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

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.

Running a positive pressure system in my house since many years, I agree that sucking in fresh air from outside and filter it with a HEPA filter makes more sense.

You probably need to make sure you use a fan that creates enough pressure to get the air through the trickle vent as well as a filter. A duct fan might be more suitable than a PC fan for this.

The pressure drop across a HEPA filter is a function of flow rate per unit area (i.e. velocity), and it can be very very low if the velocity is low. You only need some 15 cfm per person, which isn't very much. The real issue that that a HEPA filter that will achieve extremely low pressure drop is rather bulky.