Air filtration question

"Dr. Zoidberg - RIP Oppo" (thetomselleck) 09/11/2020 at 02:15 • Filed to: Doggo is displeased with lack of walks

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Apparently they’re recommending we filter the air in our homes with a furnace filter strapped to the back of a 20" box fan due to the terrible air quality. As you might imagine, most are gone already. Or the ones of irregular size with a higher MERV rating are 40 bucks apiece.

I found these above that are still in stock with a MERV rating of 5 apiece. Can I... stack them to get closer to the recommended MERV rating of 11-14? Or is even that a waste of time/money? I have to believe that SOMETHING is better than nothing, right?

If only EssExTee could be so grossly incandescent > Dr. Zoidberg - RIP Oppo 09/11/2020 at 02:21

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Just glue a bunch of N95 masks together into a sheet.

415s30 W123TSXWaggoIIIIIIo ( •_•))°) > Dr. Zoidberg - RIP Oppo 09/11/2020 at 02:27

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That’s a good question, it might filter better but would it restrict flow enough to maybe burn the motor. I have two of these with toxin/city smoke HEPA filters.

415s30 W123TSXWaggoIIIIIIo ( •_•))°) > Dr. Zoidberg - RIP Oppo 09/11/2020 at 02:36

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I saw this, they say it’s better.

Dr. Zoidberg - RIP Oppo > 415s30 W123TSXWaggoIIIIIIo ( •_•))°) 09/11/2020 at 02:43

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2 20" filters joined at 90 degrees? Is this for lower MERV ratings?

AestheticsInMotion > Dr. Zoidberg - RIP Oppo 09/11/2020 at 02:45

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I can give you 5  inches

415s30 W123TSXWaggoIIIIIIo ( •_•))°) > Dr. Zoidberg - RIP Oppo 09/11/2020 at 02:47

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I was looking at this stuff before when the fires started. I’m not sure what is true. I do see this wood set up for sale but you could do a cheap copy. I would think you have to use a couple lower rated filters to do that. I saw a comment that it would probably lower the life of the motor but the fans aren’t that expensive. I had a Levoit so I got another for the other room and changed the filter type. 

Dr. Zoidberg - RIP Oppo > AestheticsInMotion 09/11/2020 at 02:49

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Are we still taking about filters?

TheRealBicycleBuck > Dr. Zoidberg - RIP Oppo 09/11/2020 at 06:11

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The MERV rating is related to the porosity. The higher the rating, the tighter the weave. Stacking lower rated filters will slow down the air, but it doesn’t change the pore size. Small stuff will still get through, but not as much of it and not as quickly. 

SiennaMan > Dr. Zoidberg - RIP Oppo 09/11/2020 at 06:27

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My understanding is the triangle setup is so that you maintain a higher level of air flow through the fan so that you don't burnout the motor..

Aremmes > 415s30 W123TSXWaggoIIIIIIo ( •_•))°) 09/11/2020 at 07:27

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That’s exactly ho w the filters in my HVAC air handler are set up, same filter size and everything, albeit with less cardboard and tape and more sheet metal.

TheRealBicycleBuck > Dr. Zoidberg - RIP Oppo 09/11/2020 at 07:33

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I thought of a better way to describe it. Think of it like a giant game of Plinko. Stacking two filters on top of each other is the same as doubling the height of the Plinko board. The same size particles can get through, it just takes longer. Moving to a filter with a higher rating would be like adding more nails into the Plinko board, making it harder for the puck to find a way through the maze.

Putting two filters at a 90° is the same as putting two Plinko boards side by side. There are more paths to choose, so airflow isn’t as restricted, but the filtration level is still the same.

This isn’t a perfect analogy because f iltration isn’t purely mechanical. Some of the particles in the air can stick to the fibers in the filter. Slowing the airflow down improves filtration by giving more opportunities for the particles to get hung up or stick to the filter. Also, h aving more surface area gives the filter more opportunity to catch particles while also increasing the total capacity of the filter.

Finally, a dirty filter is more efficient at catching particles, but the total airflow is reduced. That can reduce the effectiveness of the filter because it isn’t processing as much air and it’s harder on the motor because the airflow keeps the motor cool.

SilentButNotReallyDeadly...killed by G/O Media > Dr. Zoidberg - RIP Oppo 09/11/2020 at 08:20

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Just how well sealed is your house anyway? If it doesn’t get rock solid c old in winter or steaming hot in summer then the answer is probably ‘quite good’. If so then ...what is the problem?

RacinBob > Dr. Zoidberg - RIP Oppo 09/11/2020 at 08:46

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I wonder how much air a prop fan will pull through a good filter. They stall at very little static. If you do that, maybe put a low merv filter in front to catch the thick stuff.

I wonder if blowing on a wet towel might also knock down fine particulate.....

Snuze: Needs another Swede > TheRealBicycleBuck 09/11/2020 at 08:57

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The MERV rating i s based on capturing particles in a specific size range - 0. 3 to 10 microns. So a low MERV rating still captures some of those particles, just not as many. So I would think stacking filters would provide some benefit, if the particle isn’t trapped in the first filter, it may be trapped by the second one. But I don’t know if stacking two MERV 5 filters equals MERV 10.

The problem with stacking is it increases pressure drop, which is hard on the fan motor. As for the 2 filters at an angle solution, that lowers the pressure drop AND slows down the air, giving the filter more time to catch particles. Im sure the ASHRAE MERV test standard is based on some airflow quantity over a given filter area, so slowing flow down * may* give a higher equivalent MERV rating for a given filter type since it could now trap more particles. But just like stacking filters, its hard to know what the exact relationship, i.e. 2 MERV 5 filters in parallel may not equal MERV 10.

I guess I could try to track down the MERV test methodology.  I wonder if ASHRAE publishes it, or if its behind a paywall.  I let my membership lapse and don't have any of the books.  We used to have all of them at my old job.  

Snuze: Needs another Swede > Dr. Zoidberg - RIP Oppo 09/11/2020 at 08:58

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See my long reply to BicycleBuck below... stacking filters OR putting them in parallel should increase their effective MERV value, but it may not be a linear relationship, i.e. 2 MERV 5 filters may not equal MERV 10.  

MasterMario - Keeper of the V8s > Snuze: Needs another Swede 09/11/2020 at 09:00

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Yea, I’m sure two MERV 5 filters in parallel gives a better rating than 5, but like you said it definitely isn’t additive to a rating of 10. I would bet two MERV 5 filters together are still closer to 5 than to 10.

Snuze: Needs another Swede > MasterMario - Keeper of the V8s 09/11/2020 at 09:11

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Based on my reading, you are probably correct.

Understanding MERV Testing

One of the big takeaways from that article is MERV ratings are pretty much all based on empirical testing at specific airflows.  So we could answer this question... if we had a test apparatus.