[HN Gopher] Ceiling Air Purifier
___________________________________________________________________
Ceiling Air Purifier
Author : pavel_lishin
Score : 222 points
Date : 2022-06-02 02:57 UTC (1 days ago)
(HTM) web link (www.jefftk.com)
(TXT) w3m dump (www.jefftk.com)
| mgraczyk wrote:
| "people often select purifiers based on their maximum flow"
|
| I don't know if that's really true. Looking at the top 20 or so
| Air Purifiers on Amazon, almost none of them mention the CADR or
| flow in the title, and very few of the reviews mention it. Most
| talk about sound, "HEPA", "ozone-free" etc.
|
| My guess would be that people care much more about physical
| appearance than maximum flow. I would personally not tape an
| octagon of dirty air filters to my ceiling because that would be
| look ridiculous and probably not filter as well as my beautiful,
| silent $200 Winix box.
| steve76 wrote:
| maxk42 wrote:
| Is nobody going to mention what a pain in the ass it would be to
| change all those filters regularly? I love in a somewhat dry and
| dusty area and need to change my filters (near the ground) every
| two to three months. I'd hate to have to climb up a ladder to
| change eight to nine filters that often.
| jefftk wrote:
| Unless your ceiling is very high you probably wouldn't need a
| ladder to change the filters on a productized version of this.
| Reach up, twist something to unlock, old filter slides out,
| slide in new filter, twist to lock. Repeat for each filter
| going around. Ten minutes every six months?
| mxfh wrote:
| Not to forget, you'll have a looming black octagon after 6
| weeks or so already depending on your environment. At least a
| reminder, that the filters are doing something useful.
| aarongray wrote:
| I love the enthusiasm and DIY ingenuity here. That said, the post
| is tagged with COVID-19. The COVID virus particles are .1 to .5
| microns in size, and these MERV-14 filters, while certainly
| better than nothing, are not going to capture a significant
| amount of these virus particles. A better approach is to not
| filter the particles, but actually rip them apart at a molecular
| level. This has the added benefit of destroying all sorts of
| other contaminents that even high grade HEPA filters will miss,
| such as mold mycotoxins. A system like the Molekule is a good
| example of this approach.
|
| https://molekule.com/technology
| picture wrote:
| Is this legit? Their website feels kinda sketchy to me, with
| the huge and somewhat tacky CG pictures, sweeping claims,
| apparent SEM photography that just looks too good to be true
| (could there really be _zero_ residue? and the background never
| change, presumably over time span of at least minutes!) This
| really reminds me of vaporware like WaterSeer, or one of the
| recent "turn kitchen garbage into dry dust" appliances that
| I've seen ads for
| fossuser wrote:
| Your intuition is correct - it's bullshit.
|
| --
|
| In the summer of 2019, we purchased a Molekule Air (the
| flagship model) and tested it. We bought an Air Mini that
| fall and tested it in February 2020. At the time we tested
| the Molekule Air, the company claimed that its
| "scientifically-proven nanotechnology outperforms HEPA
| filters in every category of pollutant."
|
| Our tests proved otherwise. And by mid-2020, that language
| had been withdrawn, after many of the company's claims were
| ruled against in a case before the National Advertising
| Division and upheld in a later appeal before the National
| Advertising Review Board. The Molekule Air turned in the
| worst performance on particulates of any purifier, of any
| size, of any price, that we have tested in the eight years
| that we have been producing this guide. The Air Mini
| outperformed it, but that's not saying much: It still
| produced the second-worst performance we've ever seen.
|
| Guide author Tim Heffernan asked Molekule CEO Dilip Goswami
| why the language was removed. He answered, "The point about
| 'in all categories' is that we see a device that outperforms
| across all of the categories. Right? So we're not trying to
| say that individually, on any particular metric, we would be
| number one. Right? What we're saying is, when you look across
| all the categories, we outperform HEPA. Right? And that's
| what we're attempting to convey with that. And so--it's fair
| to say that we needed to re-examine some of the language to
| make sure that it's saying what we're intending to say."
|
| https://www.nytimes.com/wirecutter/reviews/best-air-
| purifier...
| aarongray wrote:
| Thanks for sharing some background on Molekule, I did not
| know about this.
| aarongray wrote:
| That said though, the Molekule team themselves have said
| that their device is not optimized for filtering
| particulate matter, but rather for denaturing mold,
| viruses, and bacteria. See: https://www.reddit.com/r/mole
| kule/comments/6u7d5y/putting_mo...
|
| This is reflected as well in the testing on Molekule's
| own website: https://molekule.cdn.prismic.io/molekule/4ec
| 92005-d806-4991-...
|
| And, I will say that they have numerous tests by
| independent labs. They did do some testing with their own
| lab and with a lab affiliated with them, but this is far
| from all the testing they have done. So to say that all
| their tests were tainted or affiliated with them is
| simply not true. See: https://molekule.com/papers
| fossuser wrote:
| Brownian motion affects how small particles move around at
| small scales [0]. This is why HEPA filters can be effective
| even against smaller particles (which bounce around and get
| caught).
|
| Specifically about the Molekule - their claims seemed to be
| empirically false and the device performed worse than a
| standard HEPA filter (and they were resistant to allowing
| independent tests at all).
|
| When the tests came out awful their responses were mostly
| bullshit. [1]
|
| [0]: https://en.m.wikipedia.org/wiki/Brownian_motion
|
| [1]: https://www.nytimes.com/wirecutter/reviews/best-air-
| purifier...
| blagie wrote:
| Erm, this isn't quite correct:
|
| 1) MERV14 will capture 75%-84% of particles in the 0.3-1m
| range.
