This article represents my own opinion and may contain affiliate links. Please read my disclosures for more information.
A better more efficient and odor eliminating homemade air purifier than just taping a 20x20x1 filter to a box fan. Sometimes you need to help clear the air in a dirty environment but don't want to use an expensive air purifier such as in a room after renovating and painting. Here's a cheap and easy way to build one.
Air purifiers can be expensive and you've probably seen articles recommending to just put a 20" x 20" x 1" furnace filter on a cheap 20" box fan and POOF! instant cleaner air for not a lot of money. It really does clean the air pretty cheap.
There's a problem with this though. These fans weren't designed to be run with a filter. The filter will restrict air flow which will put a higher strain on the motor causing it to use more electricity and in worse cases could be a fire hazard. The higher the MERV rating (cleaning efficiency) of the filter the more stress it will put on the fan.
Don't worry! You can still have your cheap air purifier as long as the filter area is increased to decrease the effect of air resistance. Instead of using one 20x20x1 filter we'll use two 20x25x1 filters which increases the filter surface area over 250%. It's a little more expensive because you're using two filters instead of one but the increased filter surface area also helps the filter last longer before it gets clogged up and we're saving on energy use compared to a single filter.
I can't take credit for the design, I found it via Marshall Hansen Design but I'm using different filters.
because it has great reviews, is reasonably priced and the replacement filters aren't that expensive or a Sharp Plasmacluster True HEPA Air Purifier
or Blueair HepaSilent Air-Purification System
if I find some extra Benjamins in the couch cushions. But I don't want to put that in the dusty environment right away because I'll have to replace the more expensive filters fairly quickly because the air is so bad. I also don't want to spend the money right now but I do want cleaner air now.So I decided to get things going with a homemade air purifier that has filters that not only clean the air of particulates but also reduce VOCs and odors. Even after I purchase an air purifier I'll still have an air purifier I can use for other purposes like when I'm painting or cutting wood.
Also, don't forget to add more plants to your home. They're natures air purifiers. Not only do they convert carbon dioxide to oxygen but houseplants filter toxins and emit negative ions too!
What You'll Need
Materials
- Filtrete Home Odor Reduction Filter, 20-Inch by 25-Inch by 1-Inch, 4-Pack
(only using 2 at a time)
- 20" Box Fan
- Cardboard (from the package the filters came in) or oak tag
- Duct Tape or Clear Cellophane Tape
- Optional Plug-in Timer
so the fan isn't running constantly without manual intervention.
Tools
- A pair of scissors or utility knife
Quick Info About Filters
The nice thing about this box fan filter setup is you can use any filters you want. If you are only concerned about large particles generated from something like cutting wood, you can use cheaper, lower MERV rated filters. If you're worried about finer particles from allergens such as mold, you can use a higher MERV rated filter. To understand more about what the MERV ratings mean have a look at Table 2 on this EPA article on residential air.
I chose to go with the Filtrete Home Odor Reduction Filters which are a new type of filter from Filtrete. They contain a lot of activated carbon to help filter out odors and VOCs. They are rated MERV 11 and are equivalent to their 1000 MPR furnace filters. They should do a good job with even the fine drywall dust and the activated carbon will help a lot with odors. I bought a 4 pack for around $57 which gives me 2 sets of filters.
 |
| Filtrete Odor Reducing Air Filters |
I've been using in my air conditioning for the past few years. Plus Filtrete packs over 180 grams of activated carbon to help reduce odors.After I go through both sets of carbon filters I'm probably going to switch to the Filtrete Elite Allergen Reduction Filter
which does a much better job of filtering small particles. If odors are a problem I'll get some Cut-to-fit Carbon Prefilters
and tape them over top of the Filtrete filters.Okay, enough of the why, now for the how!
Step 1: Unbox The Filters
You're thinking "OMG! He's actually describing how to open a cardboard box?!?!?!" The best deal I found for these filters were on Amazon and with my Prime membership (or supersaver shipping) the shipping is free. Plus the packing box is going to be part of the filter so we need to keep in intact.The box is sealed very well. Not only is it taped but the seams are also glued together. To get the filters out of the box I cut the top where it's perforated with a utility knife as shown.
