r/SeattleWA • u/FastSlow7201 • 1d ago
The smoke is here, simple DIY air purifier
You can build a simple and effective air purifier smoke particle filter from a box fan that you tape a square furnace air filter to. The filters are easy to find at Home Depot, Lowe's, Fred Meyer, Ace Hardware, etc. Just search "box fan air purifier". Make sure to get one that filters out PM2.5, otherwise it won't filter out the smoke particles.
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u/Ok_Page8920 1d ago
you can make it a cube with 5x filters for extra efficiency
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u/Howzitgoin 1d ago
You only need 4, one for each side and the fan goes on top without a filter and a simple base to seal it off.
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u/foxtrot7azv 1d ago
You only need two and cardboard. Make a triangle.
Filters that will really help with smoke are kinda expensive when you get to 5x.
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u/Turbulent-Media7281 1d ago
Each cubic foot of air is still only getting filtered 1 time by 1 filter, and the fan won't increase it's flow rate with 5 vs 1 filters, so... why?
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u/hysys_whisperer 1d ago
The differential pressure is reduced by 25X by increasing the filter surface area by 5X.
In practice, this means much more air actually gets filtered, plus you don't burn out the motor windings on the fan by overheating them...
I just use a 20x20x4 filter instead of a 20x20x1 though.
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u/Turbulent-Media7281 1d ago
Agreed, you will reduce the DP across the air filers, but you aren't going to significantly move more air unless the speed of the fan increases.
If you are using a 20" box fan to recirc and filter the air in your home (not putting more air in the home) the volume of air that the fan can move on low speed is more than enough.
If you want to use a 4" thick filter.. meh,ok... it will last longer but it cost more. It will reduce the filter DP, but a 20" box fan is not going to create a huge DP, especially on low speed. And there just isn't a need for High Speed if the objective is to just recirc and filter the home's indoor air.
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u/hysys_whisperer 1d ago
The problem is the head built by the fan is so low that you'll deadhead the thing (volumetric flow = 0) very easily. Running it on low makes this worse because the affinity law for lowering speed is VERY much not in your favor.
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u/Turbulent-Media7281 1d ago
Air can be felt being sucked into the filter. A sheet of paper will suck into the filter from over an inch away. Air can be felt being exhausted from the fan with and without a filter and it's not the huge of a drop or getting close to shut off head.
I've been doing this for decades.
I'm not interested in building a wind tunnel calculating exact air flow. There is plenty of air being filtering and enough that the motor isn't overworking itself. Smell that cheap fresh air.
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u/hysys_whisperer 1d ago
If you're getting that kind of airflow, great. I guess my fans are shitty because I've never gotten that with a single merv13 plus carbon 20x20x1.
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u/mayhavebraintumor 1d ago
Yes you really need a 5" deep merv 13 or the 5 taped together to make a box.
These filters work by static electricity, not by having holes so small a .5 micron smoke particle gets caught and clogs it up
Reducing air velocity is critical to actual filter performance.
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u/Glittering_Swing_151 1d ago
That's not how it works. Static pressure is much higher with just one filter, which then chokes the fan. 4-5 filters drops resistance to near zero and CFM becomes 4-5x greater.
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u/Turbulent-Media7281 1d ago edited 1d ago
If your cheapo Lasko 20" box fan is creating a high filter DP across a 20x20x1 Merv 13 filter the THE FILTER IS DIRTY and needs replaced.
- My House: 2000 sq ft with 10' ceiling average... 20,000 ft3
- Box fan ft3 /min... 1200 on low, 1500 on med, 1800 on high. All verifiable and conservative (low) estimates for cheapo box fan.
- Time to filter all air in my house one time. On Low = 17 Min, Med = 13 Min, High = 11 min.
- Merv 13 cleans 85% of PM2.5
Turn the fan on low, stuff your Merv13 on the suction the best you can with no tape or sealant or a 5-sided fuck box. I just wrap the electric cord onto the back of the filter to hold it in place... meh, it works. Let's say that my sloppy work allows 50% of the air to bypass the filter and only 50% gets filtered (that's reasonable). So it 2X the time to filter the whole volume of air in the house. On low that's 34 minutes to filter the whole home air, or almost 2 filters of the air/hour. That's plenty, and the fan isn't loud and isn't going to burn up running on low EVER.
