Dense HVAC filters make your HVAC systems run much less efficiently. There is less airflow and less heat transfer (either cooling or heating). Anyone that has come to service my air conditioner or furnace has always checked the filter and recommended never getting a denser filter.
The filters in the HVAC system are more there to prevent your furnace from getting clogged up with larger particles than to help the indoor air quality.
if you are getting slow airflow, then you can increase the surface area of the filter medium by either buying a thicker, pleated filter or, you can duct tape multiple filters together in a cube shape with the fan being one side of the cube. That will give much more surface area for the fan to pull through.
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.
In a previous house I rented, we swapped the furnace filter for a much finer one (from 8 to 13 on whatever scale they market these things in), and it made the HVAC system run noticeably worse. Lower airflow, noisier, etc. It seemed to me that the intake system couldn't keep up with the reduced flow of the denser filter.
Could moving to a coarser filter but adding room filters be useful in that context?
When you're dealing with a machine that is exchanging air in a room a certain number of times per hour and a continuous influx of particulates, with less efficient filters you need more air exchanges which means more filters, bigger fans, etc.
The takeaway is that less particle efficient filters won't ever get you down to the same steady state as better filters in real world applications.
And from personal experience with filters and devices to measure airborne particles, it is hard enough getting good numbers with the best filters I can find.
Air molecules are a lot smaller than the particles getting trapped, so presumably a filter with a low enough density can minimize heat transfer except to things getting stuck in it.
I've had HVAC techs tell me not to use those thick filters unless your system is designed for it. They make the fan work harder to pull less air. This reduces how much air it can move.
Modern particulate filters are pleated: you take a thin filter material that is effective at blocking particles but hard to get air through, and then fold a bunch of it up in a small space to increase the surface area and decrease the pressure drop required for air to flow through it. If you don't have a suitably thin material or don't have this technique, increasing the surface area by, say, making it into a big cone would probably also work.
The miasma theory isn't quite right, but it could conceivably have provided a half decent rubric for designing a working particle filter: at least some unpleasant smells are carried by particles. (Burning incense inside the mask is an example of where this theory might lead you off course.)
Having a greater surface area to filter the air through will decrease the static pressure and be nicer on the fan motor. If OP was putting the filters in series, then yes, it should have no effect. But they are putting the filter in parallel.
We upgraded to a "media filter" when we got new HVAC at my house. IIRC they are 4 inches thick, which gives a lot more surface area due to the deep pleats. You can then run fairly tight filters without putting undue strain on the motor - plus they last quite a bit longer. It made a moderate difference in our house in regards to dust in the air, but we also have 3 pets and live in a very dry, dusty area.
You could also go with smaller filters, like 14x20 to reduce the stack size, but the prices don’t scale exactly with surface area, while the pressure does. So I’m not sure how much that really buys you.
Probably cheaper MERV 8 filter would make more sense, since for this application you're mostly concerned about those larger dust particles. Also it seems like this would be a job for the fan manufacturers rather than the PC case manufacturers. The fan just needs a little slot where you can slide in a thin replaceable filter.
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.
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