Be careful not to cut too deep and damage the filters. Keep the box someplace safe to the side while we continue.
Step 2: Hinge Filters
Start by taking 2 of the filters out of their plastic wrappers and lay them one on top of the other with the air flow arrows pointing towards each other. With the Odor Reducing Fitlers this means the black, carbon sides will be facing each other.
With the filters lined up, tape one of the short (20") edges of the filters to create a hinge between the two filters as shown.
I decided to go with white duck tape so it looks a little nicer but it makes it hard to see in the photos against the white cardboard filter frame.
With the filters lined up, tape one of the short (20") edges of the filters to create a hinge between the two filters as shown.
I decided to go with white duck tape so it looks a little nicer but it makes it hard to see in the photos against the white cardboard filter frame.
Step 3: Tape Filters To Fan
Lay the box fan face down on a flat surface to mount the filters to the back making sure the bottom of the filters are about flush with the bottom of the fan and do not extend past the bottom.
Use a strip of duck tape to secure each side to the side of the fan.
Use a strip of duck tape to secure each side to the side of the fan.
Step 4: Cut and Attach the Cardboard
Cut out one of the large sides of from the cardboard shipping box, place it over the top of the of the filters and secure it with a strip of tape to the top of the fan. Position it so that it's pretty even over both filters but it doesn't have to be perfect.
Using a pencil, trace around the filters (while pressing down the cardboard so nothing moves) to mark where the cardboard needs to be cut.
Lift the cardboard up (the front piece of tape acts like a hinge) and cut along the lines to trim the sides. By the way, I really like these Fiskars Cuts+More 5-in-1 Multi-Purpose Scissors
.
Repeat the process for the bottom cardboard. The other large piece from the box is seamed but it's taped and glued and very stiff so it shouldn't be an issue but you can run a strip of duck tape over the seem if it makes you feel better.
Also keep in mind that the bottom of the fan has feet. I positioned the cardboard underneath the feet and then pulled the feet out to tape.
After taping up the sides of the bottom cover to the filters, I added an extra strip of duck tape all the way around where the filters/cardboard meets the fan for added support. The rounded corners of the fan means there might still be an air gap there so check them and add extra duck tape if necessary. On the bottom there are also some holes, duck tape over those as well. Finally, replace the feet.
Using a pencil, trace around the filters (while pressing down the cardboard so nothing moves) to mark where the cardboard needs to be cut.
Lift the cardboard up (the front piece of tape acts like a hinge) and cut along the lines to trim the sides. By the way, I really like these Fiskars Cuts+More 5-in-1 Multi-Purpose Scissors
.
Repeat the process for the bottom cardboard. The other large piece from the box is seamed but it's taped and glued and very stiff so it shouldn't be an issue but you can run a strip of duck tape over the seem if it makes you feel better.
Also keep in mind that the bottom of the fan has feet. I positioned the cardboard underneath the feet and then pulled the feet out to tape.
After taping up the sides of the bottom cover to the filters, I added an extra strip of duck tape all the way around where the filters/cardboard meets the fan for added support. The rounded corners of the fan means there might still be an air gap there so check them and add extra duck tape if necessary. On the bottom there are also some holes, duck tape over those as well. Finally, replace the feet.
Step 5: Seal The Back
Where the hinged filters meet the cardboard in the back, add another strip of duck tape for support and to seal any gaps.
Give another look around the seams to make sure there aren't any gaps. If you find any, tape them up.
When using the fan, I always make sure that the cord isn't underneath the cardboard and instead is under the metal fan chassis, just in case something goes wrong and the cord overheats. Can never be too safe.
Replacing Filters
When it's time to replace filters, use a utility knife to cut the filters free but leave the cardboard in place. Then just hinge and attach the new filters.
Brilliant!
