Are DIY Box Fan Filters Effective?
- Dan Dearden
- Sep 9, 2025
- 3 min read
Updated: Sep 16, 2025
And how do they compare to the NovusAer?
By Dan Dearden, CAFS – Certified Air Filtration Specialist
When I first saw the Corsi-Rosenthal box fan filter design, I thought, “That’s clever. That could actually work.” And it does.
The reason is something called CADR – Clean Air Delivery Rate.
CADR is the number that tells you how much clean air a purifier can deliver. It’s calculated by multiplying:
Filter efficiency (how much pollution it captures) ×
Airflow (how much air moves through it).
For example, if you have a portable air purifier with a HEPA filter (99.97% efficiency) and a fan that moves 180 cubic feet of air per minute (CFM), its CADR would be 180.
CADR matters more than just the filter rating. A filter that’s “super efficient” but barely moves any air won’t do much to actually clean your home.
What’s in a DIY Box Fan Filter?
The Corsi-Rosenthal design is basically a box fan attached to a cube of MERV 13 filters.
At first glance, you might think:
“A box fan moves 2,240 CFM, and a MERV 13 filter is 85% efficient—so CADR should be 1,904!”
But in real life, things don’t work that neatly:
Filters restrict airflow. A box fan rated for 2,240 CFM can only move about 1,113 CFM once filters are attached.
Lab ratings don’t equal real-world results. MERV 13 filters often perform more like MERV 11–12 in home use, because air doesn’t flow evenly across the whole filter surface.
When we built and tested one ourselves, the result came out to a CADR of 834.
That’s actually excellent. It’s enough to clean the air in just about any room in your house.
For comparison:
The largest certified portable purifier on the market tops out at 800 CADR and costs around $1,399.
Our DIY model cost just $105.
So yes—DIY box fan filters are very effective for single rooms.
The Downsides of DIY Box Filters
Before you run out to build one, keep in mind:
They only clean one room at a time. To cover your whole house, you’d need one in every main space.
They’re noisy. About 70 decibels at 8 feet away (roughly like a vacuum).
They use more power. Around 57 watts each. Six units (enough for a typical home) would cost about $523 per year in electricity.
Filters don’t last long. Expect to replace them every 3 months—around $319 per year.
They can’t fight infiltration. Homes “breathe”—air leaks in and out. Only a whole-home system can address that.
DIY Box Fan vs. NovusAer Whole-Home System
It’s not exactly apples-to-apples, because the DIY box filter is a room purifier (PRAP) while the NovusAer is a whole-home purifier (WHAP).
Still, here’s how they compare:
Feature | DIY Box Fan Filter | NovusAer Filter |
Whole-home air cleaning | ✗ | ✓ |
Works in any home | ✓ | ✗ (works in most homes with central air) |
Initial Investment | $105 | $3,500 |
CADR | 834 | 1,400* |
Watts per CADR (efficiency) | 15 | 4 |
Yearly electricity (to clean full home) | $523 | $0** |
Filter replacement cost (annual) | $319 | $166 |
Noise @ 8 ft | ~65 dB | Silent |
Filter life | 3 months | 3 years |
Mitigates infiltration (house “breathing”) | ✗ | ✓ |
10-Year Total Cost of Ownership | $7,688 | $5,016 |
Medium-size NovusAer model NS14* Uses the home’s existing circulation system
Bottom Line
If you want an affordable way to clean the air in one room, a DIY box fan filter is a fantastic option. It’s cheap, powerful, and easy to make.
But if your goal is whole-home clean air, long-term savings, and quiet efficiency, the NovusAer system wins. Over 10 years, it actually costs less to own than running multiple DIY units—and it handles problems that no room-by-room purifier can.
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