|
| 2) A filter which captures 70% of particles with an air flow of
| 100cfm will capture the same amount of virus as a filter which
| captures 100% of particles with an air flow of 70cfm. Both will
| clean the room just as fast. For air filters, lower filtration
| + higher airflow is usually a better design option. Going from
| 70% to 95% to 99.9% means you'll have a more expensive, power-
| hungry, and more noisy product over one which just has a little
| bit more air velocity [1].
|
| 3) The rating is for 0.3m since that's the hardest size to
| capture. A filter will actually capture more particles below
| 0.3m.
|
| 4) COVID19 virus particles are around 0.1-0.2m, but that's
| beside the point. They're travelling on water molecules. Those
| are much bigger.
|
| 5) Even if there were a virus particle somehow floating around,
| a viral load of one virus is very unlikely to get you sick.
|
| From an engineering standpoint, something around MERV14 is
| almost certainly the sweet spot for a COVID19 room air filter.
|
| [1] High-filtrations makes sense in places like vacuums, face
| masks, and other places where the goal is to have clean air
| coming out. Vacuums shouldn't blow up dust. That's a different
| engineering design goal than a room air filter. If you'd like
| to see the impact of loading on a fan, put your hand behind
| one, and hear how much noise goes up. MERV14 has a much lower
| load than HEPA.
| aarongray wrote:
| 1) You're correct.
|
| 2) It will not clean the room just as fast, it will take
| longer to clean the room.
|
| 4) COVID-19 particles do travel on water molecules, but they
| are also airborne. The CDC has admitted this and there is a
| growing body of research proving this to be true as well.
|
| 5) This has not been proven.
| jefftk wrote:
| _> > A filter which captures 70% of particles with an air
| flow of 100cfm will capture the same amount of virus as a
| filter which captures 100% of particles with an air flow of
| 70cfm. Both will clean the room just as fast._
|
| _> It will not clean the room just as fast, it will take
| longer to clean the room._
|
| Here are two different models:
|
| A. Air moves sequentially. First you filter all of the air
| once, then you filter all of it another time etc. In this
| model, a filter with 100% efficacy will get everything in a
| single pass, and the CFM determines how long that pass
| takes, while a filter with lower efficacy will never get it
| all, but will get pretty close after a few passes. In this
| model you want high filtration.
|
| B. Air moves randomly. At each minute, the purifier selects
| air from the room at random, filters it, and spits it back
| out. In this model, a filter with 100% efficacy at 70 CFM
| is exactly equivalent to a filter with 70% efficacy at 100
| CFM, and you will often want to trade off efficacy for
| flow.
|
| I think real rooms are generally much closer to (B) than
| (A), though of course somewhere in the middle?
| blagie wrote:
| You're making statements with no backing, logic, or
| argument behind them.
|
| 2) The percent of material removed by a filter per unit
| time is the product of (1) filter efficiency with (2) what
| percentage of a room's air passes per unit time.
|
| 4) "Airborne" is generally via microscopic droplets. The
| CDC's guidance changed from large droplet transmission
| (which is relatively short-distance and short-time) to
| airborne. This doesn't mean individual viruses are floating
| around without any H20.
|
| 5) No one credible believes 1 virus particle is likely to
| infect you, except by very bad luck. Most citations give
| claims in the 100-1000 particle range. Low initial
| infectious dose also /appears/ to correspond to less
| aggressive infections. This has not been rigorously proven
| (and it's hard to do), but has a strong theoretical basis:
|
| - One virus particle is unlikely to make it past the mucous
| layer, unless you're super-unlucky.
|
| - If it does, your innate immune system can usually handle
| minor infections before they escalate.
|
| - If it can't, your adaptive immune system has more time to
| respond. You're looking at a few days before it kicks in.
| With a lower initial infectious dose, you'll still have
| that much less virus when it kicks in.
|
| If you'd like to contradict any of this, please provide
| citations. I'll read them. I'm glad to be proven wrong.
| Perhaps I'll learn something.
| jefftk wrote:
| MERV-14 filters are rated for 75% efficacy in that range:
| https://www.nafahq.org/understanding-merv-nafa-users-guide-t...
|
| The respiratory particles that transmit covid are a range of
| sizes, but likely mostly a bit larger than that:
| https://www.jefftk.com/p/how-big-are-covid-particles
| mrfusion wrote:
| Off topic but why did tic toc start showing me diy air filter
| videos right after I read this article?
|
| Is there some kind of tracking?
| atmartins wrote:
| Yes. And it's on your home IP address. I see things based on
| what others in my household have done with Tok Tok
| mrfusion wrote:
| So how would the tracking work exactly? Does this website
| have trackers that other companies can access?
|
| Is there any way to block that?
| ev1 wrote:
| yes, this website loads the full adtech parade.
| Havoc wrote:
| I like the lateral thinking. I'm currently using a pretty ghetto
| version as well - air filter is pointed at a open desktop case.
|
| I suspect the ideal version is an adaptation of the US central
| HVAC model. i.e. Push clean air into the various rooms.
|
| ...but hard to retrofit.
| harvie wrote:
| Use static electricity to attract the dust from air, completely
| quiet.
| seanp2k2 wrote:
| Cool idea but very low "wife approval factor". Just do the box
| fan + MERV 13 air filter if you want something decent for <$50.
| mreiner wrote:
| I like your concept, can imagine it to look quite aesthetic.
|
| Consider you need to create a fair pressure gradient between the
| two sides of the filter and it needs to be higher the more the
| filter clogs up. AFAIK radial fans are more efficient for that
| purpose. Downside is the noise.
|
| If you are running the ceiling fan anyway just for the turbulence
| it creates on a hot day, it's probably fine even if just a small
| percentage of the airflow is pulled through the filters, your
| data certainly looks promising.
| hinkley wrote:
| That's why those filters are full of zigzags. The CFM/m2 is
| very small, and so it can capture small particles instead of
| them zipping through. Also takes longer for them to clog up.