ReplyDeleteThank you :)
DeleteCurrently I bungee a 20x20 to the back of box fans, but I might try this instead!
ReplyDeleteVery nice idea. I got to make one of these, the wife will love it.
ReplyDeleteThanks Mr.Tom!I just built this today.... I hope my allergies improve now!
ReplyDeleteThanks!
ReplyDeletehi Tom, did you build this in the UK? I couldn't find box in amazon uk. thanks.
ReplyDeletethis is a great plan, but the information about causing the fan to pull more current when the inlet is restricted is exactly backwards. Restricting the inlet reduces the amount of air the fan can push, (less work= less current)
ReplyDeleteLess air flow at much higher resistance = more work/current.
DeleteDoubling the filter area halves the resistance caused by the filters.
Less air moved = less power consumption, but also less cooling. If the fan could force the air throuh the filter then the power consumption would increase, but that can't happen with axial flow fans.
Deletewill 2 filters reduce the filtration rate? other diy sites just attach a furnace filter (though not w/ high MERV rating) directly to the box fan. i want hepa merv 12 or above if possible for a small room.
DeleteCurrent draw isn't about air movement, its about fan motor load. More filter surface area for air to travel through=less air resistance so less load (thus better for the fan and more safe). Imagine if you use your hand to stop the fan. Zero air moving but there's still load, and a high current draw.
DeleteThe high current draw from the fan when you are holding it is obvious.
DeleteThe high current draw from the extra resistance of the filter should be obvious too. That's why you keep the outside unit of your hvac system clean and clear of debris, and why you change out dirty air filters. 2 filters won't hurt the filtration because the air still goes through the filters. If you had a cheapo filter on one side and a high filtering hepa on the other side, it would only be as good as the cheap filter though because there would be less air pulled through the higher resistance hepa than the cheapo.
DeleteBlocking the air flow decreases the load of the motor and decreases the current (amps). Period. The fan is not using any form of closed loop control trying to keep a constant air flow. The article is wrong!!! The only damage that can occur is heat built up as the air the fan is circulating is the same air being used to keep the motor cool. Please do not give false information and confuse others. What I have stated is fact! This is not an opinion, this is factual information. This confuses people all the time, even at my work. We monitor load (amps) on pumps and fans while performing rational checks against flow meters. This is done via PLC but is checking to make sure the fan / pump is performing at its best in the event a flow meter is providing inadequate information.
DeleteAgain, blocking the air flow decreases the load and the current on the motor. Period!
http://hvacrfundamentals.blogspot.com/2009/08/understanding-centrifugal-fan-motor.html
I appreciate you linking this article, it was very informative. Specifically about how CENTRIFUGAL fans decrease load as airflow is restricted. Now I don't work in the hvac industry but I do know that centrifugal fans are the most commonly used fans in the industry. So the problem is that this article is talking about using an AXIAL fan to move the air. Axial fans move air parallel to the shaft of the fan. Perhaps the difference between the way the types of fans move air could cause the box fan to draw more power as resistance is increased. Only empirical data can show us, I might update with experiment results.
DeleteI just tested this with an ammeter, the difference is negligible but slightly less current with the filter installed.
DeleteI know this is old but, gosh, I love how the empirical test flat-out ended the conversation.
Delete"It draws more amps with a filter!"
"No, it doesn't -- less load on the motor in a (slightly) relative vacuum!"
"I stand my ground. Your literature deals with axial fans!"
"Nope. Load is load."
. . . . and then some dude with a multimeter: "I just tested it. The difference is negligible, but slightly less current with the filter installed."
Not making fun of anybody here, it's just fun to see the empirical approach cause all the reasoned navel-gazing to wither and die. It's "fun" for me because most of my best-reasoned ideas often crash and burn in practice.
Haha I love this reply!!
DeleteUsed 300 mpr furnace filters. Bought 20x30x1 as they were same price as 20x25x1 or 20x20x1.
ReplyDeleteNice.