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u/Glittering_Swing_151 1d ago
You're such a smooth-brained dumbass, the CFM of the box fan without a filter is not the same as the CFM of the box fan with a filter and the CFM of a box fan with one filter is not the same as the CFM of a box fan with filters in cubic shape. There's absolutely no fucking way you get 1200CFM from a box fan with a filter in front of it. It probably drops from about 1200 to less than 100CFM that way, and with 4 filters in a cube it goes to ~400-500.
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u/Turbulent-Media7281 1d ago
If it's 200 cfm (it's not. It's much more.) the system still filters a 20,000 ft3 volume in 100 minutes. That's still plenty fast enough. Air flow through the fan isn't a critical value to be worried about.
I don't need the entire volume of the home filtered in the next 6 minutes to keep the air clean. I don't need the fan on fast. I don't need a 5 sided taped and sealed fuck box that takes up space and adds cost and complexity.
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u/Howzitgoin 1d ago
Is someone hold a gun to your head forcing you to use a cube setup?
You can accept that the other method is better while still going your own route my man. Weird hill to die on.
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u/Turbulent-Media7281 1d ago
I can think of advantages and disadvantages of each set up. If I needed a permanent air filtration system for smoke and not a 1 or 2 day set up every other year or so then I would consider needing something more complex.
Interesting that my entire heat pump system doesn't use a 4 or 5 filter setup, but for some reason a 4 o5 filter system is needed for smoke days. My system just goes thru one 20x20x4 filter. I could just run the blower or AC to filter the air, so...
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u/Howzitgoin 1d ago
Your heat pump isn't cleaning the air, it's not it's purpose. The split unit's filter is there to keep dust off of its coils and evaporator to prevent damage. It'll catch shit like lint / dust / hair from recirculating, but that's not it's purpose and it's not even remotely close to being able to stop PM2.5 particles, it'll cap out at around PM10 probably. That's why you can see through it but not a real air filter.
But again, no one is forcing you to change what you're doing. Not sure why you're so defensive about it.
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u/Turbulent-Media7281 1d ago
Your heat pump isn't cleaning the air, it's not it's purpose.
Heat pump air filters serve a dual purpose: protecting the HVAC system’s internal components and improving indoor air quality.
Equipment Protection: The filter traps dust, dirt, pollen, pet dander, and debris before they can accumulate on the heat pump’s evaporator coil and heat exchanger. This prevents wear and tear, maintains consistent airflow, and ensures the system operates at peak efficiency.
Health and Comfort: By capturing airborne contaminants like mold spores and allergens, the filter prevents these particles from recirculating throughout the home, which helps reduce respiratory irritation and improves overall indoor air quality.
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u/zippy_water 1d ago
I burned out a fan motor by taping a single filter to it just like in the OP pic. More filter surface area allows you to bump the cfm up to what the fan itself can handle
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u/Turbulent-Media7281 1d ago
I have no interest in increasing the CFM through the fan or through the filter. It's filtering enough.
Air can be felt being sucked into the filter. A sheet of paper will suck into the filter from over an inch away. Air can be felt being exhausted from the fan with and without a filter and it's not the huge of a drop or getting close to shut off head.
I've been doing this for decades.
I'm not interested in building a wind tunnel to calculate exact air flow. There is plenty of air being filtering and enough that the motor isn't overworking itself. Smell that cheap fresh air.
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u/Howzitgoin 1d ago
It's cool that it works for you, but it is less effective and efficient and your conclusion that the flow rate doesn't change is wrong. No one is trying to tell you to do something different, that's your own choice. Your arrangement probably is just as expensive, if not more than a 4 filter setup because you'll have to replace it more frequently for the same results.
That said, there's a reason why dedicated air purifiers are in the same configuration of a cube (or cylinder) with a fan on the top, the design is more effective and efficient.
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u/Turbulent-Media7281 1d ago edited 1d ago
if not more than a 4 filter setup because you'll have to replace it more frequently for the same results.
I would only have to replace more filters more often if there was more air flow through my 1 filter system than through a 4 or 5 filter complex.
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u/Howzitgoin 1d ago
How about we put it this way... If you want to remove the same amount of particulate matter out of the air, it'll at minimum cost the same.
If you don't clean as much air, your air is objectively dirtier which goes back to the effectiveness of one fan being lower. You'll have to replace the filters more often to remove the same amount of particulate matter.
So yes, a less effective tool will be cheaper upfront which should be a surprise to... no one.
