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

In this video, Ross Trethewey explains how air filtration systems work and how to build a localized DIY air filter.

Home technology expert, Ross Trethewey teaches how air filtration systems work and how to build a localized DIY air filter. Ross explains the MERV air filtration system. MERV stands for Minimal Efficiency Reporting Value, a value that measures how effectively a filter stops particulates of varying sizes from passing through and into the air system. The higher the rating, the more effective the filter is at capturing smaller particles. Ratings 1-16 are HVAC use; in general, houses usually have around a MERV 5-12 system. MERV 17-20 are HEPA filters, those are used in hospitals and laboratories. Ross clarifies that homeowners cannot just install a MERV 16 or HEPA filter into their homes. In many circumstances the HVAC systems homeowners have in place simply aren’t strong enough to handle higher rated filters, as the more filtration the air has to go through, the more resistance it has to overcome. Even if you were to get a thicker filter that increases surface area, meaning less resistance, you’ll have to check if your HVAC system has enough space to install a thicker filter. Check with an HVAC technician to check what rating your system can handle.

If you are unable to acquire a higher rated filter, Ross demonstrates how to build your own DIY air filter. By taking four MERV-13 air filters and duct taping them on all four sides of a box fan, you’ll have a localized air filter for any room you want. For about $150, you can have a little piece of mind during wildfires and other disasters that impact our air quality.

Time: 20 minutes
Cost: $150
Skill Level: Beginner

Shopping List:

Steps:
1. Cut out a piece of cardboard that’s the same size as the box fan to use for the back.
2. Place the cardboard on a table and take the four air filters and stack them on the edges of the cutout, with the airflow arrow pointing in (air intake side).
3. Take duct tape and secure each filter to the fan.
4. Duct tape the cardboard backing.
5. Place the fan on the other side and secure it with duct tape.
6. Remember that you’ll have to replace the filters every once in a while. For Ross’ DIY system, the filters should be replaced about every six months.

Where to find it?

Expert assistance with this segment was provided by Neil Comparetto.

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From the makers of This Old House, America’s first and most trusted home improvement show, Ask This Old House answers the steady stream of home improvement questions asked by viewers across the United States. Covering topics from landscaping to electrical to HVAC and plumbing to painting and more. Ask This Old House features the experts from This Old House, including general contractor Tom Silva, plumbing and heating expert Richard Trethewey, landscape contractor Jenn Nawada, master carpenter Norm Abram, and host Kevin O’Connor. Ask This Old House helps you protect and preserve your greatest investment—your home.

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

[marshacollins]

Three months ago I made two of these and began using them in our two story house. There was an immediate noticeable decrease in dust on furniture and floors. Best of all, we realized after a few weeks we no longer wake up at 3 am with sinus/allergy headaches. We are breathing easier. Thank you!

[glenniz]

I live in Southern Oregon and during the last ten years we have had the worst fire seasons ever!!! Even with all the windows closed, we still have smoke pollution entering our home. If you have ever had to deal with smoke inhalation over months at a time, you will understand how important this simple invention is...Thank You for this simple, effective, and potentially life saving device!!!

[Subgunman]

What I have done is add pre filters to catch the larger particles. This way you won't clog the finer filter with the thick stuff. Throw away the cheaper coarser filters more often and conserve the finer filters.

[gonedmtns]

I'm remodeling a house and this just gave me some ideas on how to setup a filtration system to help keep from contaminating the finished rooms with construction dust and keep the room air a bit healthier.

[dosgos]

Put basic foam or cloth outside filters as a prefilter to extend filter life. Prefilter catches larger particles. Clean prefilter from time to time with vacuum cleaner.

[coreyg]

I did a basic version of this when we were choking with wildfire smoke back in August, worked well and it was pretty obvious how much it was filtering out of the air in the house.. About the only thing I would change here is to run the fan vertically, as it was designed. Laying it horizontal like that is begging for a motor or bushing failure, depending on how that motor is designed to run. Leave it standing like it was meant to be.

[skyrocketcoast]

Northern California: Made one 2 weeks ago, the day our area started to get bad air from big Tamarack Fire 3 hrs away.
works perfectly! Did not have any duct tape, used Tvek tape instead. Air filter is placed in front of old glass door in kitchen that goes out to patio.

[ajones]

This also works with painters tape, which causes much less damage when the unit is disassembled.

[bashbroq]

This old house always putting out quality stuff, still one of my favorite shows

[pizzawarlord]

Surprised no mention about using in wood shop. Been using for years. Gotta change them a lot more often tho

[TrudiTrahanupchanOKRA]

I have standing fans, allergic to diesel, fragrances, laundry soap & my bedroom & bathroom were full of such fumes leaking in from windows and floor boards. I finally managed to seal the leaks on the floor & bedroom window but to detox the rooms I saw a TV report similar & bought Merv11 wrapped it around my stand fans and amazing this removed all the diesel fumes from my bedroom and laundry fumes from my bathroom. I keep them running and I can finally sleep without wearing a gas mask. I have paid a fortune for air filters since 1986 and Merv11 wrapped around a fan is even better than anything I ever had.

[laurabowne]

Just made this for my pottery studio because silicosis is real! Thank you !

[seanile]

Great idea! I improved upon this. Instead of 20x20 filters, I used 20x30. Made it 10 inches taller with an additional 800 inches of filter surface ( 10"x20") additional per filter (x4 sides). I used the cardboard box that the fan came in to cut out the base and taped it up with gorilla tape. Highest merv filter I could find at Home Depot was 10. Thank you for the great idea!

[ixlr]

Clear info on retrofitting a whole home unit. Thank You for making the concerns known. Been using the single filter on box fans for years in the garage. 4 sided is a wonderful idea.

[MikeTwoPoint]

I put a merv 8 4" 20x20 on a 20" box fan, it works awesome and doesn't take up so much space.

[GS-rwog]

AS AN ASTHMATIC, THIS INFO IS INVALUABLE AND M AFFORDABLE FOR MY BUDGET AND A WISE DECISION FOR ANY BUDJET, SO GRATEFUL !!!! , ,LOVE THIS CHANNEL, , , THANKYOU FROM MY HEART!!!

[bobwadsworth]

I wanted to see it in action with some smoke to see how well it draws it in and how well it filters. A follow up maybe This Old House??

[opiateL]

Very good discussion. Many people do not understand the problem with restricting air to the AC unit. We're told to change our filter to keep air flowing but then sold a highly restrictive filter. 🤔

[geronimolopez]

I really like the back to back simple conversation style, it helps understand what's really important about the subject

[TomTerrific]

I use one good filter 20 x 25 up against a box fan. Simple, but it works great.