ReplyDeleteIndoor plants decrease air quality because of the soil. Bacteria in the soil will use up more oxygen than the plant produces. Outside, sunlight kills the bacteria, but most windows block UV light. The soil also supports mold and mold spore. Also, wet soil increases indoor humidity and increases other growth away from the plant. For indoor air quality, no plants, or only hydroponic plants.
those are some very good points, thank you for the input.
DeleteI will never forget the air of an apartment that had plants on every shelf all along the walls. There were hundreds of plants, if not thousands as it was a large room. It smelled like a vibrant forest. Unforgettable. Not sure what bearing the soil had but she was old and this was her hobby. It took hours to take care of the plants every day. It could be that the soil detracted from the air quality and if the same amount of plants were using aeroponics or hydroponics the air might have been much better though its hard to imagine anything smelling better than that. I need to visit many garden stores where there are many plants packed in smaller spaces. I heard there was an office building in India that was sealed from outside air because of the air pollution and all of its air was produced by plants inside. But what about nitrogen? That is a component of air. Plants don't produce nitrogen I thought. If you could produce all of your own air from growing food indoors, that would be sweet. And would most of that oxygen that they produce by gone in the morning because they use oxygen at night and expel CO2? Maybe the answer is to sleep in a different part of the house at night than where the plants are.
DeleteOld post, but by the original comment, if you taped up the back and the front of the fan and there was no air going through the fan i guess the fan uses no electricity when it spins.
DeleteIf you have a small pipe and water goes through slow, if you want the water to get through it faster you have to exert extra load on the water to generate a stonger flow over the restricted path.
using a 20x20x4 filter would decrease air flow resistance and is a lot less work
ReplyDeleteAren't they more expensive? Significantly more?
DeleteThey are more expensive, but they need to be replaced far less often due the increased surface area, so it kinda evens out financially.
DeleteBrilliant! Shared this on my page cause our area is saturated in wildfire smoke currently
ReplyDeleteBeen thinking about doing something like this. Thanks for already inventing the wheel. Built mine today, using the box the fan came in and MERV 12 filters.
ReplyDeleteIs there anything on the market with permanent HEPA filter or does permanent HEPA exist? Something i can wash and reuse, instead of replacing these expensive furnace filters. MERV13 is ~$25-30?
ReplyDeleteI don't know about a washable filter that works well. I have seen videos on Youtube and read that some people use HEPA filters on a box fan and clean them by blowing out with high pressure air. Not sure how well it works, but you could try that. Take fan outside before removing filter and letting dust from it return to house air.
ReplyDeletegreat
ReplyDeleteyou can also do this with 4 20x20 filters, and one piece of square cardboard to make a cube. It will also allow you to use a high MERV filter without much drop in airflow.
ReplyDeleteFYI, a box fan is an axial bkade design. Restricting the inlet air actually unloads the fan reducing the power it draws. Old hvac systems used to use inlet vanes to unload large fans. The only issue would be restricting airflow to the point that the motor is nit cooled sufficiently.
ReplyDeleteBelieve it or not, I just did a current draw test on my 20" box fan and to my great surprise, if I block the intake it draws MORE - not less current!!!!
ReplyDeleteHere are thge actual figures:
- normal operation @ 120VAC: 1.84A
- intake partially blocked by a 3M Filtrete MERV 11 filter: 1.85A (no significant change)
- intake completely blocked using a plastic garbage bag: 2.02A
- exhaust completely blocked using a plastic garbage bag: 2.02A
For fun, I did the same tests by simply laying the fan flat down on the floor, measuring current for both sides. Again, I got roughly the same results.
This is totally unexpected to me. I did the same test with a vacuum cleaner a while ago and the current draw would plummet dramatically when I plugged the intake with the palm of my hand. Something like 7A down to 2A if I remember well. And the motor would start producing that caracteristic revving sound.
The box fan does not behave the same way at all, probably due to that axial blade design thing.
But still, the conclusion for this application is that it is totally negligible.
Cheers,
Norm D.