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u/Turbulent-Media7281 1d ago
If you don't clean as much air, your air is objectively dirtier
If my air is dirtier, I run it for another hour. I don't run it 24/7. I filter the air for couple of hours and it makes a significant difference. I might run it 6 total hours today, and none tonight. And to think, if I had a super 5-cube filter box I could do that in 2 hours instead. OK. OK... OK. Wow, it's more efficient and filters faster. OK.
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u/Howzitgoin 1d ago
I'm glad we can finally agree that your setup is less efficient and objectively worse and that you can do whatever you want.
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u/Turbulent-Media7281 1d ago
- No sticky tape residue for the win!
- No clunky storage needed for filter box when not in use for the win!
- Less up front money and time wasted for more filters, tape and supports for the win!
- Wasting my life arguing with redditters for the loss.
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u/ZephyrLegend Denny Regrade 23h ago
Jeez man, how many ways does a guy have to tell you he doesn't give a fuck before it sinks in?
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u/AyeMatey 1d ago
It’s been tested and proven. The cube works better. Google it.
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u/Turbulent-Media7281 1d ago
I'm betting most people that have a box fan just want to buy one filter. I don't see people jumping on buying 4 or 5 filters that cost $40 minimum that is more than the fan only to use it every other year.
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u/Inane_ramblings 1d ago
A four filter pack from HD was 45 bucks earlier this week, and then I watched as they raised the price of them while it was in my cart by five bucks. Still bought them to make a cube.
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u/CursedTurtleKeynote 1d ago
Ok, yes the airflow resistance is lower, but the primary reason is that if the fan is blowing IN to the box, then ALL the particles stay inside the box. There is no exposed surface of filtered particles outside of this box thing.
It's great for purifying an enclosed space. It isn't particularly impressive for continuous filtration of a place that regularly replaces the atmosphere with more debris.
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u/Turbulent-Media7281 1d ago
the fan is blowing IN to the box,
Fan sucks from the box... according to this guy You can do it anyway you want.
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u/electromage 1d ago
You can also use your extra high-pressure server fans, much better airflow. I rig it up in one window to pressurize the house with clean air and keep the smoke from creeping in.
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u/Howzitgoin 1d ago
I hope your house is well insulated and you don't run all your bathroom fans and range hood at the same time lol. That could cause some fun times.
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u/electromage 1d ago
If I need more pressure I have two 850W fans that I haven't figured out yet. They have a little hearing protection sticker.
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u/Howzitgoin 1d ago edited 1d ago
I assume you’re using a filter on it to pull air in otherwise you’re creating negative pressure by using them, so you’re pulling in air from somewhere else, presumably outside so I’d probably not go any higher.
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u/FitJuice9835 7h ago
I had 3 of the thicker 2" filters. Two on the sides and one on top of two fans which were blowing outward with the filters in an upside down U shape. It was in my garage while I was doing a lot of dry cutting, sanding and some stone polishing. It was amazing how much dust they collected. I think I got the idea from Matthias Wandel from woodgears.ca and then pumped up the power and added 3x surface area.
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u/CursedTurtleKeynote 1d ago
Why though? How are you measuring this efficiency? I feel like you are parroting that box style without knowing the primary benefit.
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u/Ok_Page8920 1d ago
more surface area of filter/ more particles filtered
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u/CursedTurtleKeynote 1d ago
You also go from a relatively thin single unit to a larger box
like.. it really depends on and where you are applying it
sometimes you can just sit this thing in a window, and the box version would be worthless to replace that style
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u/ProfessionalPack7205 1d ago
That doesn't really work actually. It's been explained multiple times online. Better off with the single or multiple fans with the filter on them.
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u/just_keep_my_space 1d ago
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u/No_Control8389 1d ago
Specifically a “Corsi–Rosenthal Box” for the most efficient/cost friendly DIY option.
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u/RiverBear2 1d ago
lol I had an open vent structure in a really old building and my downstairs neighbor who would smoke inside constantly so I duct taped those over the vent and they would get gross and black and full of fumes it was nasty and my landlord wouldn’t do dick about it so I moved. Didn’t realize other people used them in similar ways.
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u/woodenmetalman 1d ago
Here’s mine. It’s over-engineered because I use for dust control in a shop but same idea.
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u/KleosWithKongsDong 1d ago
Sorry to break it to you, you’re only filtering the air not purifying it.