| Way longer than the oil change place wants you to think. The
| test of a dirty filter is that it looks dark when backlit, not
| when it is covered with surface lint. When they show your your
| filter, take it out of their hands.
| WesleyJohnson wrote:
| Matthias Wandel's secondary YouTube channel has a lot of recent
| videos about retail air purifiers compared to homemade ones using
| box fans. I'm not up to snuff on the science of it all, but he
| does a lot of cool comparisons on the effectiveness of each.
| Worth a look if you're interested in this sort of thing.
|
| https://www.youtube.com/channel/UC3_AWXcf2K3l9ILVuQe-XwQ/vid...
| polskibus wrote:
| How do you measure PM 2.5 at home? Whats the best and cost
| efficient way to do it?
| mellavora wrote:
| https://www.airgradient.com/diy/
|
| Founder is very supportive, good product, easy to build
|
| The sensor will display PM2.5, CO2, Temperature and Humidity on
| the display and can optionally send the data to any server for
| data logging (e.g. the AirGradient platform or any other cloud
| backend).
|
| Parts: Wemos D1 Mini USD 2.24 Wemos OLED display USD 2.47
| Plantower PMS5003 PM Sensor USD 13.89 Senseair S8 CO2 Sensor
| USD 28.00 SHT30 or SHT31 Temperature and Humidity Sensor Module
| USD 2.55
| ryankrage77 wrote:
| I use a Nova SDS011 sensor connected a raspberry pi, then feed
| the data into HomeAssistant with a python script and MQTT.
| joelnc52 wrote:
| We run one of these (https://www.adafruit.com/product/4632) off
| of an Arduino Uno with a small LCD hat (w/ micro SD for logging
| as well). I think that's the same sensor as in the outdoor
| units that Purple Air sells. ~$150 for the whole setup, but
| could definitely run that off a smaller / cheaper board,
| display only. Going strong for 2+ years at this point.
| jefftk wrote:
| Details: https://www.jefftk.com/p/testing-air-purifiers
|
| I'm using a Temtop M2000 because it was the cheapest option I
| found with per-minute data export:
| https://temtopus.com/collections/temtop-co2-monitor/products...
| nerdbaggy wrote:
| It's not the cheapest but this is what I use
| https://www.airthings.com/view-plus
| Brajeshwar wrote:
| Can we also please add a wi-fi access point to the ceiling fans?
| I always thought that would be awesome.
| jsight wrote:
| That's kind of brilliant, tbh. The biggest downside that I can
| think of is that they tend to be electrically noisy and fairly
| space constrained. It'd have to be really small, but probably
| possible.
| hinkley wrote:
| Might be easier to do on a fan with a light fixture. The part
| that sticks down below the fan is a lot of dead space, and
| farther from the motor.
| coding123 wrote:
| Jeff, is this for covid or more for prepping for the CA fire
| season?
| jefftk wrote:
| Primarily thinking about covid, but would work for either
| sudden_dystopia wrote:
| Very cool. Now what can we do about the eyesore factor?
|
| Does this affect the fans cooling performance? The air blows more
| straight down than diffuse I am assuming due to the shroud?
| lkbm wrote:
| If you push air down, it's going to displace the air below it
| off to the sides, so I don't think it'll make a big difference.
|
| My main concern would be that the filters limit flow, and thus
| it might be more likely to pull air up, causing a tight
| vertical loop (air goes down, turns outwards, and then back up
| while still inside the filter ring).
|
| I don't really have a solid mental model of how air flow works,
| though. Some experimenting with the shape (and some colored
| smoke, perhaps) could be interesting. What if the filters
| tilted inwards to be slightly narrower than the fan's reach? Or
| if they were shorter than the ceiling-to-fan distance and just
| inside? Or does it help to put them further out? (Seems
| unlikely, but would be a good thing to test.)
| sandbx wrote:
| it could be pretty if you light it well
| pfisch wrote:
| The cost of filter replacement would be absolutely insane. Also
| filter replacement would be unpleasant.
| bombcar wrote:
| If you have a whole-home furnace you can often make it run the
| fan even if cool/heat is off - and install a high power filter
| and change it regularly and it'll filter the air reasonably well.
|
| The ecobee has a "minimum fan time per hour" setting - it can
| also help to balance the air temperature.
| post_break wrote:
| My dad worked in AC his whole life. Do not put a constrictive
| filter on your ac. He even recommends the cheap ugly ones as
| they are the least restrictive. The AC is not meant to filter
| air. The only reason it has filters is to keep large particles
| out of the evaporator coils.
| kube-system wrote:
| You don't want to restict the total amount of flow, but with
| more media _area_ you can have a tighter filter media with
| the same (or even better!) total flow.
|
| A thicker filter and/or one with more pleats can filter
| better _and_ be less restrictive at the same time.
|
| https://www.youtube.com/watch?v=RkjRKIRva58&t=454s
| flybrand wrote:
| 100% - lower merv filters, changed out more often, are the
| way to go.
| Johnny555 wrote:
| When I upgraded my HVAC system, the installer recommended an
| AprilAire filter box, I uses a 4" high pleated filter, each
| MERV-13 filter has around 16 pleats, so if you unfolded it,
| it'd be around 10 feet long.
|
| Comes with full warranty from the manufacturer, so I don't
| think it's going to shorten the lifetype of the system.
|
| I wouldn't put a regular flat MERV-13 in an existing HVAC
| system, but the pleated ones have so much more surface area
| that they seem safer.
| discreditable wrote:
| 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.
| itsoktocry wrote:
| > _I 've had HVAC techs tell me not to use those thick
| filters unless your system is designed for it._
|
| Yeah, it will also shorten the lifespan of the motor since it
| has to work harder.
|
| But is the cleaner air trade-off worth it? I think so.