Normand, there is no surprise, that's the way any motor works, well at least electric motor - slowing it down by increasing load increases current draw. And the article is factually correct about that. Your vacuum cleaner behaves differently because the motor in it is part of the centrifugal fan. And these fans are different because when the air is blocked the motor in these fans actually spins up (so the load on the motor decreases). The person above who wrote
Delete"Blocking the air flow decreases the load of the motor and decreases the current (amps). Period. The fan is not using any form of closed loop control trying to keep a constant air flow. The article is wrong!!!"
has no idea what he's talking about, reader clues would be categorical tone and exclamation marks.
You're right in the respect that with the filter current draw increased insignificantly (good), but still the idea of this article is very sound for the following reasons:
1. Power drawn from the electric grid (part of which produces heat in the motor) is dependent on current drawn and power factor and that power factor will change in the slowed down motor, so that power increase will be higher than current increase. And simultaneously less air will move around motor, so less cooling. Simply put motor loaded with one filter vs. 2 will heat noticeably more.
2. Filters will eventually get clogged. With more surface like suggested it'll reduce air flow less (or slower) and increase stress on the motor less (or it'll take longer)
3. And last but not least - when filters are not clogged this system will move more air through filters - it'll clean air faster!
Normand, there is no surprise, that's the way any motor works, well at least electric motor - slowing it down by increasing load increases current draw. And the article is factually correct about that. Your vacuum cleaner behaves differently because the motor in it is part of the centrifugal fan. And these fans are different because when the air is blocked the motor in these fans actually spins up (so the load on the motor decreases). The person above who wrote
Delete"Blocking the air flow decreases the load of the motor and decreases the current (amps). Period. The fan is not using any form of closed loop control trying to keep a constant air flow. The article is wrong!!!"
has no idea what he's talking about, reader clues would be categorical tone and exclamation marks.
You're right in the respect that with the filter current draw increased insignificantly (good), but still the idea of this article is very sound for the following reasons:
1. Power drawn from the electric grid (part of which produces heat in the motor) is dependent on current drawn and power factor and that power factor will change in the slowed down motor, so that power increase will be higher than current increase. And simultaneously less air will move around motor, so less cooling. Simply put motor loaded with one filter vs. 2 will heat noticeably more.
2. Filters will eventually get clogged. With more surface like suggested it'll reduce air flow less (or slower) and increase stress on the motor less (or it'll take longer)
3. And last but not least - when filters are not clogged this system will move more air through filters - it'll clean air faster!
Well done Norm and Vi
DeleteBelieve it or not, I just did a current draw test on my 20" box fan and to my great surprise, if I block the intake it draws MORE - not less current!!!!
ReplyDeleteHere are thge actual figures:
- normal operation @ 120VAC: 1.84A
- intake partially blocked by a 3M Filtrete MERV 11 filter: 1.85A (no significant change)
- intake completely blocked using a plastic garbage bag: 2.02A
- exhaust completely blocked using a plastic garbage bag: 2.02A
For fun, I did the same tests by simply laying the fan flat down on the floor, measuring current for both sides. Again, I got roughly the same results.
This is totally unexpected to me. I did the same test with a vacuum cleaner a while ago and the current draw would plummet dramatically when I plugged the intake with the palm of my hand. Something like 7A down to 2A if I remember well. And the motor would start producing that caracteristic revving sound.
The box fan does not behave the same way at all, probably due to that axial blade design thing.
But still, the conclusion for this application is that it is totally negligible.
An axial fan blade is an airfoil, which makes me think of one possible explanation: Blocking the airflow increases turbulence. Turbulence leads to loss of laminar flow, and loss of laminar flow leads to boundary-layer separation from the low-pressure side. Boundary-layer separation leads to stalling. On efficient airfoils, this leads to a drop in lift and a large increase in drag - and therefore power required. I did a quick Reynolds-number calculation on my 20" box fan, and they've designed it so that it's around 30,000 (low speed) to 90,000 (high speed) on the part of the blade that pushes the most air. Those are numbers at which many airfoils are sensitive to laminar boundary layer separation but fail to transition to a reattached turbulent boundary layer, and a laminar separation bubble/stalling is easy to induce.
