 fan mounts and we tested by having the fog machine discharge at the rear grille. Results w/ 140mm @ 1200 rpm:
(1) in (1) out - negative pressure fog sucked into PC
(2) in (1) out - positive pressure, no fog sucked into PC, stream pushed away
(2) in (2) out - significant negative pressure fog sucked into PC
(3) in (1) out - significant positive pressure
(3) in (2) out - neutral, barely perceptible movement in fog stream being pushed away from PC
(3) in (3) out - strong negative pressure fog sucked into PC
(4) in (1) out - very strong positive pressure, fog really getting a kick
(4) in (2) out - significant positive pressure
(4) in (3) out - slight negative pressure, some fog sucked into PC
(4) in (4) out - strong negative pressure fog sucked into PC
skipped the 5s and went to 6
(6) in (3) out - strong positive pressure
(6) in (4) out - neutral, barely perceptible movement in fog stream being pushed away from PC
(6) in (5) out - medium negative pressure significant movement in fog stream
(6) in (6) out - strong negative pressure fog sucked into PC
Went thru the same schpeil at 900 and 600 rpm.... with decreased fan speed the effect of air resistance was somewhat less. At 600 rpm for example, with (6) in (5) out, the effect was relatively neutral. At higher rpms which we didn't test, I imagine the effect of air restriction would be somewhat greater. And we did it again with 120mm fans
What fans are used is immaterial when all are the same, at the same rpm and size. As a corollary, in industrial building and kitchen design, we really don't use intake fans at all except where energy code requirements dictate using pre-heated intake air. If you exhaust air out if at space, all you need is an open window or intake louver to allow air in. Think of any window fan, kitchen hood exhaust or attic fan in your house.... you don't have a corresponding intake fan