4 ms·

Get a $20 box fan and slap a house intake HEPA filter replacement on the back of it. It will do just as well if not better than an expensive HEPA filter. If it

by jjeaff 5y ago

Get a $20 box fan and slap a house intake HEPA filter replacement on the back of it. It will do just as well if not better than an expensive HEPA filter. If it makes a difference and you want something that sounds different or has some extra features, then you could get a nicer one.

chris_va 5y ago
Probably want an activated carbon layer in there too, but that's only another $20.
bcrosby95 5y ago
This is what we did after wildfires. Way cheaper than buying a truckload of purifiers. And we could actually buy them - purifiers were completely sold out.
sonofhans 5y ago
You’re right that it’s better than nothing, but it’s a far cry from “better than an expensive HEPA filter.” The best efficacy I’ve seen claimed in rigorous testing is 87%, compared with well over 99% for a good HEPA filter: https://www.nytimes.com/wirecutter/reviews/best-air-purifier/#what-if-you-just-put-a-furnace-filter-on-a-box-fan https://www.nytimes.com/wirecutter/reviews/best-air-purifier...
- gandalfgreybeer 5y ago
  Have little knowledge on this field so this might be a dumb question: How noticable or significant is that 12% difference? On the same thought, if I had one that had a 95% efficacy, how does it compare to the 87% one and the 99% one in terms of day-to-day or long term effects?
  - sonofhans 5y ago
    I’ve spent a good amount of time in this space, mostly WRT cleaning air in wood shops. One of the problems is that the 12% difference there is mostly the smallest particles, and those are the most harmful. It’s hugely noticeable with smoke, for instance. Humans don’t have a great sense of smell, but we do very well with smoke. The difference between air with <1% leftover smoke and air with 13% leftover smoke is obvious to everyone. The EPA has a lot to say about this, if you’ve the patience to wade through it — https://www.epa.gov/indoor-air-quality-iaq/air-cleaners-and-air-filters-home https://www.epa.gov/indoor-air-quality-iaq/air-cleaners-and-...
    - blagie 5y ago
      What were you using? The filter should be MERV14 or higher. It will remove >75% of all small particles too. You shouldn't end up with 13% leftover, if you're doing it right. Air will pass through the filter multiple times, and it removes >75% each time. Below MERV14, you'll have that problem. Above, it should do as well as HEPA. The gap between 13% and 0% will be made up for with a 13% stronger fan.
- blagie 5y ago
  It's the wrong metric. The question isn't how much stuff it lets through, but how much stuff it removes. A filter with 99% efficacy at a flow rate of 30cfm will remove the same amount of stuff as a filter with 50% efficacy at 60cfm. A 99% sealed HEPA system isn't cleaning the air that's not passing through it right now, which is most of the air in the room. It reduces the pollution by some nominal rate. After it's been running for a while, the air is a lot cleaner. The major problem with box fan with filter are noise, aesthetics, and similar. Box fans aren't designed for for high loads, and just don't work great in that context. It will clean the air just fine, but a proper filter will do so much more efficiently and pleasantly. In terms of effect: I have allergies. I feel a lot better post-COVID, with HEPA filters, face masks, and similar precautions everywhere. I can't pin that down to any root cause, though. Particulate pollution? Allergens? HEPA filters at home? Masking outside. Dunno.
Johnny555 5y ago
I tried that, the box fan had pretty anemic airflow through the filter. I'm sure it was doing something, especially if it was blowing directly at me, it didn't seem like it was really cleaning the whole room. Then I got a $200 air purifier with a HEPA filter and it has much better airflow, thus more air exchanges per hour. I think fan blades aren't that efficient at pushing against the high flow resistance of a HEPA filter, while the squirrel cage blower in a dedicated air purifier is.
- pacman2 5y ago
  Doubt it. A self build filter basically outperforms all bought systems in tests. The answer is easy: A self build system blows the air through a filter. Nearly all commercial systems suck the air trough a filter. Big difference.
  - Johnny555 5y ago
    What’s the difference between blowing or sucking air through a filter?
    - strogonoff 5y ago
      The Particle Counting folks[0] blew it through, and they seem to have done their research. I’m sure they wrote about it in their blog somewhere. [0] https://particlecounting.tumblr.com https://particlecounting.tumblr.com
      - Johnny555 5y ago
        Hmm...that doesn't really answer the question and since so many high ranking purifiers (plus many industrial HEPA filters) use the same "squirrel cage fan to suck air through the filter" design, I don't believe it's true that there's a big advantage to blowing versus sucking. 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.