DeleteThat might be complete bullshit, but it would be an interesting thing to test for. You could probably do it by attaching a strip of tissue paper to the leading edge of the blade and seeing whether it stays sucked against the intake side of the blade in free flow vs. blocked conditions.
Norm wins. I love facts and stats.
ReplyDeleteHaha.... I was gonna say hands down but how about FANS down instead.
DeleteThe motor on the fan is set to run and a particular speed regardless of load as it even spins at the same speed if you take the blades off entirely. When you restrict airflow it make the motor work harder because of the lack of air movement to achieve its RPM that it is designed to run at. When you think about this you can relate it to breathing thru a straw, you still need the same amount of air but the restriction is making you work harder or paddling a canoe as it is easier to paddle when the boat is moving then from a dead stop.
ReplyDeleteYou always use more energy if you restrict airflow and the fan is trying to achieve a certain RPM.
A physical resistance on an electric motor will simply burn out the motor, the motor does not pull more current as a result. Current is "pushed" by Voltage, not pulled by the working components. Come on
ReplyDeleteThink of what you're experienced in the past. How many times have you seen motors under load heat up and die. You don't think they are "working harder" under load and by worker harder wouldn't draw more current? They heat up, right? When something heats up its drawing more current.
DeleteVoltage is what "pushes" current, hence the original name of "electromotive force". The current is unaffected by the component pulling it, kinda basic science guys. The motor will simply burn itself out, it will not "pull" more current to compensate.
ReplyDeleteWe use to use old direct drive "squirrel cage" fans out of used furnaces to create powerful air-movers to cool down our HVAC shop. When we plugged them in, we had to screw a piece of sheet metal to the outlet of the fan to choke down the exiting air so that the fan would not freewheel and burn up. I remember putting an amp meter on one of the motor lead wires and the amps increased when the fan outlet was left open, but decreased as I choked down the outlet.
ReplyDeleteTom, great DIY instructional.
ReplyDeleteHowever, any DIY'er should know that it is DUCT tape, not "duck" tape. It was invented for sealing around the joints in ducts.
Ducks are fantastic, beautiful birds. Duck tape is just a cruel thing and should never be used.
And while there is a "Duck Tape" brand, it's still DUCT tape.
Tom knows (the supplies list says "duct tape") and uses the colloquialism because it's cute. Try being less of a dull, prescriptive pedant and you might pick up on that.
DeleteIt's very touching that you would say this in response. I almost had to grab a kleenex.
DeleteThis is a very topical post given the state of the air in the San Francisco Bay Area presently (usually very good, but terrible in the past week because of wildfires in the north bay).
ReplyDeleteCurious why this design chooses to pull air through the filters by mounting the filters to the back of the fan. Would it be more effective to trap the particulates inside the fan and push air into the enclosed filter space?
It would continually blow off. Air gets filtered through it from either direction.
DeleteBreathe easy from Portland!!
I can't believe people are not understanding that there is a limit for the current draw. That test that mentions it draws more when it was obstructed forgot to tell us that they turned it into the higher setting to get that higher number. That is the only way those results were obtainable. The person who wrote this is def not an electrician.
ReplyDeletecom'on people... wracked those brains... which is what...? we need need the truth....
ReplyDeleteDo you place the fan facing out of the window do draw air out, or do you turn it so it draws fresh air in? Can it be placed anywhere?
ReplyDeleteI keep it in an enclosed room with no windows open so it filters out the dust.
DeleteDepends where your pollution is coming from. If you're filtering outdoor air coming in obviously the filter will be facing inwards. If your pollutants are being generated from the room itself you need to filter that through cleaning and moving the air throughout the room.
DeleteAnyone that believes that physically restricting an electric motor by any means will make it run more efficiently is ignorant to both mechanical and electrical principles and hopefully not a tradesperson. Thank God for fuses!
