DIY Air Filter: How to Build Your Own Air Purifier

In an era of increasing air quality concerns, from wildfires to airborne viruses, many homeowners are looking for ways to improve indoor air filtration. Professional-grade air purifiers can be expensive, but there’s a cost-effective DIY solution that can help clean the air in your home.

In the video above, This Old House home technology expert Ross Trethewey demonstrates how to build a simple yet effective DIY air filter using a box fan and four MERV 13 filters. In the guide below, we’ll give you the background information you’ll need to understand your new filter and then help you complete the project yourself.

Understanding Air Filtration and MERV Ratings

MERV stands for Minimum Efficiency Reporting Value, a scale that measures how effectively an air filter captures particles of various sizes. The MERV scale ranges from 1 to 20, with higher numbers indicating better filtration.

• MERV 1-4: Captures large particles like pollen and dust mites
• MERV 5-8: Traps mold spores and pet dander
• MERV 9-12: Filters out fine particles like legionella
• MERV 13-16: Captures bacteria and smoke particles
• MERV 17-20: Filters out viruses and the smallest detectable particles (typically used in hospitals and clean rooms)

Most residential HVAC systems use filters with MERV ratings between 5 and 12. However, for improved air quality—especially if you’re worried about smoke and airborne viruses—a MERV 13 filter is recommended. Ross explains that MERV 13 filters can capture particles as small as 0.3 to 1 micron, which is about 1/50th the size of a human hair.

Can’t I Just Upgrade My HVAC Filter?

While it might seem logical to simply upgrade your existing HVAC filter to a higher MERV rating, doing so can cause a few problems.

• Increased air resistance: Higher MERV filters have a denser network of fibers to capture smaller particles. This structure creates more airflow resistance—like trying to breathe through a thick cloth compared to a thin one. 
• Reduced airflow: As the air resistance increases, your HVAC system has to work harder to push the same amount of air through the filter, which often means less air flows through the system.
• Coil freezing: Your AC functions by removing heat from the air as it passes over the cooling coil. Without adequate airflow across the coil, moisture from the air may condense and freeze on the coil’s surface, leading to ice buildup.
• Motor damage: The increased air resistance caused by a high MERV filter also puts strain on the HVAC system’s motor, as it has to work harder to maintain airflow. This extra wear and tear can cause the motor to overheat or even fail.

Before upgrading your HVAC filter, we recommend consulting with an HVAC technician. They can assess your system’s capabilities and determine if it can handle a higher-rated filter without compromising performance or causing damage.

Building Your Own DIY Air Filter

If upgrading your HVAC system isn’t feasible, creating a DIY air filter can be an effective alternative. This project, inspired by the Corsi-Rosenthal Cube, or Comparetto Cube, uses readily available materials to create a portable air purifier.

Materials Needed

To build your DIY air filter, you’ll need the following items:

• One 20-inch box fan
• Four 20x20x2 MERV-13 air filters
• Duct tape
• Cardboard cutout (20×20 inches)

Step-by-Step Instructions

Follow these steps to assemble your DIY air filter:

1. Prepare the cardboard: Cut out a piece of cardboard to match the size of the box fan (20×20 inches).
2. Arrange the filters: Place the cardboard on a flat surface and arrange the four MERV-13 filters around the edges, with the airflow arrows pointing inward.
3. Secure the filters: Use duct tape to secure each filter to its adjacent filters, creating a cube shape.
4. Attach the cardboard: Tape the cardboard backing to the bottom of the filter cube.
5. Position the fan: Place the box fan on top of the filter cube, ensuring it’s centered.
6. Seal the fan: Secure the fan to the filter cube using duct tape, making sure there are no gaps.

In the demonstration video, Trethewey highlights the importance of proper filter placement (with the airflow arrows pointing inward) and secure taping to create a tight seal.

Enhancing Your DIY Air Filter

Here are a few ideas to make your DIY air filter even more convenient and effective:

• Filter protection: Consider adding pre-filters to capture larger particles before they reach the MERV-13 filters. This step can prolong the life of your primary filters and increase overall efficiency.
• Noise reduction: Box fans can be noisy. To reduce noise, you can place foam padding around the fan or use a quieter, more efficient fan model.
• HEPA filtration: For those looking to achieve even higher air quality, incorporating HEPA filters into your DIY setup can provide an added layer of protection. This step is particularly useful for those with severe allergies or respiratory conditions.

Using Your DIY Air Filter

Your new Comparetto Cube air purifier can be surprisingly effective. This DIY setup can move approximately 900 cubic feet of air per minute on the fan’s highest setting. In a typical 2,500-square-foot home with 8-foot ceilings, this equates to about three air changes per hour.

Tips for Optimal Use

Though this DIY air purifier won’t provide whole-house coverage, it is portable. That means you can leave it in your living room or home office during the day, then move it to your bedroom at night. 

To get the most out of your DIY air filter, consider these practical tips:

• Placement: Position the air filter in a central location to maximize air circulation. Avoid placing it in corners or near obstructions that could hinder airflow.
• Run time: For optimal performance, run the air filter continuously, especially during wildfire seasons, allergy seasons, or when air quality alerts are issued in your area. This keeps the air in your home consistently filtered.
• Fan Speed: Adjust the fan speed based on the room size and air quality needs. Higher fan speeds will filter more air but may also be noisier.

Care and Maintenance

Trethewey recommends replacing the filters approximately every six months, depending on usage and air quality conditions. It’s a good idea to check the filters periodically. If they appear dirty, replace them sooner.

Comparing DIY and Commercial Air Purifiers

Comparing DIY air filters to commercial air purifiers will help you decide if this project is a good fit for your needs.

• Cost: DIY air filters are generally more affordable than commercial units—especially compared to a professionally installed whole-home air purifier. You can recreate this DIY setup for $100–$150, while mid-range or high-end commercial air purifiers can cost hundreds of dollars.
• Effectiveness: Commercial air purifiers often come with advanced filtration technologies, including HEPA filters and activated carbon filters, which can capture a broader range of pollutants. 
• Maintenance: DIY air filters require regular filter replacement and occasional inspection to ensure they are functioning correctly. Commercial units may have longer-lasting filters and indicators to alert you when a replacement is needed. 
• Portability: Both DIY and commercial units can be portable, but DIY filters are often lighter and easier to move around due to their lightweight design. However, commercial units may come with additional features like wheels or handles for added convenience.
• Customization: DIY air filters offer a high level of customization, allowing you to tailor the design to your specific needs. 

Our Conclusion

Building a DIY air filter is a practical and affordable way to improve your home’s air quality. While it may not replace a professional-grade air purification system, it can help remove smoke particles and potentially harmful aerosols. With simple materials and easy assembly, this project provides a quick solution for cleaner, healthier indoor air.