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A 20 × 20-inch box fan fitted with a properly sized MERV 13 (or higher) filter can reduce airborne wildfire-smoke particles in a closed room. The safest basic build uses a fan manufactured in 2012 or later with a UL or ETL mark, a filter attached to the fan’s intake side, and sealed edges so air cannot bypass the filter. A cardboard shroud or additional filters can improve measured cleaning performance, but every homemade unit varies and should be used as one part of a broader cleaner-air plan.
What you need
Gather the parts before assembling the cleaner. EPA’s published designs use a nominal 20 × 20-inch box fan and filters that match the fan’s intake dimensions. Measure the actual fan grille and filter because nominal sizes can differ slightly between products.
- Box fan: Choose a 20 × 20-inch model manufactured in 2012 or later with a UL or ETL safety marking. Do not use a damaged, cracked, overheating or malfunctioning fan.
- HVAC filter: Use one 20 × 20-inch MERV 13 or higher filter. A 2- to 4-inch-deep filter may offer better airflow and particle removal than a one-inch filter when it fits your fan, room and budget.
- Sealing and fastening material: Use duct tape, clamps or bungee cords to hold the filter tightly against the fan and close gaps.
- Cardboard: Save a clean, rigid piece large enough to cover the fan’s front corners if you plan to make a shroud. Multi-filter designs need additional cardboard supports or panels.
- Scissors or a utility knife and a marker: These are useful for marking and cutting the shroud. Cut on a stable surface and keep blades away from the fan cord and grille.
Look for the airflow arrow printed on every filter. It must point in the same direction as the fan’s airflow: from the room, through the filter, and into the fan. A filter installed backward can restrict airflow or reduce the intended performance.
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The basic configuration is the quickest to assemble and needs the least floor space. It is also the reference design against which EPA compared shrouded and multi-filter versions.
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- Unplug the fan. Place it on a level surface with the intake side facing you. The intake is the side that draws room air in; on many box fans, the motor and support struts are more visible on the exhaust side.
- Inspect the filter. Confirm that its face covers the intake opening and that the airflow arrow points toward the fan.
- Position the filter flat against the intake. Keep the filter pleats straight and avoid crushing the frame. If the filter does not cover the opening, do not rely on tape to bridge a large mismatch; obtain a correctly sized filter.
- Fasten all four sides. Apply duct tape around the perimeter, or use clamps or bungee cords that hold the frame firmly without bending it. Leave the fan’s controls and cord accessible.
- Seal bypass paths. Look around each corner and along the filter frame. Cover visible gaps so the fan cannot pull substantial air around the filter. Do not cover the motor vents or obstruct the fan blade.
- Check stability. Stand the unit where it cannot tip, and make sure the filter cannot fall into the blades if a fastener loosens.
- Test at low speed first. Plug the fan into a properly grounded outlet, listen for rubbing or rattling, and inspect the filter for movement. Turn it off immediately if the cord, plug, housing or motor becomes unusually hot or if you smell an electrical odor.
During a smoke event, run the cleaner as often as practical while people occupy the room. Higher fan speeds generally move more air and clean faster, but they also create more noise and air movement.
Add a cardboard shroud for more effective airflow
A shroud reduces the amount of air that recirculates around the fan’s corners instead of passing through the blades. EPA’s tests found a higher wildfire-smoke clean air delivery rate (CADR) for a shrouded one-inch-filter design than for the same basic design without a shroud.
- Measure the fan face. Place the cardboard over the front of the fan and mark the outer edges. Keep the cardboard clear of controls, handles and the power cord.
- Mark a circular opening. Center the opening over the blade area. The opening should expose the usable blade region without cutting into the fan housing or leaving an unnecessarily large gap.
- Cut the opening safely. Remove the cardboard circle with a utility knife or scissors while the cardboard is away from the plugged-in fan. Reinforce weak edges with tape if they flex.
- Attach the shroud to the front. Tape or clamp the four corners and perimeter to the fan frame. The shroud should remain flat and secure when the fan is running.
- Check clearances. Make sure no cardboard can touch the blades, block motor vents or cover the fan’s controls. Run the fan briefly at low speed and watch for vibration.
EPA reported a 40% CADR increase when its tested basic configuration received a cardboard shroud. That percentage describes the agency’s tested units and conditions, not a guaranteed increase for every home-built cleaner.
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More filter area or a deeper filter can increase particle-cleaning capacity, but the trade-offs are higher material use, a larger footprint, more assembly and potentially more noise. The following values are EPA measurements from selected tests of simulated wildfire smoke; the uncertainty shown is the reported measurement uncertainty. They are not promises for a particular fan, room or assembly.
