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Show HN: I built an under-the-door fan duct to lower bedroom CO2

(www.lepekhin.com)
64 points bizzz | 2 comments | 24 Jun 25 14:17 UTC | HN request time: 0.788s | source

Hi HN,

This is my attempt to fix groggy mornings by lowering my bedroom CO2 from 1700 ppm to ~900 ppm.

I designed a simple, 3D-printed fan duct that directs air under the door to improve air circulation.

A note on the design process: I used CadQuery, a Python library for creating parametric 3D models. I didn't know tools like this existed and highly recommend it if you can program; it turned out to be more convenient than manually iterating on prototypes in a GUI.

The project is open source and the post explains the full build. Happy to answer any questions.

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amluto ◴[24 Jun 25 18:47 UTC] No.44369469[source]▶
Given the effort involved (unusually-shaped 3D printed duct and housing for the whole gizmo), ISTM it would make a lot more sense to fit the duct to the trickle vent and operate it in positive pressure mode. This would have lots of advantages:

- With positive pressure, the intake air doesn't pick up contaminants from the wall cavities and other nooks and crannies within the building envelope.

- A fresh air supply fan can be fitted with a high-quality filter (HEPA or near-HEPA) to supply air with approximately zero PM2.5. (This is a very, very effecive way to get indoor air that is cleaner than the outdoor air without substantial power consumption.)

- You can open the door without moving the fan! You can leave it on with the door open! And it may improve the air quality in the rest of the living space more than the under-the-door scheme.

In a very cold climate, it might make more sense to put something like this in a different room so that the unconditioned supply air could be mixed with conditioned indoor air farther from where anyone is trying to sleep.

For what it's worth, there are a couple vendors of paired decentralized ERVs that alternate which one is supply and which one is exhaust and blow all the air through energy recovery cores that buffer both temperature and humidity. They're intended to be installed in holes in the walls, but it ought to be possible to fit them to windows or trickle vents with some creativity to operate them without any permanent modifications to the structure. In a climate with serious weather, energy recovery can substantially improve comfort and efficiency compared to using unconditioned supply air.

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1. bizzz ◴[25 Jun 25 06:13 UTC] No.44374107[source]▶
Thanks for the detailed thoughts! You've given me a lot to chew on for the next version of this project.

I did briefly look at ERVs, but for a single room in an apartment building with central heating, that level of energy recovery is probably overkill. I'd be simulating the result of keeping the window slightly open, I don't mind the cold it's even better for sleep (and the heat is not a problem here).

But your core points about the system's location are a real lightbulb moment for me. Having the fan up by the window instead of as a trip-hazard on the floor would be a huge improvement.

I can remove the housing for the trickle vent. It's screwed on, so I can put it back later without violating my rental agreement. Then, I could 3D print a custom adapter to fit that opening perfectly, designed to hold one of the Noctua fans and a small HEPA filter like you suggest.

That feels like a much more elegant solution. Thanks for the inspiration! You've got the gears turning again.

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2. amluto ◴[25 Jun 25 13:50 UTC] No.44377343[source]▶
I would consider a less-than-HEPA filter, too — it might be a better balance of pressure drop and filtration. IKEA will sell you an “UPPÅTVIND” for $15, which supposedly blocks 99.5% of particles.