| larrywright wrote:
| 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.
| mr337 wrote:
| I have been told a different take. The amount of airflow
| over the coils is important as the compressor system also
| expects a certain thermal exchange (think air conditioning)
| and thus a certain pressure on the return.
|
| Less air over the coils can inhibit the cooling properties,
| this was conveyed to me as high "head" compression pressure
| that can lead to premature compressor failure in the
| outdoor AC part, the expensive part. This is more important
| as the higher the SEER level, generally newer the system,
| the more sensitive they are to this issue. HVAC tech sold
| me with high head pressure expect failure of compressor in
| a few years.
|
| Also you are running as efficient since the AC has to work
| longer burning electricity.
| malfist wrote:
| I've always heard this, but I don't know that that's true.
| The thing that dies first on an HVAC system isn't the
| motor, it lasts a long damn time, it's the capacitor, or
| contactor or some other fault not really related to the
| motor work load.
|
| Plus, your "motor" isn't an engine, it's not variable
| speed, it only has one "hardness" that it works at. It
| might push less air with the thicker filter, but it's not
| going to ramp up and consume more wattage or anything like
| that.
| foobarian wrote:
| >Plus, your "motor" isn't an engine, it's not variable
| speed, it only has one "hardness" that it works at.
|
| This is incorrect - electric motors draw more current
| when under load. I would suggest researching this or
| playing with some toy electric motors run from a battery
| through a current meter.
| bityard wrote:
| > This is incorrect - electric motors draw more current
| when under load.
|
| Which is a true statement but incomplete and therefore
| misleading. A fan motor is under the highest load when it
| is doing the most work. That is, moving the MOST air. If
| you restrict airflow (e.g. with a dirty filter), the
| motor is under less load and draws LESS current. A lot of
| people get this wrong because it's counter-intuitive on
| the surface of it.
|
| You can test this very easily with a box fan and a kill-
| a-watt. Turn the fan on High in the center of the room
| and read the power. Now move it against the wall. You
| will hear the fan get louder because it is spinning
| faster, because it's doing LESS work. The meter will also
| show less power being drawn.
|
| It is true that a dirty filter restricts airflow through
| the whole HVAC system. This extends the system's "on"
| cycle, which reduces the whole system's efficiency. But
| the popular claim that a dirty filter will burn out the
| motor is bunk.
| creaturemachine wrote:
| That's true, but it's not so simple anymore. Furnace
| blowers are all electrically commutated brushless DC
| motors now, and will ramp to keep a constant torque.
| malfist wrote:
| I have a degree in electrical engineering, thank you for
| telling me to do my research, I spent four years doing
| that.
|
| Sure, some motors have characteristics that can be
| tweaked to run at different work loads either with PWM,
| or allowing them to pull more amps to drive higher loads.
| Hell, you can even overvolt them and make them actually
| work harder.
|
| That's not how HVACs work though. Have you ever opened
| one up and did a repair? The circuitry is dead simple,
| there's no current limiting setup or PWM to control how
| much the motor is pulling or spinning, there's no CFM
| measuring device to give the motor more volts or a higher
| amp limit.
|
| The motor is simply pushing air. Air isn't something like
| a solid load where a motor might lift something or move a
| lever or gear, it's fluid. The motor is going to run at
| max and be done.
| bombcar wrote:
| Modern furnaces have variable-speed fans - Blower Motor
| Variable-speed constant airflow full-featured ECM
|
| https://www.bryant.com/en/us/products/gas-furnaces/987m/
| anarticle wrote:
| The vast VAST majority of hvac systems are still bang
| bang systems.
| [deleted]
| foobarian wrote:
| > I have a degree in electrical engineering, thank you
| for telling me to do my research, I spent four years
| doing that.
|
| Apologies for the tone!
|
| > Have you ever opened one up and did a repair?
|
| Well funny you should ask, but yes! Two times. One time,
| a power relay on the control module shorted and blew a
| hole in the circuit board. Had to replace the control
| module there. Another time, the start capacitor needed to
| be replaced on the giant 2KW squirrel cage motor. I
| removed the blower, disassembled it, lubricated moving
| parts, and of course tested it out on a stand. That much
| blowing power is quite impressive when right next to your
| face.
| devenson wrote:
| Even a motor without a fancy control system will consume
| more power under load. See:
|
| https://en.m.wikipedia.org/wiki/Counter-
| electromotive_force
| malfist wrote:
| I'm not saying a motor turned off will consume the same
| power as a motor turned on, that's foolish.
|
| Back pressure doesn't impact the load of a fan motor, at
| least not as the scale we see in HVACs, we're not making
| vacuum chambers.
| foobarian wrote:
| Maybe our disconnect is that we're talking about two
| different things, one being the rated design point of the
| blower system, which is presumably constant (though newer
| systems seem to be more sophisticated), and the other
| being the exact load and power consumption that depends
| on backpressure. A higher backpressure will certainly
| increase the motor power consumption on a classic
| induction motor - if not by 100% then by some non-zero
| amount.
|
| Anyway at the end of the day I agree with the original
| characterization of wear and tear - motor and blower will
| probably outlast most other components. In the case of
| the one I mentioned this seemed to be true. You just may
| end up paying a little more for the electricity to run
| it.
| bombcar wrote:
| It's usually a misunderstanding of how furnaces "work" -
| the blower motor does its thing and doesn't really "spin
| up or spin down" depending on the load - it just moves
| more or less air.
|
| The problem is (especially with older furnaces; modern
| ones have safety features to prevent this) that if you
| have too much back-pressure you don't get enough airflow
| over the heat exchanger (or air conditioner coils) and it
| can crack - allowing dangerous exhaust gasses into the
| airflow (or freezing the coils for the AC which isn't as
| bad).