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| Configuration tested by EPA | Wildfire-smoke CADR | What changes | Best fit |
|---|---|---|---|
| One 1-inch MERV 13 filter, no shroud | 111 ± 1 | Smallest and simplest design; baseline for the comparison | Limited space, lowest assembly effort |
| One 1-inch MERV 13 filter with cardboard shroud | 156 ± 4 | Same filter with a cut-and-attached shroud | Most people who want a low-cost performance improvement |
| One 4-inch MERV 13 filter with cardboard shroud | 248 ± 15 | Deeper filter and shroud; needs more clearance behind the fan | Higher performance without adding multiple filter panels |
| Two 1-inch MERV 13 filters with cardboard shroud | 263 ± 22 | More filter surface and a wider support structure | Rooms with additional floor or table space |
| Four 1-inch MERV 13 filters with cardboard shroud | 401 ± 31 | Large cube-like assembly with four filter faces | Maximum tested performance among these designs when space and materials allow |
EPA also reported that the tested shrouded single 4-inch-filter design produced a 123% higher CADR than the tested basic single-filter design. Because fan model, filter resistance, sealing, room layout and maintenance all affect results, use these numbers to compare design direction rather than to predict the air changes in your room.
When the basic design is enough
Use the one-filter build when you need a cleaner quickly, have little floor space or want the fewest parts to inspect. A shroud is a sensible next step if you can cut and secure cardboard without blocking the fan. The basic build is not automatically inferior for every room: a correctly sealed, well-maintained small unit may be more useful than a larger cleaner that cannot be placed safely or operated consistently.
When to use a deeper filter
A 4-inch filter provides more pleated media than a one-inch filter and had a substantially higher CADR in EPA’s shrouded test. Confirm that the filter frame does not press against the fan blades, that the fan remains stable, and that the added depth does not block a wall, doorway or air return.
When to build a two-filter wedge or four-filter cube
Two-filter wedges and four-filter cubes expose more filter surface to the fan. Match the filters, orient every airflow arrow toward the fan, and seal each joint. These builds require supports that keep the panels square and prevent them from collapsing into the fan. They are harder to move, consume more filters, and occupy more space than a single-filter unit. A four-filter cube produced the highest CADR in the EPA table, but that result came from a specific tested assembly and should not be treated as a universal rating.
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How to assemble a two- or four-filter design
- Plan the footprint. Lay the matching filters on the floor and decide where the fan will sit. Keep the exhaust side open and leave room to reach the controls.
- Orient every arrow. Arrange all filters so their airflow arrows point toward the fan’s intake. Mark the intended orientation with a pencil before fastening anything.
- Build a rigid frame. Use cardboard panels, folded flaps or other nonflammable supports to hold the filter frames at the intended angle. The supports should bear the structure’s weight rather than the filter pleats.
- Join the filter frames. Tape or clamp the edges together without crushing the cardboard filter frames. Avoid placing tape where it could be sucked into the fan.
- Seal the seams. Close visible gaps between filters and around the fan intake. Air should have a clear path through filter media, not through open corners.
- Secure the fan. Prevent the fan from sliding or tipping as vibration changes with speed. Do not suspend the assembly from the power cord.
- Run a staged test. Start at low speed, check for movement and rubbing, then increase speed while watching the supports. Stop if any panel flexes toward the blades.
Safety rules that matter
EPA and UL evaluated five fan models under selected conditions. In those tests, component temperatures stayed below recognized safety limits and no observable fire hazards occurred. The agencies caution that the test does not represent every fan, filter combination or operating condition.
- Use only a fan with a UL or ETL mark and a manufacture date of 2012 or later. EPA’s concise instruction is: “Only use certified fans with UL or ETL marking (2012 models or newer).”
- Follow the fan manufacturer’s instructions and keep the motor housing, vents and controls unobstructed.
- Keep the cleaner away from curtains, bedding, paper piles, heaters, water and other objects that could block airflow or contact the motor.
- Place it on a stable, level surface with enough clearance for the exhaust air to circulate. Do not wedge it tightly against a wall.
- Never operate a unit with a frayed cord, damaged plug, cracked housing, loose blade or persistent overheating.
- Do not leave a cardboard shroud or filter assembly where it can be pulled into the blades. Recheck fasteners after moving the cleaner.
- Use a smoke alarm and follow local wildfire evacuation instructions. An air cleaner does not make a fire-threatened building safe.
Operate it as part of a cleaner-air room
A box-fan cleaner reduces airborne particles; it does not remove every pollutant or replace evacuation, ventilation decisions or medical care. EPA’s cleaner-air-room guidance recommends combining filtration with source control.
- Choose one room where people can spend time and keep doors and windows closed when outdoor smoke conditions make that appropriate.
- Reduce indoor particle sources such as smoking, candles, incense, frying and activities that stir dust.
