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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteBuild a DIY wildfire-smoke air cleaner with a 20-by-20-inch box fan, a same-size MERV 13 (or higher) HVAC filter, and a careful seal. Attach the filter to the fan’s intake side, confirm the airflow arrow points toward the fan, tape or clamp every gap, and add a cardboard shroud over the fan’s front corners. The result can substantially reduce airborne particles in a room when operated continuously, but performance varies by fan, filter, construction and maintenance. Use it as one part of a cleaner-air plan—not as a substitute for closing windows, reducing indoor smoke sources or using a certified portable cleaner when one is available and affordable.
What you need
The basic cleaner uses commonly available parts. EPA’s dimensions are a starting point; measure the actual fan and filter before assembling because nominal sizes can differ.
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- 20-by-20-inch box fan: Choose a model manufactured in 2012 or later with a UL or ETL safety mark. EPA’s guidance states: “Only use certified fans with UL or ETL marking (2012 models or newer).” Do not use a damaged, overheating or malfunctioning fan.
- One 20-by-20-inch HVAC filter: Select MERV 13 or higher. A one-inch filter works for the basic build; a two- to four-inch filter can improve performance if it fits the fan and your budget.
- Cardboard: Use a clean, rigid piece large enough to cover the fan’s front corners for a shroud. Multi-filter designs need additional cardboard supports or panels.
- Sealing and fastening supplies: Duct tape, clamps or bungee cords can hold the filter securely. Tape is also useful for sealing leaks around the frame.
Check the arrow printed on the filter. It must point in the direction air travels through the fan—from the room, through the filter, then through the fan and back into the room. Installing it backward increases resistance and defeats the intended filtration path.
The Tool Desk
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- Inspect and position the fan. Unplug the fan while assembling it. Place it on a stable, level surface with the intake side facing the room and enough clearance for air to enter. Keep the cord and controls accessible.
- Fit the filter to the intake. Center the 20-by-20-inch filter on the back of the fan. If the filter frame does not sit flush, position it so the entire intake area is covered without blocking the motor housing or controls.
- Confirm the airflow direction. Read the arrow on the filter frame and point it toward the fan. If the arrow is difficult to see, use the manufacturer’s printed airflow marking rather than guessing from the filter’s appearance.
- Secure the filter. Fasten it with duct tape, clamps or bungee cords. The attachment should hold the filter against the fan during operation without crushing the filter media or blocking the fan blades.
- Seal the perimeter. Run tape around visible gaps between the filter frame and fan. Air follows the path of least resistance; unsealed openings allow smoke to bypass the filter.
- Test before a smoke event. Plug in the fan, start on a low setting and listen for rubbing, rattling or unusual heat. Stop immediately if the fan behaves abnormally. Once it runs normally, check that air is being pulled through the filter rather than around its edges.
This configuration is the simplest and usually takes the least floor space. Its cleaning rate depends on the specific fan, the filter’s resistance and depth, the quality of the seal, room size and operating speed.
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Add a cardboard shroud for better airflow use
A shroud covers the fan’s front corners and leaves a circular opening over the blade area. It reduces the amount of air that recirculates around the blade tips and encourages more useful airflow through the filter.
- Place a piece of cardboard over the front of the fan, covering the four corner regions.
- Mark a circular opening centered over the blade area. The opening should expose the fan’s effective outlet without touching the blades or obstructing the grille.
- Cut the opening carefully and remove loose scraps that could enter the fan.
- Secure the shroud to the fan frame with duct tape or another firm fastening method. Keep the controls, cord and motor vents accessible.
- Run the fan briefly at low speed, then inspect the shroud for flexing, contact with moving parts or loose edges. Reinforce it before using a higher speed.
In EPA’s controlled testing, a shrouded one-inch MERV 13 design produced a higher wildfire-smoke CADR than the same basic design without a shroud. The agency’s published results show why sealing and airflow control matter, although a home-built unit will not necessarily match those measurements.
Choose the configuration that fits your room
More filter area can increase cleaning performance, but it also raises material cost, footprint and assembly effort. A thicker filter can offer more media in roughly the same footprint. Higher fan speed generally cleans faster while producing more noise and air movement.
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|---|---|---|
| Box fan with one 1-inch MERV 13 filter | 111 ± 1 | Smallest, simplest baseline; least filter area. |
| One 1-inch MERV 13 filter with cardboard shroud | 156 ± 4 | Same filter footprint with a shroud; EPA’s result was about 40% higher than its tested basic design. |
| One 4-inch MERV 13 filter with cardboard shroud | 248 ± 15 | Uses a deeper filter and shroud; more material depth but no additional side panels. |
| Two 1-inch MERV 13 filters with cardboard shroud | 263 ± 22 | More filter surface and a wider assembly; requires support and careful sealing. |
| Four 1-inch MERV 13 filters with cardboard shroud | 401 ± 31 | Highest value in EPA’s table; occupies substantially more floor space and needs more construction. |
These are EPA measurements from selected fan, filter and assembly conditions, not guarantees for every homemade cleaner. EPA reports a 123% increase for its shrouded single four-inch-filter design versus its tested basic single-filter design. Differences in fan model, filter brand, dimensions, seal quality, room conditions and filter loading can change the result.
