# Ceiling Air Purifier

Imagine a high-capacity air filter that is extremely quiet and doesn't get in the way. I think I see how to build one; here's a prototype:

[![](https://www.jefftk.com/ceiling-fan-air-purifier-prototype.jpg)](https://www.jefftk.com/ceiling-fan-air-purifier-prototype-big.jpg)

By putting a fan on the ceiling you can have very long blades. This is important for keeping noise down: the longer your blades are the more air you move at a given rotational velocity, which is a major determinant of noise. Testing my prototype, it has a CADR of ~180 CFM and is only 33dB. By contrast, the Wirecutter's top-recommended [air purifier](https://www.nytimes.com/wirecutter/reviews/best-air-purifier/) ([Coway AP-1512HH](https://smile.amazon.com/dp/B01728NLRG/)) has a CADR of 233 CFM (31% higher) at 54 dB (21dB louder) or 110 CFM (38% lower) at 36 dB (3dB louder). With some tweaks it should be able to match the commercial purifier's performance, without being louder.

Volume is very important here: people often select purifiers based on their maximum flow, but then run them on a lower setting or even turn them off because they are too loud.

[![](https://www.jefftk.com/ceiling-air-purifier-from-below.jpg)](https://www.jefftk.com/ceiling-air-purifier-from-below-big.jpg)

Note that while my purifier is an octagon of 12x20 filters around a 52" fan, a nonagon would have been better. Not only should that get you 12% more filtration, but you don't have to fill gaps between the filters.

[![](https://www.jefftk.com/ceiling-fan-filter-and-person.jpg)](https://www.jefftk.com/ceiling-fan-filter-and-person-big.jpg)  
*Jeff for scale*

One downside of using this much tape, especially some of it with the sticky side bare, is that you get an adhesive smell. The smell fell below where I could detect after about a day, but Julia could still smell it for about five days.

To determine the CADR of the prototype I followed the approach I described in [Testing Air Purifiers](https://www.jefftk.com/p/testing-air-purifiers), where I created smoke by burning matches and then tracked how quickly the purifier could reduce pm2.5 levels:

[![](https://www.jefftk.com/mighty-vs-ceiling-decay.png)](https://www.jefftk.com/mighty-vs-ceiling-decay-big.png)

[![](https://www.jefftk.com/mighty-vs-ceiling-derivative.png)](https://www.jefftk.com/mighty-vs-ceiling-derivative-big.png)

To determine volume levels, I used a Sennheiser 835s dynamic microphone with a foam windscreen, connected to an AudioBox 1818VSL to record samples of each purifier option and silence. Here are the samples:

([Silence](https://www.jefftk.com/silent-room-sample.wav))  
[![](https://www.jefftk.com/silent-room-spectrogram.png)](https://www.jefftk.com/silent-room-spectrogram-big.png)

([Ceiling Purifier](https://www.jefftk.com/ceiling-fan-high-sample.wav))  
[![](https://www.jefftk.com/ceiling-purifier-high-spectrogram.png)](https://www.jefftk.com/ceiling-purifier-high-spectrogram-big.png)

([AP-1512HH Medium](https://www.jefftk.com/purifier-medium-sample.wav))  
[![](https://www.jefftk.com/purifier-medium-spectrogram.png)](https://www.jefftk.com/purifier-medium-spectrogram-big.png)

([AP-1512HH High](https://www.jefftk.com/purifier-high-sample.wav))  
[![](https://www.jefftk.com/purifier-high-spectrogram.png)](https://www.jefftk.com/purifier-high-spectrogram-big.png)

To compute the loudness of each sample I ran:

ffmpeg -i sample.wav -af\
  loudnorm=I=-16:dual\_mono=true:TP=-1.5:LRA=11:print\_format=summary\
  -f null -

This gives loudness in LUFS, which is K-weighted dB. I don't have any calibrated tooling, but I can report values in loudness units (LU) relative to silence. Trusting the manufacturer's rating of 53.8 dB for the AP-1512HH on High, however, lets us determine absolute loudness for the other options:

| Sample | Relative | Absolute |
| --- | --- | --- |
| Everything Off | normalized to 0 LU | 25 dB |
| Ceiling Purifier High | 7.3 LU | 32.6 dB |
| AP-1512HH Medium | 10.8 LU | 36.1 dB |
| AP-1512HH High | 28.5 LU | 53.8 dB |

I think it ought to be possible to get even better performance out of the ceiling purifier by building a [shroud](https://www.texairfilters.com/how-to-improve-the-efficiency-of-the-box-fan-and-merv-13-filter-air-cleaner/). The ideal shroud is generally quite a bit narrower than I would guess: I've seen [people measuring](https://twitter.com/DavidElfstrom/status/1427792853636460544) peak performance with only a 14" hole for a 20" fan. They report ~40% more airflow without increased noise, but I would want to run tests and verify that here.

With ~12% improvement from switching to nine filters and ~40% from better shrouding, this ought to get us from a CADR of ~180 CFM to ~280 CFM. Even if we only get half that improvement, a CADR ~230 CFM gets us even with the commercial purifier, at far lower volumes.

In terms of cost, materials for one fan are nine MERV-14 filters for [$110](https://smile.amazon.com/dp/B007KPY0VS) plus ~$5 for tape. The ceiling fan was a [Hunter Mariner 52"](https://www.homedepot.com/p/Hunter-Mariner-52-in-Indoor-Outdoor-White-Ceiling-Fan-59127/205223776), which seems to cost $106 now, though it's not something I installed for this project. A real-product version of this would replace the tape with a frame you attach to the ceiling, into which you could slide filters. Maybe $100? If someone wanted to build and sell something I'd be pretty happy to see that!

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