Let’s be real—good air purifiers aren’t cheap. The better ones, especially those designed for wildfire smoke or allergens, often cost over $300. That’s a lot to ask when you’re trying to stay healthy during a bad air day, especially if you’re renting, on a tight budget, or just hate spending unnecessarily.
Now, what if you could get similar results using something you might already own—a twin window fan? Add a couple of furnace filters, some tape, and a little patience, and you’ve got a homemade air filtration system that filters outdoor air before it even gets inside your home.
This isn’t just about saving money. It’s about being in control. Whether you’re dealing with wildfire smoke drifting into your bedroom, city smog, seasonal pollen, or even musty old-house dust, this setup gives you an easy and surprisingly effective way to clean your air—without overcomplicating things or relying on high-end gear.
What You’ll Need to Make Your Window Fan Air Filter
You won’t need much to get started. Most of what’s required is affordable and easy to find at your local hardware store or online. Here’s your basic parts list:
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Twin window fan: These usually come with two side-by-side blades. Holmes, Bionaire, and Genesis are all solid brands. If you already have one, that’s perfect.
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Two 20″x 20″ furnace filters: Look for filters with a MERV 13 rating. These are designed to catch fine particles like smoke, bacteria, and pollen. If your fan struggles with airflow, MERV 11 filters are a good middle ground.
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Duct tape or industrial Velcro: For attaching the filters to the fan securely. Tape creates a tighter seal, but Velcro makes filter changes easier.
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Weather stripping or foam tape (optional): Helps fill gaps between the filters and fan frame for better sealing.
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Scissors or a utility knife: You might need to trim the filters slightly depending on your fan model.
If you live somewhere with frequent wildfire alerts or pollen seasons (like California or the Southeast), you may want to keep extra filters on hand. They clog up faster when the air is really dirty.
How to Build Your DIY Window Fan Air Filtration System (Step-by-Step)
You don’t need any engineering skills to do this—just follow these steps:
Step 1: Set your fan to “intake” mode
This means the fan will pull outside air into the room. Most twin fans have a switch or a button that controls the airflow direction. Make sure the air is coming through the filters before entering your home.
Step 2: Line up the filters on the back (intake side)
That’s the side of the fan that will be facing outside when placed in your window. Position one filter over each fan blade section. Ensure that the airflow arrow printed on the filter points into the fan.
Step 3: Secure the filters tightly
Use duct tape around all four edges of each filter. You want an airtight seal, so air is forced through the filter rather than sneaking around it. If you prefer a reusable option, industrial Velcro works well but may not seal as tightly.
Step 4: Fill in gaps
If there are spaces where air might slip around the filters (common in older fans), seal them with weather stripping or foam tape. Even small leaks can let in unfiltered air, defeating the purpose.
Step 5: Plug in and power on
Once your filters are firmly attached and everything is sealed, turn on the fan. Start at medium speed and increase if needed. Within a few hours, you’ll likely feel a difference in your air quality—less stuffiness, fewer odors, and even a drop in dust levels.
🛠️ Pro tip: Place the fan in a window that doesn’t face a major road or chimney, especially during heavy smoke or pollution events. Cleaner air in = cleaner air inside.
Where to Place It: Intake vs. Exhaust Setup Explained
This part confuses people sometimes, but it’s simple once you understand the goal.
Use “intake” mode if you want to filter outdoor air as it comes in. This is the most common use—especially during wildfire season or high pollen days when you still want fresh air but don’t want to invite in pollutants.
Use “exhaust” mode to push indoor air out, possibly through filters, to create negative pressure and remove stale or polluted indoor air. This works best when paired with other filtered intake sources.
If you’re trying to clean up one specific room, say your bedroom or home office, go with intake mode and keep the door closed. The fan will slowly replace the room’s air with filtered air from outside.
Which Filter Should You Use? MERV 13 vs MERV 11
Let’s talk filters—because the quality of your setup depends heavily on this choice.
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MERV 13 filters are considered ideal. They capture smoke particles, fine dust, pet dander, mold spores, and even some bacteria and viruses. They’re the best pick if you’re dealing with smoke from wildfires, poor urban air quality, or if anyone in your home has asthma.
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MERV 11 filters still do a good job. They catch pollen, larger dust particles, and pet hair, and allow for better airflow, especially if your fan isn’t very powerful. They’re a great compromise for those worried about motor strain or noise from restricted airflow.