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One-filter, one-inch build
Choose this when you need the lowest material use, smallest footprint or fastest assembly. It is a sensible emergency option if smoke is arriving and you already have the parts. Add a shroud and seal carefully to get more from the same fan.
One-filter, four-inch build
Choose a deeper filter when the fan can support it without blocking controls or creating an unstable stack. EPA’s tested shrouded four-inch design had a CADR of 248 ± 15, higher than its one-inch shrouded design. A four-inch filter may cost more and be harder to fit securely, but it provides more filter media without the width of a multi-filter cube.
Two-filter wedge
A wedge uses two matching filters arranged at an angle behind the fan, usually with cardboard sides or a triangular support. It increases filter area while remaining lower and narrower than a four-filter cube. Make sure both filters’ arrows point toward the fan, support the joints, and tape all seams. The assembly must not sag into the blades or motor.
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A cube surrounds the fan with four or more filters, leaving the fan’s outlet open. It provides substantially more intake area and can reduce the resistance each filter presents to the fan. The trade-off is a larger footprint, more filters to buy and replace, more joints to seal, and a heavier structure that needs stable support. Use matching MERV 13 filters and align every airflow arrow toward the fan.
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How to compare cost, performance, space and noise
| Priority | Most suitable choice | Trade-off |
|---|---|---|
| Lowest initial material use | One 1-inch filter, preferably with a shroud | Less filter area and lower tested CADR than deeper or multi-filter designs. |
| Higher performance in the same general footprint | One 4-inch filter with a shroud | Deeper, potentially more expensive filter; fit and stability must be checked. |
| More performance when floor space is available | Two-filter wedge or four-filter cube | More filters, supports, sealing work and storage space. |
| Least assembly effort | Single filter attached to the fan | Requires especially careful edge sealing; adding a shroud takes another step. |
| Lower operating noise | Use a lower fan speed or a design with more filter area | Lower speed reduces air cleaning rate; large builds still produce motor and airflow noise. |
Current retail prices are not established by EPA’s guidance and vary by location, filter depth and availability. Compare the total cost of the fan, filter(s), tape and replacement filters rather than the fan alone. Do not substitute a smaller filter simply because it is cheaper; an uncovered intake area can create bypass leakage.
Safety requirements and placement
- Use a newer certified fan. Select a fan manufactured in 2012 or later with a UL or ETL mark. Follow the fan maker’s instructions and electrical requirements.
- Inspect before every smoke event. Do not operate a fan with a damaged cord, cracked housing, seized blade, burning smell or intermittent power.
- Keep airflow paths clear. Leave space around the filter intake and fan outlet. Do not push the unit against a wall, curtain, bed, pile of clothing or other obstruction.
- Prevent tipping. Put the cleaner on a level surface. Multi-filter cubes and wedges can be top-heavy; add stable cardboard supports and keep them away from children and pets.
- Protect the motor and controls. The filter and shroud must not cover motor vents, switches or the electrical connection. Never allow cardboard or tape to touch moving blades.
- Watch temperature and sound. Stop the fan if it becomes unusually hot, smells electrical, sparks, vibrates excessively or makes a new grinding noise.
EPA and UL tested five fan models under selected conditions. EPA reports that component temperatures stayed below recognized safety limits and that no observable fire hazards occurred in those tests. That testing does not represent every fan, filter, assembly or operating condition, so it is not a blanket safety guarantee for homemade units.
Operate the cleaner during wildfire smoke
- Prepare a cleaner-air room. Choose a room where people can spend time with windows and doors closed when it is safe to do so. Place the DIY cleaner where air can circulate freely.
- Start early. Turn it on when smoke begins to affect outdoor air, not only after indoor odor becomes strong. Run it as often as possible while the room is occupied.
- Use the highest practical speed. A higher setting generally increases particle removal but also increases noise and air movement. If noise prevents people from staying in the room, use a lower setting rather than turning the unit off.
- Limit indoor sources. Avoid smoking, candles, incense, frying and other activities that add particles. Keep windows and doors closed when outdoor conditions and ventilation needs allow.
- Check the room, not just the fan. A cleaner can reduce airborne particles but cannot remove every smoke gas or prevent smoke entering through leaks. Follow local wildfire and health advisories.