|
| Once the heat exchanger is gone the furnace is basically
| trash and has to be replaced (you can replace the heat
| exchanger but it's rarely worth it).
| danans wrote:
| > The problem is (especially with older furnaces; modern
| ones have safety features to prevent this) that if you
| have too much back-pressure
|
| An everyday way in which people induce this scenario is
| by shutting the heating vents in particular rooms because
| they get too hot. Even on the newer furnaces, this
| results in the automatic controls shutting off the burner
| when the pressure and temperature gets too high, and then
| the fan starts pushing around cold air.
|
| A secondary negative effect of this is that it
| pressurizes the ducts causes them to leak more, resulting
| in reduced efficiency, and also quicker failure.
|
| A well designed system which has been configured to
| deliver the correct amount of heat to each room doesn't
| experience the same issues. Unfortunately, most older
| homes and even newer production built homes have poorly
| designed HVAC systems.
|
| Heat pumps don't experience as many of these issues
| because they just don't get air as hot, and instead rely
| on higher throughput of lower temperature air to heat
| spaces, but that makes them far more reliant on good duct
| system design.
| danans wrote:
| > Plus, your "motor" isn't an engine, it's not variable
| speed, it only has one "hardness" that it works at.
|
| Many newer AC motors are variable speed or at least
| multi-speed.
| [deleted]
| bombcar wrote:
| You can have them do a test and adjust the system depending
| on the filter that's installed to make sure it's moving
| enough air (though that is mainly important when heating to
| prevent the heating element from overheating).
| dr_orpheus wrote:
| It is also important in the cooling cycle for two reasons.
|
| 1. If there is not enough air moving over the air-
| conditioner coils there is the possibility that they will
| ice over. This has a cascading effect where there is less
| transfer through the coils and more ice builds up until the
| whole thing is a block of ice. This most often happens
| though when people buy condensers that are too big for
| their system (the thinking being the bigger the better the
| more cooling, but it is a balance within some tolerance)
| but can also be done on the other side by reducing airflow
| too much.
|
| 2. Less airflow can also mean too much condensation in the
| air which can end up sitting in the ductwork and causing it
| to rust out. This happened to my parents when they
| installed extra filters in their HVAC system. I guess this
| is similar cause as #1, just different potential effects.
| SigmundA wrote:
| A good system like an AprilAire [1] will have a 4" thick
| filter vs the standard 1" thick filters. This allows much
| more filter area than a standard filter allowing proper
| airflow even with a finer more restrictive filter medium.
|
| You can't just swap filters, the 4" thick one usually go in
| right before HVAC air handler intake rather than at returns.
| I have had an AprilAire installed for over 10 years in my
| HVAC when it was replaced last time, recommended by HVAC
| installer, its has worked very well and I do have my blower
| fan cycle constantly regardless of need for heat/cool to turn
| over and filter air.
|
| 1. https://www.aprilaire.com/whole-house-products/air-
| purifiers...
| sha256sum wrote:
| Great idea. What's worked for me is simply taking a 20"x20"x1"
| filter and sliding it into the back cover of a standard $25 Lasko
| box fan. Cheap, easy option for air filtration in the home.
| sudden_dystopia wrote:
| Can confirm, I have made like a dozen of these at this point.
| Works great but is loud. Good white noise at night though.
| sfteus wrote:
| I did this for a cheap dust filter when demoing a ~7 cu ft
| concrete shower pan in my house. Pretty incredible how much
| material it pulled out of the air; I ended up running through
| 3-4 filters total throughout the whole process.
| euroclydon wrote:
| I prefer to mount a high-power fan in the bathroom window for
| demolition.
| fady wrote:
| Reminds me of this video that came from University of Michigan
| during one of California's fire season when many fires were
| ablaze and air quality was terrible:
|
| https://www.youtube.com/watch?v=kH5APw_SLUU
| TedDoesntTalk wrote:
| That's a great idea. What kind of filter do you use?
| TurkishPoptart wrote:
| Is that enough for allergens, smoke, and pet dander? I don't
| care about virus filtration (which is not going to happen
| anyway, if it's something I'm making out of cardboard and
| duct-tape)
| tgsovlerkhgsel wrote:
| You're not getting a "guaranteed certified safe"
| environment with cardboard and duct tape, but there is no
| reason why you shouldn't be able to get a significant risk
| reduction.
|
| You're trying to reduce the viral load in the air, not
| build a BSL4 lab.
| jefftk wrote:
| If you're doing one filter per box fan, I'd go with a
| MERV-14: https://www.jefftk.com/p/merv-filters-for-covid
| formerkrogemp wrote:
| This is very similar to what hobbyist mushroom growers use
| in our setups to inoculate substrate in a 'sterile'
| environment. Other methods include using sterile, hot oven
| air, buying very expensive equipment, and making a still
| air box at home with a plastic tote box. Good times.
| titanomachy wrote:
| In college we used to work under a bunsen flame, is that
| not enough space?
| formerkrogemp wrote:
| I think a lot of those options are born out of
| creativity. Inoculation typically happens from jar to jar
| or from a jar to a block of substrate for instance. A
| Bunsen burner would probably work well for many
| applications in mycology. I only ever used one in cell
| bio, and only a mini Bunsen in my mushroom hobby.
| newsclues wrote:
| Standard furnace filters, pick the level of filtration you
| desire/can afford.
| bombcar wrote:
| I've had surprisingly good luck in my area finding brand-
| new furnace filters at Goodwill for quite cheap. HEPA, too.
| If you're making your own setup the size doesn't matter as
| much so that can be a good way to go.
| tiahura wrote:
| https://www.uofmhealth.org/media/19281
| totetsu wrote:
| Some really good discussion of this has happened on HN last
| time California was on fire.
| seanp2k2 wrote:
| Given the wildfire risk for this summer[1], I'd say this
| article is quite timely.