by jjeaff 5y ago

chris_va 5y ago

Probably want an activated carbon layer in there too, but that's only another $20.

bcrosby95 5y ago

This is what we did after wildfires. Way cheaper than buying a truckload of purifiers. And we could actually buy them - purifiers were completely sold out.

sonofhans 5y ago

You’re right that it’s better than nothing, but it’s a far cry from “better than an expensive HEPA filter.” The best efficacy I’ve seen claimed in rigorous testing is 87%, compared with well over 99% for a good HEPA filter: https://www.nytimes.com/wirecutter/reviews/best-air-purifier/#what-if-you-just-put-a-furnace-filter-on-a-box-fan https://www.nytimes.com/wirecutter/reviews/best-air-purifier...

gandalfgreybeer 5y ago
Have little knowledge on this field so this might be a dumb question: How noticable or significant is that 12% difference? On the same thought, if I had one that had a 95% efficacy, how does it compare to the 87% one and the 99% one in terms of day-to-day or long term effects?
- sonofhans 5y ago
  I’ve spent a good amount of time in this space, mostly WRT cleaning air in wood shops. One of the problems is that the 12% difference there is mostly the smallest particles, and those are the most harmful. It’s hugely noticeable with smoke, for instance. Humans don’t have a great sense of smell, but we do very well with smoke. The difference between air with <1% leftover smoke and air with 13% leftover smoke is obvious to everyone. The EPA has a lot to say about this, if you’ve the patience to wade through it — https://www.epa.gov/indoor-air-quality-iaq/air-cleaners-and-air-filters-home https://www.epa.gov/indoor-air-quality-iaq/air-cleaners-and-...
  - blagie 5y ago
    What were you using? The filter should be MERV14 or higher. It will remove >75% of all small particles too. You shouldn't end up with 13% leftover, if you're doing it right. Air will pass through the filter multiple times, and it removes >75% each time. Below MERV14, you'll have that problem. Above, it should do as well as HEPA. The gap between 13% and 0% will be made up for with a 13% stronger fan.
blagie 5y ago
It's the wrong metric. The question isn't how much stuff it lets through, but how much stuff it removes. A filter with 99% efficacy at a flow rate of 30cfm will remove the same amount of stuff as a filter with 50% efficacy at 60cfm. A 99% sealed HEPA system isn't cleaning the air that's not passing through it right now, which is most of the air in the room. It reduces the pollution by some nominal rate. After it's been running for a while, the air is a lot cleaner. The major problem with box fan with filter are noise, aesthetics, and similar. Box fans aren't designed for for high loads, and just don't work great in that context. It will clean the air just fine, but a proper filter will do so much more efficiently and pleasantly. In terms of effect: I have allergies. I feel a lot better post-COVID, with HEPA filters, face masks, and similar precautions everywhere. I can't pin that down to any root cause, though. Particulate pollution? Allergens? HEPA filters at home? Masking outside. Dunno.

Johnny555 5y ago

I tried that, the box fan had pretty anemic airflow through the filter. I'm sure it was doing something, especially if it was blowing directly at me, it didn't seem like it was really cleaning the whole room. Then I got a $200 air purifier with a HEPA filter and it has much better airflow, thus more air exchanges per hour. I think fan blades aren't that efficient at pushing against the high flow resistance of a HEPA filter, while the squirrel cage blower in a dedicated air purifier is.

pacman2 5y ago
Doubt it. A self build filter basically outperforms all bought systems in tests. The answer is easy: A self build system blows the air through a filter. Nearly all commercial systems suck the air trough a filter. Big difference.
- Johnny555 5y ago
  What’s the difference between blowing or sucking air through a filter?
  - strogonoff 5y ago
    The Particle Counting folks[0] blew it through, and they seem to have done their research. I’m sure they wrote about it in their blog somewhere. [0] https://particlecounting.tumblr.com https://particlecounting.tumblr.com
    - Johnny555 5y ago
      Hmm...that doesn't really answer the question and since so many high ranking purifiers (plus many industrial HEPA filters) use the same "squirrel cage fan to suck air through the filter" design, I don't believe it's true that there's a big advantage to blowing versus sucking. 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.