I built a "four filter fan box" this year when the smoke started rolling in (off MERV 13 filters, I think?), and have also gone about getting the measurement devices to verify that, yes, it really does work! I used a round fan, and a lot of tape to seal things to the fan, though it appears this might not matter as much as I thought. The filter area is sufficient that the fan doesn't really struggle - fan motors burning out from a single filter (as someone else reported) is a well known failure mode, and a good reason not to just strap filters flat to a fan. They rely on the airflow to cool, and it's easy to screw that up.
Air quality (in terms of PM2.5 and PM10, the stuff my testers measure) coming out of the fan is very good - far lower than the room, and the filter will start pulling down air particulates in a room fairly quickly. If the room is around 5ug/m^3 PM2.5, the output from the fan is sub-1, and it moves enough air to do quite a few air exchanges per day. It really does make a difference.
I'm using a couple Temtop meters to measure air quality, and while they're Chinesium, they do seem to work fairly well.
Also, if you ever wondered: Use your stove's vent fans! The air quality difference in PM2.5/PM10 between cooking without those running, vs cooking with them on, is HUGE! I'll see spikes in the kitchen up well past 100ug/m^3 PM2.5 after browning meat, which is worse than it is outside in heavy forest fire smoke!
Filter's airflow restriction is primarily a function of surface area and filtration medium layers. More surface area results in a better airflow and more filtration medium layers results in a worse airflow.
To maximize surface area of a filter, you can get a thicker accordion filter. This will generally provide good airflow and filtration characreristics.
Sorry about video links below, I liked them at chart times, so you don't have to watch the video to check out the summary.
Here is an example of airflow restriction vs filter. Note that 4 inch Honeywell filter only provides small static pressure rise over no filter option: https://youtu.be/RkjRKIRva58?t=456
Thicker filters cost more, but also last longer. General rule seems to be that when you double the filter thickness, time between replacements doubles as well.
It’s more a function of surface area to volume ratio of the filter. More surface area to volume means more energy is lost due to flow friction between the air and filter walls. However the trade-off is that a higher surface area to volume ratio results in higher filtration efficiency so you have to optimize between flow rate and filter efficiency to get a
overall optimum filtration performance.
We saw a huge improvement in air quality (less smoke, less steam, less grease) when I installed a vent to the outside so the microwave fan over the stove could vent outside instead of just circulating the smoke through some charcoal filters.
That makes sense during normal times, but when the air quality outside is hazardous, venting the air outside means it's pulling unfiltered air inside your house. So use sparingly.
The last time I lived in a place with an unvented stovetop microwave fan, the previous owner never bothered to install the filters, so it was just recirculating unfiltered air.
It pull in outside air, yes, but things are more dynamic than that. Most likely you don't have perfect seals on all your windows, walls, doors, pipes, fans etc.
Secondly, the air from cooking is generally very bad, especially if you're using nonstick surfaces or using much of anything with oil. It's probably better to trade it for smokey air outside.
As part of your testing, did you notice a big difference in the amount of airflow when you attached one versus four filters onto the fan? I wish to know how important that surface area is for air flow.
Air quality (in terms of PM2.5 and PM10, the stuff my testers measure) coming out of the fan is very good - far lower than the room, and the filter will start pulling down air particulates in a room fairly quickly. If the room is around 5ug/m^3 PM2.5, the output from the fan is sub-1, and it moves enough air to do quite a few air exchanges per day. It really does make a difference.
I'm using a couple Temtop meters to measure air quality, and while they're Chinesium, they do seem to work fairly well.
Also, if you ever wondered: Use your stove's vent fans! The air quality difference in PM2.5/PM10 between cooking without those running, vs cooking with them on, is HUGE! I'll see spikes in the kitchen up well past 100ug/m^3 PM2.5 after browning meat, which is worse than it is outside in heavy forest fire smoke!
https://www.sevarg.net/2021/08/28/temtop-air-quality-sensors...