|
| 1. https://www.predictiveservices.nifc.gov/outlooks/monthly
| _sea...
| hinkley wrote:
| Tends to burn out the motor though.
|
| Vornado used to sell an air filter that took two 10x20" house
| filters. Before I discovered that model I was trying to build
| my own using computer case fans (my intent was to hide the loud
| box under furniture).
|
| Home Depot used to carry those in the 1500 classification which
| was great, but those are getting harder to come by too, and
| even the regional chain that had them doesn't as much anymore
| and I have to use the 1200 or once in a while the super basic
| ones. Or buy them from Amazon and fuck Amazon.
|
| Air filters became a rent seeking gambit before the term rent
| seeking was even invented. We should just be able to use
| furnace filters. And speaking of furnace filters, if you have
| central air, do yourself a favor and change the filter
| religiously. Also helps slow down accumulation of dust in your
| ductwork, which makes it hard to ever get the house clean,
| though you can hire someone to come out and vacuum them out.
| They have something like a softer version of a chimney sweep's
| brush with a longer handle. Last place we had them do that on
| like day 2 to avoid any problems from the previous owners.
| wang_li wrote:
| The EPA disagrees with you regarding duct cleaning. It makes
| sense since anything that is sitting in your duct can't be
| removed by the airflow your hvac generates. If it could be
| then it wouldn't be in there in the first place.
|
| https://www.epa.gov/indoor-air-quality-iaq/should-you-
| have-a...
| bredren wrote:
| > Tends to burn out the motor though.
|
| Do you have data on this? Adding a fan shroud to the Corsi-R
| supposedly does not increase energy usage or motor
| temperature:
|
| https://www.texairfilters.com/how-to-improve-the-
| efficiency-...
|
| I'm looking for data on the power usage and motor temperature
| difference from a 20" Hurricane in normal operation, with a
| 20" merv 14 on the back, a C-R and a Comparetto.
| hinkley wrote:
| Same author https://www.texairfilters.com/a-variation-on-
| the-box-fan-wit... chose 4 filters due to higher
| throughput.
|
| He has an addendum at the bottom where they tested the fans
| and saw no problems with a single filter. I... don't
| understand how they come to those conclusions based on the
| charts in the paper. All of the fans were >10oC hotter and
| one model went up by 20. 15C is a lot for cheap, planned
| obsolescence consumer electronics.
| kolencherry wrote:
| Yes! This is the basis for the Corsi-Rosenthal Box [1]. Five
| 20"x20"x1" filters and a box fan, with duct tape to seal the
| edges. Not the prettiest, but it's effective.
|
| [1] https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box
| TurkishPoptart wrote:
| How is this significantly better than the parent comment of
| attaching one filter to a box fan?
| alexpw wrote:
| > _An updated design, also known as a Comparetto Cube, [1]
| uses four filters and a cardboard base that can sit directly
| on the floor._
|
| I never understood the purpose of the bottom filter in the
| Corsi-Rosenthal Box, so it felt like a waste of a filter, but
| I'm a big fan of the Comparetto Cube (no put intended). And 4
| packs of filters are readily available -- 20x20x2 is
| suggested for the Comparetto Cube.
|
| [edit] Also, thank you. I couldn't recall the Corsi-Rosenthal
| name/link, but thought of the wiki immediately.
|
| [1] https://www.thisoldhouse.com/green-home/22231148/diy-air-
| fil...
| happyopossum wrote:
| > I never understood the purpose of the bottom filter in
| the Corsi-Rosenthal Box
|
| It's for when the box fan is not sitting on the floor -
| either raised up on feet (which many have), sitting in a
| window, or hung somewhere. That said, you're right - 4
| packs are easier to come by and probably work nearly as
| well in the real world.
| KennyBlanken wrote:
| It's easy to add "legs" to the C-R device - a few dollars
| at most in wood dowels and no tools, for example.
| Maximizing filter area is really important because flow vs
| static pressure for a fan like this is usually
| logarithmic...and static pressure rapidly rises as the
| filters get used (their efficiency goes up, but flow
| drops.) That's one reason you see a lot of squirrel fans
| used in air filtration units; they can generate much more
| static pressure.
|
| In theory, if you mounted it fan-down and placed some towel
| or blankets underneath, you could also dampen a fair amount
| of the noise coming from the fan.
|
| It's probably more effective and cheaper to get thicker
| filters. A 20x20x5 filter has five times the filter surface
| area of a 20x20x1 filter, but costs $36 - about 2-3x as
| much as a 20x20x1. Two 20x20x5 filters would provide twice
| the filter surface area.
|
| But...these solutions were all intended mostly for
| emergency situations where purpose-built air filtration
| units were in really constrained supply. Folks should
| really just buy a regular air filtration unit that uses
| much less electricity and is quieter, especially if it
| auto-adjusts speed depending upon need.
| jefftk wrote:
| _> Folks should really just buy a regular air filtration
| unit that uses much less electricity and is quieter_
|
| This post is about how one of these made with a ceiling
| fan can be much quieter than a regular air filtration
| unit: "Testing my prototype, it has a CADR of ~180 CFM
| and is only 33dB. By contrast, the Wirecutter's top-
| recommended air purifier has a CADR of 233 CFM at 54 dB
| or 110 CFM at 36 dB. With some tweaks it should be able
| to match the commercial purifier's performance, without
| being louder."
|
| _> especially if it auto-adjusts speed depending upon
| need_
|
| I see how that works for wildfire smoke, but how would it
| work for covid?
| bredren wrote:
| I have been focused on HVAC / air purification recently.
|
| Its amazing to me that these designs are less than two
| years old when the tech has been available for decades.
|
| One of the biggest problems with the C-R and comparetto is
| that they are large and hideous to look at.
