Building a Box-Fan Air Cleaner (MERV 13)

What it's for

On smoke days, you want one room in the house with cleaner air. The EPA says a do-it-yourself air cleaner made from a box fan and a MERV 13 furnace filter is a cost-effective way to cut smoke particles indoors. Our list's smoke-day air cleaner is either this build or a HEPA purifier. The Fixer builds it before the season, not during.

Buy a portable air cleaner if you can. The EPA says commercial units with a known clean air delivery rate (CADR) are usually preferred, and that DIY units vary more with how they're built and are often louder. If you buy one, the EPA says choose a CARB-certified model (little or no ozone) with a CADR for tobacco smoke of at least two-thirds of the room's square footage.

Before you need it (practice)

Supplies (EPA, simple design):

• A 20" x 20" box fan with a UL or ETL mark, 2012 model or newer. The EPA says newer certified fans have thermal fuses that cut the risk of overheating.
• A 20" x 20" furnace filter, MERV 13 or higher. The EPA says thicker is better: 2 to 4 inches deep rather than 1.
• Duct tape, clamps, or bungee cords.
• Cardboard for a shroud (the fan's box works).
• Spare filters. The EPA says keep extras on hand.

Build it in the off season and run it for an hour to check noise and fit.

Step by step

Simple one-filter build (EPA)

1. Check the filter's airflow arrows. Air should go through the filter first, then into the fan. Put the filter on the fan's intake (back) side, pleats vertical, arrows pointing toward the fan.
2. Attach it with duct tape, clamps, or bungee cords.
3. Seal every gap so air goes through the filter, not around it.
4. Make a shroud. Tie a string to a pencil, measure from the fan's center to the tip of a blade, draw that circle on cardboard, and cut it out. Tape the cardboard over the front of the fan so air only moves through the circle where the blades are. The EPA says this no-cost step raised performance by about 40% in its tests.
5. Run it in your cleaner-air room with doors and windows closed (see Smoke Days at Home).

Bigger builds (EPA)

• Wedge: two filters taped into a V against the fan, cardboard triangles sealing top and bottom.
• Cube (Corsi-Rosenthal style): four or five filters taped into a cube with the fan on top, arrows pointing in. The EPA says this had the highest clean air delivery rate of the designs it tested. With five filters, set it on sturdy blocks so air can get in the bottom.

How the designs compared (EPA tests, wildfire-smoke CADR)

Common mistakes

• Filter on backwards. Arrows point toward the fan.
• Gaps around the filter edges.
• An old fan running overnight. The EPA says if you must use an older fan, never leave it running while you're away or asleep.
• A dirty filter. EPA testing found DIY units were almost completely ineffective with smoke-loaded filters.
• Blocking it. The EPA says keep it clear of obstructions.

Care & storage

• Change the filter when it looks dirty or starts to smell (EPA). During a long smoke event that may be often.
• Store the fan, shroud, and spare filters together in a dry spot, with the tape.
• The EPA notes air cleaners are only part of the plan; they don't replace ventilation when the air outside improves.

Related gear

• Smoke-day air cleaner (box-fan filter or HEPA purifier) · Duct tape
• See also: Smoke Days at Home · Respirators and Mask Fit
• Category: Air & Smoke · Role: Fixer

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