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.
>The larger the surface of the filter the easier time the fan will have to push the air through it and more efficient it will be.
This is quite true. Assuming the media in your particle filter is of sufficient quality, the most important metric to look at is the aggregate surface area. The larger the surface area, the more air you will be able to move through it and the more particulate matter it will hold before requiring replacement.
Particle filters use pleated media for this reason. It's not uncommon to see a filter with 1 square foot face having 50 square feet or more of media. Without specs, the thickness of the filter and the density of the pleating will give you a decent estimate of the filter area. To improve performance and longevity further, it's also typical to have a coarser, pre-filter.
The machine itself is just a fan to push air through the filter and as noted, any reasonably powerful fan will do a sufficiently good job. High quality machines are only really required where you're filtering air before it enters a given area (e.g. LCD/semiconductor plants), and thus cannot afford to have any unfiltered air leak past the filter.
Just run highly rated MERV filters. I use a bit of a bulky MERV 14, but as long as you can cycle the total air volume of the space you're cleaning quickly, anything with 20+% uptake on small particles will work over time.
The experiments and data on that site (I assume it's yours?) don't give me nearly the same conclusions you do - Almost all decent filter setups will hit the same particulate equilibrium asymptote with the only real difference being the duration it takes to get there - a negligible plus or minus half an hour. Speed is not aim. It's the maintenance of the steady-state that is.
If the filter stays on, the air stays clean (barring total capacity issues - you can't filter a huge house with a single box fan).
So the only real roadblocks are human factors that would 1) make people skip making a filtering setup and 2) make people want to turn off the filter. This is where the real UX comes into play.
It’s a trade off though, because good filters will keep dust out of the HVAC systems innards, which according to the techs I’ve talked to makes a difference in longevity.
There are plenty of good products out there that are basically a fan with a HEPA filter and an activated charcoal filter. Even at fairly low airflow it does continously filter the entire room air, and can radically reduce particle counts within half an hour.
Efficiency in terms of airflow goes down, and static pressure difference goes up - but the filters trap more of the particles going through them. Changing filters on furnaces and AC units is really important, since lack of airflow hurts efficiency.
Wirecutter (which is usually trash) confirmed this in testing; versions of units that had been running for months or longer (ie one they bought a year ago and used in someone's home, vs a new unit bought new with little run time) tended to perform better in terms of how many particles they completely removed.
The unit starts using more electricity to do the same work, has to run at higher fan speeds, etc. So it's a tradeoff between that and the cost and waste generated by buying more filters.
This is like theoretically true but mostly useless.
There is filtration beyond even HEPA.
For example, MERV-20 is 99.9999% efficient at 0.1->0.2um particle sizes.
It is 99.99% efficient at particle sizes like 0.01um.
The bigger issue is not whether you can get filters to do it, it's whether your system was built for the pressure drop you'd need to do it.
IE UV's advantage is that it doesn't really drop pressure (but does require some dwell time), not that you can't filter air well enough.
That's not the point though - that part is technically true (EPA filters are 'Efficient Particulate Air' filters and HEPA are 'High Efficiency Particulate Air' filters, and the E and H correspond to those respectively).
The point of the article is that the Wirecutter authors don't understand the physics of air filters and gives the difference more emphasis than what actually matters - it doesn't actually make a massive difference in this particular application. For a purifier that intakes and exhausts in the same space, getting more airflow through the filter per hour can mean over time it's basically the same effectiveness, and using a slightly lower spec filter can be a good design trade-off because it doesn't require as much pressure so it can use less power per unit volume of air filtered.
Of course, in other applications, like bringing air into a cleanroom, it makes a massive difference, but that's not what we're talking about.
It's about power efficiency, not total power draw. Use less powerful fans with an inefficient setup and you're not cleaning your air as well. Engineers put thought into chassis design and motor/fan specs.
For this same reason I decided to get a used box from a reputable, long lasting company with top of the line CADR, and I can find plenty of aftermarket HEPA filters that are compatible.
The quality of the filter is important if you want to remove very fine particles.
Even good filters do not typically remove all particles, only particles above certain size. If you have cheap filter it may not even capture a portion of the particles you are interested to remove, it may remove none no matter how many times you cycle the air.
I think IKEA is on the right track with their lower efficiency filter. For a standalone air filter, you care about the rate of removal of contaminants, which is roughly (concentration of contaminants) · (flow rate) · (filter efficiency), and you probably end up caring the most about the particle size at which the filter is least efficient. So a real HEPA filter has .9995 for that last factor, and .99 is barely worse. Even MERV 13 at 75% or so isn’t so bad.
For a given amount of power consumption, you end up with (concentration of contaminants) · (flow rate) · (filter efficiency) / (power), which is (at fixed contaminant level and a fixed quality of fan) roughly proportional to (filter efficiency) / (pressure drop at design flow rate) and this is where IKEA is making the right tradeoff: those non-HEPA filters have considerably less pressure drop and only fail to filter a percent or so of the air going through.
Your home should be seeing multiple air exchanges per hour. It doesn't much matter if particles are filtered on entry or after, but (paradoxically) an indoor filter which accomplishes multiple passes of air through the filter membrane is likely more effective than an intake filter which only conditions the airstream once.
Most homes or bedrooms should see 5-6 exchanges per hour:
DIY box-fan + furnace filter methods can be effective at reducing indoor particulate levels significantly. Not quite as effective as manufactured HEPA filters, but at a fraction of the cost.
I would assume the majority of people (in the US, anyway) are already filtering their air enough through their central air to make any additional "room-sized" filtering negligible.
Similarly they make some very dense HVAC filters if standard ones aren't catching the level of particles you need, so I'm not sure what the purpose of these filter-fan things are for most people.
Just raise the air exchange per person and or per volume by a significant factor and mandate a very fine particulate filter. You could do many measurements and lots of theory but nobody is going to engineer their buildings like cleanrooms anyway so the results of a lot of science wouldn’t be so good regardless.
If you are taking air, running it once through a filter, and using the air that comes out for an application that needs very few particles, then a 99.99% filter is “10x” as efficient as a 99.9% filter in the sense that the air coming out will have 1/10 as many particles. For example, a 99% efficient face mask is “10x” as efficient as a 90% efficient mask (assuming both fit perfectly, which they don’t, although a PAPR approximates a perfect fit).
But an air purifier doesn’t do this at all. It continuously sucks in air, removes particles from it, and sends the filtered air right back into the room to mix with all the other air. The performance of a 95% filter in this context is barely distinguishable from that of a hypothetical 100% filter. Your characterization would have the 100% air purifier being “infinitely” more efficient.
> A self build system blows the air through a filter. Nearly all commercial systems suck the air trough a filter. Big difference.
But if you’re strapping a filter to a fan, it’s probably more efficient to blow through, since the air will be more focused (plus, it’s easier to mount due to front being flatter).
> I can believe that some home built box fan purifiers out perform some commercial units, but not as a blanket statement that a home made filter outperforms all commercial units.
Agreed. I recall seeing the stats they published and it looks like there are good purifiers among the more expensive ones (US$1000+), but apparently even in that price class they’re hit or miss and don’t always perform as well as a DIY solution could. Generally there’s a lot of information asymmetry, manufacturers want their margins so commercial air purifiers might often use something custom rather than a standard-compliant HEPA filter (either worse or slightly better but 10x more expensive than it should be) and make it difficult or impossible to swap it for something from a trusted third party. They’re counting on very small percentage of customers having properly calibrated particle counting equipment set up at home, they get away with providing misleading tech specs like CADR (or not providing them at all), and so on.
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.
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