ReplyDeleteI would think a 20x20x4 or 20x20x5 would also do this, without having to build such a contraption?
ReplyDeleteIt's about $8 dollars cheaper per filter change going with the 1 inch goes and the odor reduction filters are only available in 1 inch.
DeleteThe 4 and 5" filters will last significantly longer before needing to be replaced, though.
DeleteIt's usually recommended to replace a 1" filter every month in a traditional HVAC system, while a 4" will last 6 months, and a 5" can go nearly a year or more before needing to be replaced.
I know this post is OLD, but I'm awake and nothing to do. SO if we go with "Anonymous" statement that decreasing the air flow decreases the load on the motor, then the opposite would be true that if we force air through the fan then the load would INCREASE!? I think NOT!.
ReplyDeleteOkay so what would happen in these cases:
ReplyDeleteWith the assumption that the box fan motor is "dumb". It doesn't actively try to maintain a certain number of RPM's or a certain volume of air moved per minute. It just spins at whatever rate 120 volts makes it spin. Then outside factors such as drag and friction slow it down.
1. Block the fan blades from moving. Turn fan on. The load on the motor is high, so the current goes up right? Therefore wattage goes up?
2. Remove the fan blades from the motor. Now it's just an electric motor spinning a shaft. Less weight/load on the motor. Same voltage applied.
Won't it spin faster? And use less current (therefore fewer watts)?
3. Fan blades are back on. Put the box fan in a vacuum chamber. There's no air to be moved. No air impeding movement of the fan blades. So similar to #2, there's less load on the motor. So again, it'll spin faster? Use less current (and watts)?
3. Similar to #2 and #3, putting an hvac filter on the fan would reduce the rate at which a volume of air is capable of passing across the fan blades. Wouldn't this be reducing the work of the motor? Kinda/sorta like a partial vacuum chamber? Again, reducing the current and wattage?
As others have stated it's pretty clear that in all my cases above, the reduced airflow would increase the rate of temperature rise in the motor, shortening the life of the motor.
I just hooked a 20in box fan up to a meter and it drew the same amount of power whether I had a filter attached to the fan or not. Although it moved a lot less air with the filter attached, but it used the same amount of power..
ReplyDeleteFantastic Post! Lot of information is helpful in some or the other way.
ReplyDeleteCleaning Materials
The important thing to discuss is what is the optimum angle for airflow for the filters? They are designed for air to flow through them straight, right? They might work better that way. But its a lot harder to measure then measuring current unfortunately!
ReplyDeleteThanks for all the MERV info I understand micron's but not MERV , also I picked a couple of Honeywell floor HEPA fan units ( 15$ at goodwill ) these motors would probably work to offset the load problem
ReplyDeleteMentioned in the comments HEPA I thinks .003 micron something like that but the filters are expensive your idea looks like a good option for the DYIers ��
Nice sharing!
ReplyDeleteThanks for this post! its sept of 2020 and wildfires are raging on the west coast. HORRIFIC air quality and my apartment smells like food and funk cause I can't open the windows. Hope this helps my indoor air quality by filtering out smells and also leakage from my no doubt shittily (sp?) sealed apartment windows/patio sliding door). Thanks so much!
ReplyDeleteI slapped a 20x20x1 odor and allergy filter to my 20 inch box fan and now I can't sleep worried it will burn out. But it works. There are wildfire smoke at 214 air quality in Seattle and we all have asthma, the filter got dirty in a few hours and all doors and windows have been closed for 2 days and my house is clean. So, now I am afraid the fan is going to burn up due to the comments above. Sigh.
ReplyDeleteDid my steps in a bit of a different order, and didn't have a large box so I needed to put some smaller ones together, but I built one just now using the recommended filter size. Feeling a lot better about the indoor air now even with all the wildfire smoke outside.