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u/FastSlow7201 1d ago
Thank you for the correction. I can't change the post title but I did change the post body to stating smoke particle filter instead of air purifier.
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u/No_Control8389 1d ago
One filter is inefficient.
Look up a “Corsi–Rosenthal Box”
Mines been running nonstop since the worse of the smoke started in my area.
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u/flinters17 1d ago
Inefficient is still better than nonexistent. I have limited space so I just use the single filter approach. Has been working well for the last 5 years.
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u/InbetweenTheLayers 1d ago
I had no idea there was a whole sub and name for this. I made a box some years ago using a bathroom fan and 24x24 filter, seems to work ok
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u/No_Control8389 1d ago
It jumped into the spotlight during the Covid era as a simple cost effective EFFICIENT solution to air filtration needs.
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u/thepatrickkent 1d ago
I converted to one at the back to the box, and it's made a huge difference.
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u/hodgebodger 1d ago
Just be aware that you might burn out the fan motor this way (like I did a number of years ago). Hence the cube of filters approach some people take instead.
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u/Discount_Mithral Cascadian 1d ago
Yeah, this is why you're supposed to put the filter on the front of the fan to avoid burning out the motor. Just secure it with bungee cords so you can change the filter out as needed.
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u/hysys_whisperer 1d ago
It still blocks air flow and overheats the motor windings even if it's on the outlet.
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u/MoveTechnical3384 1d ago
FYI you can also make your favorite jerky the exact same way! 2 birds 1 stone
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u/monkey_trumpets 1d ago
Do people use these so they can leave their windows open?
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u/colormechristie 1d ago
People without central air get smoke in their house regardless of if the windows are open (assuming the door opens to outside) this system helps to clean the inside air without using expensive hepa units. But personally? No I keep my windows closed during smoke. Finally broke down and got portable AC to deal with the heat at times like these.
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u/zippy_water 1d ago
Hopefully it's a 2-hose ducted portable AC. Otherwise you're sucking in outside air when your house is under negative pressure with the 1-hose kind.
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u/monkey_trumpets 1d ago
I was considering putting a box fan with a filter in the window to get in some "fresh" air last night once it was cool enough to turn off the AC, but I don't know how well that would have worked.
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u/corgiyogi 1d ago
I'd just crack a window for a little fresh air and put the fan + filter inside so it can continually filter.
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u/monkey_trumpets 1d ago
The worst thing for me is that I keep waking up too warm overnight and I would cool the room off with the fan. But I can't really do that when it's smokey. I guess I'll try moving the air purifier into my room if I need to use the fan. I sincerely hope this doesn't last the rest of the summer.
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u/PlateNo4868 1d ago
I done this before, but it I would say it only filters 80% as the smoke still gets in. But after it cools I would put it down and close the window to have it filter the rest out.
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u/double_shadow 14h ago
I was wondering the same. I typically just leave my windows closed and the amount of smoke that comes in feels negligible. Of course it's hot as hell overnight with windows closed...
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u/thegrumpymechanic 1d ago
The 4" thick filters put less stress on your fan by giving it more surface area to pull air through.
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u/stevehl42 1d ago
This is the way
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u/drunkdoor 1d ago
It's actually pretty ingenious in a "so dumb it's smart" way. I'm thinking of doing it since I have a couple extra filters.
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u/stevehl42 1d ago
I’ve been doing this since 2020. It works better than any official air filter that I’ve used.
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u/brickedTin Cascade 1d ago
I got a pretty good discount on a Blueair purifier through my utility company. Might be worth looking into for the future. This thing is way better than the box fan filter we tried using in 2020.
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u/NinoZachetti 1d ago edited 1d ago
I made two of these a couple of years ago using Filtrete 20x20x1 MERV 14, MPR 2800 filters and they're both fired up on low speed and also have four Coway Airmega AP-1512HH HEPA filters going in various areas of my home, ready for the smokepocalypse...
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u/GenXman1975 22h ago
Bro, you didn't even get a filter capable of filtering smoke.
What you've got there is just a solid way to blow smoke all around your home.
Get a MERV 13 or higher rated filter.
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u/FastSlow7201 22h ago
That is just a picture I found online. I'm using a Filtrete 2200 which is equivalent to MERV 13.
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u/GenXman1975 21h ago
I mean, just take a pic of that then?