|
| I have been working on a design that makes the C-R or
| comparetto or a smaller version than the 20" something that
| can be attractive enough to hang in the corner of a room as
| both an air purifier and room lighting.
| donthellbanme wrote:
| rapunkill wrote:
| Not a new idea[1] I'm surprised it even has a name [1]
| https://www.youtube.com/watch?v=Uzpn09OIyqw
| ark03 wrote:
| My intuition says most airflow comes from the center(and drops
| linearly towards the tip of the blade and picks up at the blade)
|
| i.e. there's no need to build a filter the size of the fan; you
| can cover "most" of the airflow by using something similar but
| extends 50-60% of blade length(pure guess) and the whole
| arrangement can be above the rotating plane (so easier build?)
| leobg wrote:
| Isn't the dust just going to rain down on you as soon as you turn
| the fan off?
| jefftk wrote:
| Mostly no: it's tiny particles that get embedded in the filter.
|
| One way to see this empirically is that pm2.5 levels don't go
| up again after turning off the fan
| blagie wrote:
| I think people are missing the point. YES this is bad engineering
| for a finished product:
|
| - The fan is not designed for high static pressure. It won't do
| well pulling air through filters.
|
| - It's ugly. It will get uglier as filters get dirty.
|
| - It's expensive to replace filters.
|
| - Etc.
|
| But it's not a finished, industrial project. It's a prototype.
| Those sorts of issues are universal for prototypes. They're easy
| to address:
|
| - Switch to a rotor design with higher static pressure
|
| - Provide prefilters. Ideally, there's a nice cover, followed by
| a washable pre-filter, followed by a carbon filter, followed by
| the fancy high-filtration filter.
|
| With an appropriate frame, it could be beautiful -- nicer than a
| normal fan. The filters could actually reduce noise. The frame
| could guide air (much as in a Vornado), increasing airflow.
|
| Good job OP. It's a clever idea and a good design. It's a proof-
| of-concept implementation, but that's the point of a proof-of-
| concept.
|
| (Unless you like the DIY aesthetic, which some do)
| Pxtl wrote:
| If we're looking for a finished product, we're leaving a tool
| on the shelf -- filters aren't the only option. Pipe the air
| through a duct flooded with deadly UV-C and sterilize the air
| instead of just filtering it.
| jryb wrote:
| This also creates ozone though, which is detrimental to human
| health. Review here:
| https://link.springer.com/article/10.1007/s11356-017-9239-3
| hinkley wrote:
| One way to reduce static pressure is to increase the number of
| filters. I saw one design last year, during the fires, that was
| a cube. Lasco fan on one side, and five filters tied into a box
| shape.
|
| A couple problems. One, space - this thing was bigger than a
| dorm fridge. Two, availability - no way you were finding five
| filters during a wildfire. Three, etiquette - if you found five
| filters that meant two other people weren't getting any at all.
| Four, diminishing returns. Two filters reduce the pressure by
| half. Another only reduces it by a third, and so on.
| jefftk wrote:
| _> Two, availability - no way you were finding five filters
| during a wildfire. Three, etiquette - if you found five
| filters that meant two other people weren't getting any at
| all._
|
| Filters last a long time unopened: buy them in advance and
| store them until you need them, rotating stock.
| hinkley wrote:
| Sure, but that's hard to do when reading a tutorial that
| was posted after the sky turned orange.
|
| Edit: To your point though, help out the distribution
| networks by buying yourself an extra filter or two now, and
| put it on the shopping list every time you use one.
| turtlebits wrote:
| This is just an air filter with more filters, the installation
| location is part of the prototype, which is not practical at
| all.
| [deleted]
| russnewcomer wrote:
| I wonder if there is going to be an issue with the structural
| integrity of the filters, since they are designed to rest on an
| object rather than hang? Wouldn't think it would be a big issue,
| but possibly? If you were wanting to do this long term, feels
| like a series of light wood frames to support would not be too
| expensive/difficult to construct or possibly 3d print, and have
| advantage of reduced adhesive smell.
| usefulcat wrote:
| As long as the filters are adequately supported at the edges it
| shouldn't be a problem. I know because my house has two 12x24
| HVAC filters which are only supported at the edges and are
| almost certainly subject to much higher rates of airflow.
| Kadin wrote:
| Most filters (especially the 1" and thicker ones) are
| surprisingly sturdy across most dimensions except for torsion.
|
| Thinking about 3D printing, you could probably print a nice
| corner bit, that would hold the corner of 3 filters together
| and square to each other. That would make the whole assembly
| easier to put together and seal up with tape.
| jaggederest wrote:
| Very cool, if I was going to build something like this de novo I
| would start with a squirrel cage fan. When you oversize them and
| run them at low RPM they can produce a startling amount of
| airflow with virtually no sound at all. You could also bump it up
| to a MERV 16 with enough intake filter area.
| UI_at_80x24 wrote:
| I too am a fan of the squirrel cage fan, and have collected a
| lot over the years. One excellent location to source them for
| 'reuse' are photocopiers. Old analog copiers can have as many
| as 4 fans while newer digital ones seem to have 2-3 depending
| on size/capacity. Mostly DC too, but I have also come across
| some big A/C fans.
|
| They almost always are designed for higher voltages (i.e.
| 20+VDC) so when combined with a common 12VDC power source it's
| already been stepped down.
|
| (source: used to work for a photocopier company, and when
| people bought newer units, we would pickup the old units and
| usually junk them.)
| Kadin wrote:
| Old furnaces nearly always have big-ish squirrel cage blowers
| in them, and many are replaced before the fan motors fail for
| various reasons. In my area they are a reasonably frequent
| sight by the side of the road on bulk garbage day. If you get
| there before the scrap metal guys do, you could easily nab a
| blower.