ReplyDeleteUsing only a single air filter will cause a strain on the motor, reducing the life of the motor...but would not increase the load/current drawn. This makes the motor less efficient but does not cause the motor to pull more amperage. Changing the speed of the motor via the speed control switch/dial causes the motor to pull more amperage. But again, there are no home-use box fans that would work so hard as to pull enough amperage to start a fire. Compare household heaters that we plug in to our outlets...ie, think high wattage heaters such as 1100 or 1500 watts (Watts equals Voltage X Current). If you can safely run an 1100 W heater I feel pretty certain that you can attach a Merv 11 filter to the back of a 20" box fan without worry that you are going to start a fire.
ReplyDeleteNice post
ReplyDeleteI'm so glad this post is helping folks, especially as we enter the apocalypse. Lotta folks mentioning wildfires!
ReplyDeleteAfter several years a new argument; Is it better to locate the filters on the intake or the exhaust?
ReplyDeleteFilter on exhaust will not keep dust and dirt out of fan. You will need to clean inside of fan periodically to maintain fan efficiency.
DeleteOn exhaust side will allow the fan to collect dirt and dust and create a periodic cleaning job.
ReplyDeleteCan anyone address the filtration rate please?
ReplyDeleteThis box fan air purifier system, while it is filtering the indoor air, will also be stirring up a huge amount of dust particles just by having a fan on. We would prefer not to be in the room when this is taking place.
So regarding the important filtration rate, do you know how long would it take to filter the air in small 200 square foot room, so that all the indoor air has been purified, with time for whatever remaining "dust to settle" so to speak? An hour, a day? And how many times, by per how often would this cycle need to take place to maintain clean air? If you don't know these kind of technical effectiveness questions, can you recommend where to find out?
Thank you : )
You're missing a room dimension if you want to calculate filtration rates, since you're dealing with volumetric airflow ("CFM") through the fan/filter assembly.
DeleteLet's assume that your 200 square foot room has a 10 foot ceiling, which means your room has a volume of 2,000 cubic feet. I haven't measured it, but I've seen estimates of airflow through a homebrewed box fan/filter combination in the range of 600-850 CFM when the fan is running at the highest speed. Using the 600 CFM value, this means that your fan/filter is giving you 36,000 ft^3/hour of filtration. Dividing this 36,000 ft^3/hour by your 2,000 ft^3 room volume, gives you 18 room air changes per hour.
Using the CDC's air contaminant removal calculation given in Table B1 of their Guidelines for Environmental Infection Control in Health-Care Facilities, https://www.cdc.gov/infectioncontrol/guidelines/environmental/appendix/air.html, it would take 18 minutes to achieve a 99% reduction in air contaminants.
All this should be tempered with the notes that (a) the filter the author of the article linked to is a MERV-11 filter which isn't hugely efficient in catching particulates -- MERV-13 was the mantra for COVID and wildfire smoke particulate protection, and the CDC guidance was really intended toward healthcare facilities either ventilating with 100% fresh air or filtering air through a HEPA filter which is hugely more efficient than a MERV-11 (the calculations don't care, however -- math is math). Additionally (b) the calculation is theoretical, assuming that the fan/filter assembly is providing perfect mixing of air within the room whereas in reality air in the corners of the room is going to be filtered less than air next to the fan assembly, and air will stratify in higher ceiling rooms. Nonetheless it gives you a decent estimate.
If you want more precision, use a particle counter. You can get a decent one for about $4,000.
Just made one of these to combat the heavy wildfire smoke in eastern Ontario. Feeling refreshed already.
ReplyDeleteFor the love of god, do I put the filter and pre filter on the intake or out-take side?
ReplyDeleteI put the filter on the back side so it keeps the fan clean too.
DeleteWe have three of these running on each floor of our modest brownstone and keep them set to low as not to burn out the motors.
ReplyDeleteOccasionally I will take the one on our 3rd floor into our en suite bathroom of our master bedroom. I turn the fan to high for the 30-45 minutes I’m on my porcelain throne dropping the Cosby kids off at the pool so my wife doesn’t taste the biscuit when she comes to do the needful herself.
They work most excellent.