- Run the DIY cleaner whenever the room is occupied, using the highest speed people can tolerate. Lower speed may be a practical compromise when noise interferes with sleep or communication.
- If the building has forced-air heating or cooling, use a compatible high-efficiency HVAC filter according to the equipment manufacturer’s requirements. Do not install a filter that the system cannot move air through safely.
- When available and affordable, consider a commercially manufactured portable cleaner with a measured CADR. EPA says those ratings make performance more standardized than it is for homemade units.
Keep the unit in the room where it is needed. Moving one small cleaner repeatedly between rooms gives each room less operating time than leaving it running continuously in a selected cleaner-air room.
Filter inspection, replacement and disposal
Wildfire smoke can load a filter quickly. Inspect it frequently during smoke events instead of waiting for a fixed calendar interval.
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- Inspect the filter face: Replace it when the media looks visibly dirty, when airflow drops noticeably, or when the filter begins to smell like smoke.
- Do not rely on a dirty filter: EPA found that heavily dirty filters made tested DIY cleaners almost completely ineffective.
- Remove it gently: Turn off and unplug the fan, then loosen tape or fasteners slowly. Avoid shaking the filter, which can release collected material.
- Bag it: Place the used filter in a disposal bag, close the bag, and wash your hands afterward. Follow local waste rules for household filters.
- Recheck the assembly: After installing a new filter, confirm the airflow arrow, seal and fan stability again.
Troubleshoot common problems
The fan is loud
Noise rises with fan speed and with the amount of air being moved. Set the unit on a stable surface, check for loose cardboard or tape, and make sure no filter frame is touching the blades or grille. If the room cannot tolerate the highest setting, use a lower speed continuously rather than switching the cleaner off for long periods.
Air seems to bypass the filter
Turn the fan off and inspect all four filter edges, shroud corners and multi-filter seams. Add tape, clamps or bungee tension where the frame is not flush. Do not seal motor vents or create a tight enclosure around the motor.
The filter collapses or bows inward
Stop the fan and add rigid support around the filter frame. A deeper filter or a multi-filter structure may need a separate cardboard brace. Replace a damaged filter rather than attempting to hold torn media in place.
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The unit vibrates or moves
Check that the fan and supports sit level, the shroud is centered and no panel is loose. Reduce speed until the cause is corrected. A moving unit can open bypass gaps or tip into nearby objects.
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The room still smells smoky
Close outdoor air pathways when safe, eliminate indoor smoke sources and verify that the filter is not saturated. Odor can persist in fabrics and surfaces even after airborne particles fall, so filtration alone may not remove every smell. If smoke conditions worsen, use public health guidance and evacuation instructions rather than trying to compensate with a larger homemade fan.
How to decide which build to make
| Your priority | Recommended direction | Trade-off |
|---|---|---|
| Fastest assembly and smallest footprint | One 1-inch MERV 13 filter, sealed to the fan | Lowest EPA-tested CADR in this comparison |
| Better performance with little extra material | Add a cardboard shroud to the single-filter build | Requires careful cutting and secure attachment |
| Higher performance without four separate filters | Use one shrouded 4-inch MERV 13 filter | Deeper, heavier and potentially more expensive; current retail prices are not established here |
| More filter surface and available floor space | Build a two-filter wedge | More supports, seams and filter replacements |
| Highest value in the EPA table | Build a four-filter cube with a shroud | Largest footprint, greatest material use and most assembly effort |
There is no single best configuration for every home. Choose the largest design you can place safely, seal well and run often. A smaller unit that operates continuously in an occupied room may be more useful than a larger unit that is too noisy, unstable or difficult to maintain.
What the measurements do—and do not—tell you
CADR, or clean air delivery rate, combines airflow and particle-removal performance into a measure of how quickly a cleaner can supply particle-free equivalent air. EPA’s figures came from controlled testing of simulated wildfire smoke and specific fan, filter and shroud combinations. They help show the effect of a shroud, deeper media and additional filter area, but they are not a certification for every home-built device.
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Actual results depend on the fan, filter resistance, filter condition, seal quality, room size, ceiling height, placement, outdoor smoke leakage and operating speed. The numbers also do not establish removal of gases or odors. Treat them as comparative evidence for selecting a design, not as a promise that your room will reach a particular particle concentration.
Bottom line
For most readers, start with a 20 × 20-inch UL- or ETL-marked box fan made in 2012 or later, a matching MERV 13 or higher filter, and a fully sealed intake. Add a cardboard shroud if you can attach it securely. Move to a deeper or multi-filter design only when the extra footprint, noise, material cost and maintenance are acceptable. Run the cleaner in a closed cleaner-air room, inspect the filter throughout the smoke event, and keep evacuation and public-health guidance ahead of any DIY equipment.
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