EPA generally prefers commercially available portable air cleaners with measured CADR when they are available and affordable because their performance is more standardized. A DIY unit is useful when a certified cleaner is unavailable or unaffordable, provided it is built and operated safely.
Filter inspection, replacement and disposal
Wildfire smoke can load a filter quickly. Inspect it frequently during a smoke event. Replace it when the media looks dirty, airflow has fallen noticeably, or the filter begins to smell strongly of smoke. EPA reports that dirty filters made tested DIY cleaners almost completely ineffective.
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- Turn the fan off and unplug it.
- Support the filter before releasing tape, clamps or bungee cords so it does not fall or shed particles.
- Remove it gently. Do not shake, brush or blow out the used filter.
- Place it directly into a bag, seal the bag and dispose of it according to local waste rules.
- Wash your hands after handling the used filter.
- Install a fresh filter with the airflow arrow again pointing toward the fan, then reseal the edges.
Do not wash a disposable HVAC filter unless its manufacturer specifically identifies it as washable. Reusing a loaded filter can sharply reduce airflow and cleaning effectiveness.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Troubleshoot common problems
The fan is loud or vibrates
- Confirm the shroud or cardboard is not touching the blades.
- Check that the fan is level and the filter is secured without pressing against the housing.
- Inspect for a damaged blade or loose grille. Stop using the fan if the vibration persists.
Smoke odor remains
- Check every filter edge and tape gap for bypass leakage.
- Make sure the filter arrow points toward the fan.
- Replace a visibly dirty or smoke-smelling filter.
- Close windows and doors when appropriate and reduce indoor particle sources. A particle filter does not make a room airtight or remove every odor-producing gas.
Airflow seems weak
- Inspect the filter for loading and replace it if dirty.
- Remove objects blocking the intake or outlet.
- Check whether a dense or overly deep filter is restricting a fan that cannot handle it.
- Look for gaps that let air bypass the filter; bypass can make the fan sound normal while providing little filtered airflow.
The cardboard structure is unstable
- Add cross-braces or side panels and place the unit on a level surface.
- Reduce the configuration from a cube to a wedge or single filter if it cannot stand securely.
- Keep the assembly away from children, pets, curtains and walkways.
Use the DIY cleaner as part of a broader smoke plan
A box-fan cleaner addresses airborne particles in the room where it runs. It does not stop smoke from entering, replace outdoor-air decisions, or provide the same standardized performance as a portable cleaner with a published CADR. During severe smoke, combine the unit with a designated cleaner-air room, closed windows and doors when safe, reduced indoor emissions, and a compatible HVAC filter if your central system can use one.
People with asthma, heart or lung disease, children, older adults and pregnant people may be more vulnerable to wildfire smoke. Follow public-health alerts and seek medical advice for breathing difficulty, chest pain, severe coughing or other concerning symptoms. The cleaner is a practical particle-reduction tool, not medical protection.
Frequently asked questions
Can I use any box fan?
No. Choose a fan manufactured in 2012 or later with a UL or ETL safety mark, and do not use one that is damaged or malfunctioning. Safety testing covered selected models and conditions, not every fan or homemade configuration.
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Does a higher MERV rating always make the DIY cleaner better?
MERV 13 or higher is the recommended target for this design. A higher-resistance filter can reduce airflow on some fans, so match the filter depth and resistance to the fan, keep the seal tight, and monitor airflow and temperature.
Which design should I build if I have very little space?
Use one 20-by-20-inch filter attached to the fan and add a cardboard shroud if you can. A four-inch filter can improve the tested CADR without adding the width of a multi-filter cube, but verify that the fan remains stable and unobstructed.
How often should I replace the filter?
There is no universal interval because smoke concentration, fan speed and filter size differ. Inspect it frequently and replace it when it looks dirty, airflow drops or it smells like smoke. A loaded filter can make the cleaner almost ineffective.
Can I leave the cleaner running unattended?
Use the fan according to its manufacturer’s instructions and inspect the assembly before operation. Do not run a unit that is unstable, obstructed, overheating or showing electrical or mechanical problems. Extra caution is appropriate for cardboard multi-filter structures.
Will this remove all wildfire-smoke pollution?
No. It can reduce airborne particles in the room, but results vary and it does not seal the room or remove every smoke constituent. Keep outdoor smoke out when possible, reduce indoor sources and use a measured-CADR portable cleaner when available and affordable.
Bottom line
A properly sealed 20-by-20-inch fan-and-MERV-13 filter is a low-cost way to reduce wildfire-smoke particles when a certified portable cleaner is not practical. Add a shroud, choose a deeper or multi-filter design when space permits, run the fan at the highest tolerable speed, and replace loaded filters promptly. Safety-rated equipment, stable construction and a cleaner-air room matter as much as the filter itself.
Quick Recap
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