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

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

#21

Post by bizzz »

amluto wrote: post #7

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…

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.

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

#22

Post by bizzz »

amluto wrote: post #17
ahaucnx wrote: post #14

Earlier quoted context omitted.

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.

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

#23

Post by bizzz »

ahaucnx wrote: post #16

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…

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.

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

#24

Post by bizzz »

turtlebits wrote: post #11
bizzz wrote: post #8

Earlier quoted context omitted.

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).

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

#25

Post by bizzz »

Havoc wrote: post #18

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).

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

#28

Post by amluto »

bizzz wrote: post #21
amluto wrote: post #7

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…

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…

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.

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

#30

Post by sprobertson »

nancyminusone wrote: post #12

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.
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