A quick test: If your fan starts making a loud whirring noise or airflow drops drastically when you attach MERV 13 filters, try switching to MERV 11. It might just be the better match for your fan model.
What Kind of Results Can You Expect From a DIY Fan Air Filter?
Let’s manage expectations but also give credit where it’s due. A DIY twin fan air purifier won’t beat a $600 HEPA purifier, but it does do a surprisingly good job at tackling real problems—especially with high-quality filters and proper sealing.
Here’s what you’ll likely notice within a day or two of use:
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Less dust is collected on surfaces, especially near the fan.
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Improved breathing comfort, particularly for allergy or asthma sufferers.
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Reduced lingering odors—cooking smells, smoke, pet funk, etc.
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Less eye or throat irritation if you’re dealing with wildfire smoke.
If you live near wildfires or industrial areas, you might even see your filter turn gray or brown in a couple of days. That’s gross—but it’s proof that the filter is working and keeping all of that junk out of your lungs.
One user who lives in Oregon during fire season said their room smelled better, felt cleaner, and their chest tightness disappeared within hours of running their homemade setup. That kind of feedback has been echoed in dozens of DIY communities and Reddit threads.
How to Maintain Your Fan and Filter Setup
Maintenance is easy, but it matters. Even the best filters clog up, especially when they’re working hard.
Here’s how to stay on top of it:
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Inspect the filters every 2–3 weeks during high smoke or pollen seasons. If they look dark or dusty, it’s time to replace them.
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Replace filters every 30–90 days depending on use. More if you live in a clean area, less if you’re battling heavy pollution or smoke.
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Wipe down the fan blades and outer casing once a month. This helps the fan last longer and keeps the airflow strong.
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Unplug the fan when not in use to save energy and avoid dust buildup inside the motor housing.
Some filters now come with change indicators—a little strip that turns color when it’s time to swap. Handy if you don’t want to guess.
Other DIY Air Purifier Alternatives Worth Considering
This twin fan air filter setup is great for people who want clean air without spending hundreds, but there are other DIY options worth knowing about:
🔹 Corsi-Rosenthal Box
Uses a box fan + four to five MERV 13 filters in a cube shape. Offers massive air cleaning for larger rooms. Bulkier and louder, but highly effective.
🔹 Single-Filter Box Fan Setup
Just a 20” box fan with one furnace filter taped to the back. Not as strong as the twin fan, but it works in a pinch or smaller spaces.
🔹 HEPA filter hack
If you have an old HEPA filter lying around from another purifier, you can sometimes attach it to a window fan—just make sure the airflow still moves easily.
Each method has trade-offs—noise level, filter availability, energy usage—but all can improve your indoor air quality significantly when done right.
What to Avoid (Common Mistakes to Watch Out For)
It’s easy to mess this up without realizing it. These common slip-ups will reduce how effective your system is—or even break your fan:
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Using HEPA filters designed for machines instead of HVAC filters – HEPA filters are too restrictive for most box or window fans. Stick with MERV 11–13 HVAC filters.
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Mounting filters backward – Every filter has an airflow direction arrow. If you install them backward, they won’t work properly.
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Forgetting to seal the sides – Gaps between the filter and fan let unfiltered air in. Use tape or foam to block them.
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Running the fan on high speed only – Strong airflow is great, but it can reduce filtration effectiveness. Medium speed is usually ideal.
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Reusing old filters indefinitely – A dirty filter becomes a clogged filter, and eventually just a dirty decoration. Don’t wait too long to replace.
Is It Worth It? Our Verdict After Testing and Research
Yes—worth it for most people. If you’re in a smoke-prone area, allergy season, or just trying to breathe cleaner without shelling out for premium machines, this homemade filtration system punches far above its weight.
What makes this project so great is its flexibility. You’re not stuck with one setup. You can:
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Change filter ratings based on need (MERV 11 vs. 13).
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Customize the airflow direction.
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Adapt it to different fan brands or window shapes.
You control the cost, the materials, and the output. And when you breathe easier afterward? That’s the win.
What’s Next? Take Action Now
Now that you know how to build and maintain a reliable DIY twin window fan air purifier, the next step is simple: start gathering the materials today. If you already own a twin fan, you’re halfway there.
Grab a couple of MERV 13 filters, some tape, and give it a go. Even if you’re skeptical, this is a low-cost, low-risk project that might dramatically improve how your home feels—especially during smoky or allergy-heavy months.
You’ve got the knowledge. Now it’s time to put it to use. Clearer air might be just one DIY away.