How to Make a DIY Air Filter | Ask This Old House

[Music] ross it is the best time to be talking about air filtration tough year very important for good air yes it is we've got the wildflowers out west not good there with all the smoke right some of the worst air quality in history right in california and portland um and then we've got coronavirus right cover 19. you know so a lot of reasons for a lot of people to be thinking about filtering the air in their house that's right and we're also fortunate that a lot of people use an hvac system for heating and cooling so they're actually already moving air that's right and they're already filtering air that's right question is what are they filtering with and can we do any better yeah so here's the typical murph mirv one rating so let's just talk about mirv mirv is the system that we use to rank filtration yeah so merv is a rating scale from 1 to 16. one is the lowest level of filtration 16 being the highest on that scale higher the number goes the better off we get that's right so merv this is a merv one merv ones are good for you know maybe uh cat or dog hair maybe some dust yeah right through it yeah it's not gonna do much for you so but this is actually pretty typical you see them in a lot of houses right so um not that great right and you work your way up from there to merv fours and eights you know but then where it really gets interesting is merv 13. what happens at 13. so 13 you're actually going to get the really small particulates all right so if you think about it you know particulars come in different sizes from 0.3 micron to 1 micron is the really small stuff and with that we're talking about 150th the size of the human hair so one micron is 150th the size of a human hair or smaller gotcha okay so hang on a second um before we get to 13 okay am i doing anything for covid like one through twelve am i getting what i need to get to know that i'm safe you're not gonna get a great level of filtration because where 12 or smaller is not going to get those fine particulates it's going to get the medium size and the larger size so it might be into 13 or more you need to be 13 or higher to really get smoke and to really get the aerosols that we breathe out which is what the virus basically you know hitches a ride with perfect to transmit so what does it what does a 13 look like is this a third so this is a mr13 this is a mr13 20 by 20 it's a two inch thick filter so 20 inches wide 20 inches long and then two inches that pleat size that's right they come in one inch they come in four inch right so here's an example of a four inch version 20 by 25 but four inches thick so if 13 is my number i'm going home right now thank you for buying me this and i'm taking my probably a six or eight out of my system and i'm putting this in and i'm good to go not so fast why not so with this you're going to increase the resistance of the air going across it so you think about the higher levels of filtration the more pressure drop you have to the more resistance you have to get and overcome i don't care i don't care if my fan slows down because i'm trying to save my family yeah i know so it's not a good idea so if you put two restrictive a filter on you're either going to freeze up your coil or you could seize the motor in that system so you really want to have an htc technician take a look at it before you actually go retrofit all your systems and upgrade them to 13 so i do care you do care yeah you don't want that to happen yeah so in an ideal world everybody would have that murph 13 or higher right so you just got to take a look at that so is there any work around to something like this i mean there's any way that i can reduce the resistance to the air flow right so if we go from a two inch pleated filter that we have here to a four inch pleated filter like that right you pull that out so like this guy right so you have twice the thickness which means that you have twice the surface area so these pleats are twice there's twice the amount of surface area in this filter than there is and that filter if i were to unfold this and unfold that that would be bigger that's right more surface area means less resistance that's right so maybe i could go to a 13 with my existing motor and system if i picked a properly sized mirv 13 right so it could be a four inch or could even be a five inch deep filter like this that could slid into a filter box so you have to have the space to be able to accommodate this but if you could then you could actually upgrade your system and if i don't have the space then i'm back to the same old problem which is i'm i can't go to 13 right without maybe risking damaging my motor that's right yeah so what your next best solution after that is potentially a home air purifier or air cleaner buying one off the buying one off line yep or we have one here that we're gonna actually assemble okay like we're gonna make our own we're gonna make our own all right so this is this is called the comparator cube it's using four of these filters with a box fan all for less than 100 bucks do we know copperado or yeah yeah his name's neil he's out of virginia and he took the the typical design was this take a two inch merv 13 filter put it on a box fan like that turn it on turn it on right and let it let it do its thing that's brilliant then you get murphy infiltration but he came up the idea of taking this and adding four filters by doing that you're going to increase the surface area which means you decrease the pressure drop means we move more airflow oh all right well let's build it you show me how to build it okay what do we do so we start off with the cardboard here in the bottom and we're gonna stack these two inch filters like this with the airflow arrow pointing in in okay there's another one in there there how do you want this guy there there yep and last one you want it to come to that that's right all right yep just like that i'm going to get some duct tape duct tape right here of course there's duct tape involved hey this is diy right here this is this is great this is definitely what people are going to be doing in this apocalyptic year right now this is great so the bottom cardboard is there so we're not blocking any of the filters that's right do we get the duct tape this down too yep awesome it's portable it's portable give me a sense of how much air we are moving with this homemade cube filter here so this one is this is a three speed fan so we're going to be moving somewhere up around 900 cfm on high speed 900 cubic feet per minute that's on high speed that's right and um can you give me a sense as to how much cubic feet is in a i don't know 2500 square foot house roughly yeah so a 2500 square foot house eight foot ceilings you're talking about you know uh maybe 20 000 cubic feet of volume and you said we're moving about 9900 or so yeah so we're going to get about almost uh one air change in that house um every every 20 minutes three air changes an hour wow that's pretty good that's really good actually oh the thing is though this is localized right and it's not ducted so it's going to work for localized air cleaning wherever it is if you're in the kitchen living room where it's located that's great if you go to bed at night you can take it with you but um it's not a bad thing it's great turn it on all right turn it on so we have got airflow this way so coming in through four filters and coming out through the fan perfectly clean which means clean enough that we're taking the smoke out of the air and coronavirus that's the great thing about 13 filters they get the small stuff and the big stuff this thing is awesome love it now keep in mind like any filter it's going to load up right so this could get surface loading which is gonna increase that resistance so you're gonna have to replace these filters every once in a while on this system it's gonna be about six months good enough for me terrific ross thank you what a great idea thanks for watching this whole house has got a video for just about every home improvement project so be sure to check out the others and if you like what you see click on the subscribe button make sure that you get our newest videos right in your feed

Channel: This Old House

Views: 602,787

Keywords: This Old House, DIY, Construction, Home Improvement, Ask This Old House, Building, air quality, Ross Trethewey, Kevin O'Connor

Id: aw7fUMhNov8

Length: 7min 40sec (460 seconds)

Published: Mon Jan 18 2021

Reddit Comments

[u/yusrandpasswdisbad]

Also this one - you can get everything you need at Walmart.

https://twitter.com/DavidElfstrom/status/1429526273009889286

[u/NorCalWeirdo]

Quality HEPA filters can be found on Amazon for less than $100 and work way better than these though.