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.

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.

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.

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.

Thank you both! I'll start with a small research into available HEPA filter sizes, pressure drops they entail, and noise vs power ratio of available fans. Then I'll see if any combination fits my trickle vent setup and theoretically gives enough air as an output.

Achim, founder of AirGradient here.

Nice to wake up and see a hacker news post benefiting from our open source air monitor designs, especially with the original design that we started with so many years ago! Makes me realize how much AirGradient developed since these early days.

Did you consider linking the fan with the CO2 data and making this a demand controlled system? Like you control the speed (or just switch on / off the fan) based on CO2 levels?

I think this could have a few advantages in terms of energy savings, noise level etc.

Achim, AirGradient was the key that unlocked this whole project for me, so thank you for creating it. Also, I see that you're still so active in the forums helping users (including me) out, double kudos!

That's a great suggestion about making it a demand-controlled system! It's definitely on the long-term roadmap. Right now, though, my system is still a bit underpowered for that. I'm in an "all fans on" mode just to get the numbers down to a barely reasonable level. If I could easily get the CO2 to a steady 500-600 ppm, then absolutely, dialing it back automatically would be the next logical step.

So, my immediate focus would be on improving the raw efficiency. Following a great suggestion in this thread (https://news.ycombinator.com/item?id=44369469), I plan to design a new mount to put a fan directly on the trickle vent itself to actively pull fresh air in. It feels like a more direct approach, and I'm curious to see what that does to the numbers.

Generating negative pressure in a room isn't great for fresh air. You'll be sucking air through any opening- wall outlets, floor registers, ceiling light fixtures, gaps in baseboard trim, etc.

Might be simpler to augment your fresh air intake with a fan.

Good point for standalone houses! I live in an apartment, so everything you listed is backed by some concrete blocks. So, realistically speaking, the air in any meaningful amounts can only come through the window/trickle vent of the door.

You'd be surprised where air can leak in. Your house/building is not a closed system. You should assume that any/all wall penetrations will infiltrate air, via electrical wiring/conduit, through insulation, plumbing, hvac ducting.

Thank you, I did not consider it, but looks like positive pressure is a best practice and I'll be looking into that (mounting fans directly onto the trickle vent).

I had no idea CO2 could mess with mood like that, makes sense though. Definitely something I need to try out. Thanks for sharing, and nice project btw!

I wonder what the market size for this is. If it were a real product that wasn't super ugly in the bedroom, we'd use it. My BR is ~1500 ppm with two sleepers in a 10x12 room, and opening the window isn't an option.

Heh, never really thought there's a market for it. Though now I can imagine it being sold at some TV shopping channel. A solution to a problem you never knew you had.

Anyway, I think there are plenty of people who do not want to open windows and still have fresh air. But the amount of people who can't modify their room or for whatever reasons don't want to open their bedroom door is bound to be much less.

Now I'm curious, why don't you open your bedroom door? Me, I'm a light sleeper and other apartment dwellers will wake me up if I open the door.

We don't like to open the door because of habit, light leaking, and noise.

Software people will do anything but learn to use traditional CAD...

Just kidding. But would you mind sharing what you find so compelling about CadQuery?

As a mechanical person, these code to CAD tools make as much sense to me as suggesting you should write programs with a compass and straight edge.

I have used OpenSCAD not CadQuery, to me (as a vim person) the appeal is not having to do a bunch of mousework. But it doesn't hold up for long, at some level of complexity it is certainly easier to just go learn SolidWorks/OnShape/Fusion.

Mainly, that all the data (constraints etc) are discoverable in one place instead of hidden behind various gui features. Also being able to use a battle tested version control system like git instead of whatever ball of duct tape comes with the tool.

I've used OpenSCAD for a few small projects. The main draw for me is avoiding all the "fiddly mouse stuff". I figure it's a trade-off between simple stuff being more complicated, and complicated stuff being easier to bulk edit and organize.

Cool project. Will need to do similar soon at new place to deal with tech in a cupboard for heat.

900 is still quite high though - upper end of normal for indoor

Thanks! Yep, the goal is stable 500-600 ppm; now thinking about making it into a positive pressure setup (https://news.ycombinator.com/item?id=44374107).

Or just open a window, you'll also sleep better

Yeah, if you can do it, I'm all for it!

Its 95 degrees outside. Ill take the AC.

I don't want to use Python if I can help it, OpenSCAD is another "CAD as code" language. How does it compare?

there are other possible marketing oportunities for this, and other gasses. "The remaining trace (<1% volume) compounds contribute to the odor of flatus. Historically, compounds such as indole, skatole, ammonia and short chain fatty acids were thought to cause the odor of flatus. More recent evidence proves that the major contribution to the odor of flatus comes from a combination of volatile sulfur compounds.[2][28] Hydrogen sulfide, methyl mercaptan (also known as methanethiol), dimethyl sulfide, dimethyl disulfide and dimethyl trisulfide are present in flatus. The benzopyrrole volatiles indole and skatole have an odor of mothballs, and therefore probably do not contribute greatly to the characteristic odor of flatus."

https://en.wikipedia.org/wiki/Farts

Thanks for sharing, this is interesting to me.