[schiffern]

HEPA gets all the hype, but if you can afford to recirculate the air for multiple passes (ie regular filtering, not a surgical theatre) then MERV-13 is the GOAT. It strikes a near perfect balance between particle interception efficiency and airflow volume.

Paradoxically, a 'more efficient' filter will generally achieves lower real-world performance (CADR) because the airflow drops so much.

The MERV rating are often hidden, instead you may need to look for numbers from a company-specific rating system like 3M's 'MPR 1900' or Home Depot's (yes, really) 'FPR 10.'[0]

(I almost don't want to include this last paragraph of info, because by sounding like an ad it will automatically 'taint' everything else, but fuck it....)

I find myself generally buying the 3M version (which is ~30% more expensive locally) which I observe has a much higher pleat count. That means more surface area (lifetime) and better pressure drop (airflow). By my math I come out ahead in cost per area of medium, which for me is a better metric than cost per filter.

[0] https://airfiltersdelivered.com/blogs/helpful-tips/merv-mpr-...

[tempestn]

Aprilaire 413 are our MERV 13 go-to. One other note not everyone may be aware of: if you have a central blower (aka furnace) that takes a standard 1" filter cartridge, it's usually not too expensive to add in a larger filter housing in the adjacent ducting. You just leave the default slot empty, or if you like, put a low-filtration backup filter in there. (Low filtration so as not to further restrict airflow.) So basically just because your existing unit only takes 1" filters doesn't mean you're stuck with that. Upgrade to 4" and you'll get better filtration and much longer filter lifespan.

[schiffern]

Great tip. I should compare the cost-per-area across 4" filters too. Often I'm just making a Corsi Box[0], so the filter thickness is unconstrained.

Anyone got recommendations for a reputable vendor online? Something other than BezosMart?

[0] https://cleanaircrew.org/box-fan-filters/

[tempestn]

It might be a bit more challenging to make one of those with 4" filters, but yeah, worth a look at least. The other reason I like it is that our furnace is in the crawlspace, so having to get down there less often to change the filters is a pretty significant benefit. I'd do it even if the total cost were equal or higher.

[JoshTriplett]

> put a low-filtration backup filter in there

It isn't just a backup; you want it there before the expensive fine-grained filter, to catch the big stuff and extend the life of the more expensive filter.

[tempestn]

That's not always feasible, as the built-in filter is usually integrated with the air handler. The add-on will generally have to be in the inlet ducting, so it will come first. I'm not sure that approach would even reduce cost though; much of the benefit of the large filter is that it has way more surface area, and so it lasts longer before restricting airflow too much. If you put a 1" filter in front of it, now you'll have two filters you need to replace regularly. You might preserve the life of the larger filter a bit, but not enough to make up for the cost of the whole extra filter. So I think I'd prefer to either just use a 4" alone (which is what we indeed do), or have the 1" as a backup, and basically use its condition to test whether anything significant is getting past the main filter. It would very rarely need to be replaced, as the primary filter should be catching everything.

[schiffern]

For a prefilter I sometimes wedge in a fiberglass "stick filter" to catch larger dust and pet hair. At service time I vacuum it off (or just bang it off in the trash) and reuse.

I wouldn't use a prefilter with any real pressure drop thought. Why not? Well...

In theory a two-stage filter is ideal, because you can cycle the filters through: swap the (mostly clean) post-filter over to the pre-filter during filter changes, optimizing both filtration level and using the full capacity of each consumable filter element. This is the procedure when changing the ISS water filters, incidentally.

There's a downside, of course...

Essentially it's the same as series and parallel resistors, so for two filter stages in series (to achieve the same rated pressure drop) you need double the rated size for each of the stages, therefore 4x the total filter area and size. In practice, nobody really wants to install that in their basement.

Some of the Chinese positive pressure systems have seemingly the ultimate low-consumables design: a washable stainless prefilter, washable electrostatic filter, two stages (supports cycling) of HEPA filter, and last a refillable granular activated carbon stage. Spent activated carbon could be used as a soil amendment, or returned to a local facility for regeneration into new activated carbon.

Very low consumables, but very costly up-front.