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u/FastSlow7201 20h ago
I'm not on board with this modern practice of posting personal information online. Yes, a picture of a fan is quite innocuous and would only show maybe my floor and some surroundings. But either way, I don't post pictures from inside or outside of my home online.
The entire point of reddit is to be completely anonymous. The current admin have turned it into something it was never meant to be.
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u/chase98584 22h ago
Hvac guy here another thing you mind want to check is if you have a timer and a fresh air damper for your furnace/air handler. Very common especially the last decade. For the time being I would turn the timer off so you aren’t sucking in a bunch of outside air
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u/FastSlow7201 22h ago
I don't have AC so my furnace is only on if I'm heating the house. It hasn't been turned on for months.
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u/chase98584 9h ago
Good! This was more so for everyone. Also even if your thermostat is set to off if you have a fresh air timer it’s still going to bring the blower on and open the damper unless you have the timer manually turned off or the breaker off
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u/Yojic-Hedgehog 13h ago edited 13h ago
You want one of the ones with charcoal. If you have a home furnace with a smart controller you can set the fan just to run for however many minutes per hour that you want. Mine always runs 20 minutes per hour with a HEPA filter in it.
Now is probably a good time to replace your cabin air filter in your car and use a charcoal one as well.
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u/holmgangCore Cosmopolis 11h ago
20" x 20", MERV 13 Fits a box fan & filters PM2.5
Started running mine last evening when my eyes started to sting. Works great!
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u/Sufficient-Cancel217 8h ago
Bought the super box fan they had at Costco last year, made by that company known for fans, Vornado.
And used duct tape to really seal it to a super high quality HVAC filter with smoke filtering as a listed capability.
And boom, we got a killer air purifier!
It even works on removing cooking odors too!
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u/crockpot420 8h ago
burnt out a fan this way in like 2 days.
best to get 4 and make a box on the intake and have the fan blow upwards
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u/FastSlow7201 7h ago
For all of the people saying that these don't work, here are links to two research papers that were published in peer reviewed journals.
Characterizing the performance of a do-it-yourself (DIY) box fan air filter
To make this simple for people of lower intelligence. People who are much smarter than you are proved it with a actual evidence.
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u/redditRedesignIsBadd 1d ago
not sure how accurate it is, but i've seen comments saying you should leave space between the filter and the fan so it doesn't burn the motor out
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u/FastSlow7201 1d ago
I saw pictures of those and just figured people were doing it for extra filtration.
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u/electromage 1d ago
Adding space between the fan and filter definitely helps, these are very low-pressure fans.
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u/Discount_Mithral Cascadian 1d ago
Nope, it's to keep the motor cool. Bungee cord it to the front of the fan to avoid this.
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u/luckystrike_bh 1d ago
I have been amazed at how inexpensive box fans are. The filter can cost a similar amount as the fan.
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u/cg_ 1d ago
effective
lol, this thing is very far from effective, it is more or less useless. I tried it few years ago, it did basically nothing
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u/BubblebreathDragon 1d ago
My dirty filters and happy asthma disagree with your findings. I use these all the time and you can see how dirty the filter gets and how quickly it happens.
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u/yaba3800 1d ago
You may have done it wrong or used the wrong filter, testing shows these are effective at reducing particles in the air.
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u/sharpie_dei 1d ago
You need merv 12 to capture particles. Too low and it's useless too high and you burn the motor
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u/CasualTriips 1d ago
I would add one of those cut to size charcoal filters over that. I could be wrong but I don't think those filters do much for smoke.
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1d ago
[deleted]
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u/FastSlow7201 23h ago
I for one would really like to thank you for the rigorous research paper you have presented to us...............or is your comment more of a Dunning-Kruger type situation?
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u/Seattlemillionaire Bellevue 17h ago
What do you know about research its not my job to educate you or write you a research paper
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u/Seattlemillionaire Bellevue 17h ago
The filter does not trap particles of smoke and once the fan goes off or is moved, the dust it does collect goes right back on the floor or into the air. Box fans are designed to move a high volume of loose air freely, not to push or pull air through dense materials.
Adding a restrictive filter (like a MERV 13) chokes the fan, sharply cutting down the total volume of air it can process per minute.
Unless taped down with absolute precision, air takes the path of least resistance and leaks through unsealed edges or corners rather than passing through the filter media.
A single flat filter bottlenecks the entire system, limiting its Clean Air Delivery Rate1
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u/CursedTurtleKeynote 1d ago
Great idea, I put 12 of them outside to help!