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

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

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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:
20-inch box fan []
Four 20x20x2 MERV-13 air filters []
Duct tape []
Cardboard cutout []

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?
Ross built the air filter using four, 20x20x2, MERV-13 air filters, which Ross got from Filterbuy ( ). He duct taped the filters to a cardboard box and a 20-in 3-Speed Box Fan, which is manufactured by Lasko ( ), though any 20” fan would work for this application.

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

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