|
| They're typically 120V single-speed devices, although some
| newer ones may have variable-speed fans. Not 100% sure how
| most of them vary the speed; my bet is that they just have
| some capacitors switched inline with the fan motor to add
| reactance to the circuit and slow it down. (That would
| probably be how I would slow down a single-speed one to make
| it quieter, at any rate.)
|
| There are a lot of projects online that you can find, showing
| how to build fairly big workshop air cleaners with a furnace
| blower and some cheap plywood/MDF/whatever. Basically you
| just build a 5-sided box around the blower with a hole for
| the outgoing air, and then mount a furnace filter on the open
| face of the box.
| tallanvor wrote:
| There doesn't seem to be good control tests to say what
| difference the filters really make - seems one was added in in
| the comments, but it's missing important details (is the control
| with everything off? What happens if the fan is running on medium
| or high, etc.?
|
| And, of course, it's not really practical in that most people
| really don't want to be sticking stuff on their ceiling, but it
| could be an interesting experiment with more details provided.
| SamBam wrote:
| I was thinking the same thing. He has graphs for how quickly
| the pm2.5 levels reduce compared with a commercial machine, but
| not compared with running the fan alone, without filters.
|
| I believe that it is quite possible the majority of the air is
| not going through the filters. It seems that much of the air
| might spill out down the insides of the filters, rather than
| being forced through. In that case, the graph could be equally
| explained by the fan causing outside air to be sucked in.
| sdflhasjd wrote:
| It's not clear how this performs over having no fans and
| filters at all.
| flybrand wrote:
| Agreed - and another wish list item would be a basic room air
| purifier "RAP" off the shelf, sitting in the corner.
|
| OP will also want to track performance over time. How long
| does it take the filters to load.
| jefftk wrote:
| The first chart in the post compares performance against a
| RAP (Coway AP-1512HH Mighty, the Wirecutter's top pick):
| https://www.jefftk.com/mighty-vs-ceiling-decay-big.png
| jefftk wrote:
| _> How long does it take the filters to load_
|
| This is basically the same question as "how long does a
| filter cube last". See
| https://www.texairfilters.com/testing-the-efficiency-of-
| merv... where they found performance was still good at 6m,
| though decreasing at 10m.
| dylan604 wrote:
| What happens when someone flips the switch and reverse the
| rotation of the fan?
| SamBam wrote:
| Ooh, it sucks the air through backward, dislodging the
| particles, and showering the person below with dust.
| 0daystock wrote:
| That looks extremely cheap and ugly, I would be embarrassed to
| have company over. I'll stick to my True HEPA Coways, which you
| can get for roughly the same price as this DIY contraption.
| lkbm wrote:
| High ceilings will help. Next step would be to add paint the
| joints and latticework (or run some colorful tape), and maybe
| add some LEDs.
|
| Black light tape along the latticework, black light at the top
| of the room. Other lighting is all dim lighting in the lower
| half (warm lamps).
|
| I dunno. It'll be hard to make this not look weird unless you
| do something crazy, like a drop ceiling so the fan and filters
| are inset. If the aesthetics are important, I'd put it on the
| home office (out of view from Zoom) rather than the living
| room.
| LinuxBender wrote:
| I going for aesthetics I might just put them in the ceiling
| and have what appears to be low profile vents that one might
| not even notice. That would keep most of the noise in the
| ceiling. One could build an enclosure that the HEPA filter
| unit sits in and routes the intake/output to different
| ceiling vents.
|
| With a little extra effort the filter could be swapped out by
| removing/lowering the vent intake.
| [deleted]
| havblue wrote:
| An ac filter is pretty gray after just a few months. Granted this
| will take longer to get dirty but I suspect it will look ugly
| before you know it.
| SOLAR_FIELDS wrote:
| A good reminder to change it at least. If this were a
| purchasable product, I envision making the filters replaceable
| and the frame compatible with some standard furnace filter
| size, so that you can just run to the store and grab new
| filters when you can visually see the filter needing
| replacement.
|
| Contrast this with the furnace filter, which needs some other
| mechanism (often, a schedule) to determine when to change the
| filter since it's not readily visible on a daily basis.
| sampo wrote:
| I read on the internet, that HEPA filters actually just
| filter better when they start getting clogged. But clogging
| increases air resistance, so you need higher power level in
| the fan to get the same throughput. So the economics
| calculation is, the cost of new filters vs. the cost of
| electricity of running the fan and eventually the cost of a
| new fan. And the comfort factor is more noise when the fan is
| running at higher power level.
|
| So running the filters a long time before replacing, might be
| the cost-saving option.
| DJBunnies wrote:
| Did you see the tape?
| jstream67 wrote:
| Neat idea however air filter changes would be extremely expensive
| as its about 5x the air filters of a conventional purifier -
| which already seem to cost around 50-100 dollars to replace.
| jefftk wrote:
| The filters are MERV-14, not HEPA, so they're much cheaper
| ("materials for one fan are nine MERV-14 filters for $110").
| See https://www.jefftk.com/p/merv-filters-for-covid for why
| MERV-14.
| eli wrote:
| But they'd last 5x as long to move the same amount of overall
| air, right?
| post_break wrote:
| I mean if it works, but it's also the ugliest thing I've seen.
| It's up there with those plastic covers for furniture. A
| standalone air purifier in the corner would be less noticeable
| and ikea even makes a table with a hidden one:
| https://www.ikea.com/us/en/p/starkvind-table-with-air-purifi...
|
| Aesthetics matter. Now if you could somehow make the blades the
| filters, that would be badass.
| flybrand wrote:
| I agree - I love the enthusiasm for IAQ, the data is fantastic
| - but find a conventional white good stand alone room air
| purifier "RAP" and it will be as effective, better designed,
| and likely in the same cost range once changeouts are
| considered.
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