Initial DIY cleanroom experimentation | Hacker News Reader

Initial DIY cleanroom experimentation

jefftk.com

29 pointsluu23 comments

I live in an apartment over a busy road, and my building doesn’t have forced-air ventilation to the units, so I need to pull in fresh air somehow. Just leaving the windows open results in black soot accumulating in the carpets (and is probably why both my SO and my cat developed asthma while living here.) Regular air purifiers (we have two) only do so much.

What ended up making a difference for us, is buying a portable industrial air scrubber, of the kind construction/remediation sites use to clean the smoke and dust (and mold, and asbestos, etc) they’re generating out of the air before it gets vented outside (see e.g. https://a.co/d/01lTtyuZ); and plumbing this machine into our apartment in reverse: putting it out on our (covered) balcony, with it exhausting the “scrubbed” air into the apartment. (I made a little sliding-door shim of the type people use to pipe portable AC units outside, just with the hose on the other side.)

Generally works great. We run it on a very low speed, so it’s surprisingly quiet (no louder than other fans we have for air circulation, and a more pleasant frequency because of the large diameter of the fan it has inside.) The pre-filters get absolutely caked with exhaust and road dust after ~2wks, and start to let through an odd stench at that point, so we’re actually replacing those faster than even is recommended for the industrial use-case. But the actual main HEPA filter inside the unit seems to last longer than in industrial usage.

Cars are the best form of transportation ever invented.

Your prefilters only lasting two weeks, plus two household members developing asthma, might make me think very seriously about moving somewhere less polluted, and quickly.

You could probably improvise something cheaper using the same filter arrangement, though now that you have it, sounds like the best thing to do might be just including an extra sacrificial initial filter to increase the lifespan of the proper ones.

The machine's pre-filters (https://www.amazon.ca/AlorAir-MERV-10-Replacement-CleanShiel...) basically are that "extra sacrificial initial filter." I'm not sure what a "pre-pre-filter" would look like that would make it any different than one of these.

Were you maybe thinking some kind of wire mesh, like the "washable pre-filters" in air purifiers tend to be? I did initially consider this; but, after running the air scrubber for a few months and seeing what actually builds up in the pre-filter, it seems like the pre-filters don't actually end up being coated in anything (dust/hair/etc) that can actually be rubbed/scratched away or even vacuumed out of them. Rather, just end up gradually blackening (due, I think, to fine [PM2.5] particulate making it partway through them before gradually embedding deep inside them); and gradually feeling more and more "skudgy" to the touch (due perhaps to some of those particulates being hygroscopic, and due to also trapping things like pollen/mold spores.) You can actually touch the surface of the fouled pre-filter and your fingertip will come away clean (but touching it won't be a nice sensory experience.)

I think, for helping with that kind of particulate, a wire mesh wouldn't do much if anything. The only "passive" thing that would help is a non-woven cross-linked fiber material, porous enough to not overwork the air scrubber's fan to the point of making it work harder / be louder / burn out sooner, yet layered and "internally labyrinthine" enough to capture particulate despite that porosity. I.e. the same material used in the pre-filter.

(If I could add an active component, I might introduce an intake-side ionizing air purifier, where particulate gets charged and then preferentially deposits onto the fins of a washable heatsink-like grille. AFAIK, those don't normally work at the sort of CFM being pushed by the air scrubber. But if anyone wants to design a better mousetrap here, that might be a direction to look into.)

There was some talk on here during Covid about the feasibility of making all structures up to BSL4 standards. Seems kind of nuts, but it would certainly contain a lot of biosafety threats. With AI it now seems borderline urgent.

I was very disappointed that COVID did not inspire more interest in indoor air quality, especially in public spaces.

I don't know how general it is, but my dentist and kid's school kept the HEPA filters after covid. It's at least... a little better?

(Speaking personally, my home indoor air quality is now awesome and I love it. Wish it hadn't taken a pandemic to clue me in, though.)

Agreed. It was literally the wake up call for the fact that air quality standards are as important as soap and handwashinhg are, and yet... nothing happened.

And so the question of what effects getting rid of several-yearly respiratory infections would have on our health and the development of children remains open.

AirFanta creator Adam Wong has a version using a 3D-printed adapter to 8" circular duct.[0]

In newer builds he places the purifier box indoors and simply reverses the orientation of the fans and filters. This keeps the box protected from weather, and the duct can be run in such a way as to keep out rain or dripping.

The AirFanta is actually rated as EPA E11 (95%) not HEPA H13 (99.97%), so if you're trying to keep out everything in one single pass like a clean room then AirFanta might not be best. It may be a good idea to check the real-world interception performance with a particle counter.

[0] https://x.com/Engineer_Wong/status/2034098395376455946

https://www.reddit.com/r/crboxes/comments/1ef3dly/positive_p...

An ERV with HEPA filter may be a better, less improvised option. Most systems can be adjusted for positive pressure, although the intent is balanced airflow.

If the airflow isn't balanced, there's little point to using an ERV. If half the air is escaping elsewhere, none of that heat is being recovered. You might as well delete the ERV and just use half the airflow, because that will get the same positive pressure for the same energy loss.

What if it's only 1% of air escaping

Then it's 100x oversized for the purpose of pressurizing the structure. ;)

ERVs are great for increasing air exchange (two-way flow), but unnecessary when pressurizing because that's one-way flow by definition.

If you already bought an ERV it makes sense to combine the functions, simply because you already bought the expensive ERV hardware. But if you weren't planning to install an ERV anyway, it doesn't make sense to install one just for this project, because it won't do anything to reduce the heating/cooling loss from this type of system.

The one-way component of flow is still 100% non-recuperated. If you install a cheap 100 CFM positive pressure system and it burns an extra $100, then setting your ERV to be unbalanced by 100 CFM will cost that same $100. Same leak rate, same heat loss, same cost!

This is one of those deceptive math problems where it seems like it should work, but really it works out "six of one and half-dozen of the other."

I didn't understand the purpose as pressurising the structure. I understood the purpose as ventilating the interior, providing a central place to attach a filter, and preventing air from coming in from other places.

Yes, exactly! If you're ventilating the interior then using an ERV makes sense. If you're preventing air from coming in from other places, then the ERV doesn't save anything, because (intentionally) 100% of the incoming airflow is leaking out somewhere, so there's nothing for the ERV to do.

I edited my post above to hopefully make it a bit more clear, but it seems you get it right away! Cheers

Right but they didn't want to stop air coming in. They just wanted all incoming air to be filtered.

  >they didn't want to stop air coming in

They did, though! The entire point of positively pressurizing a structure is that any small leaks will instead result in clean air leaking out, which prevents dirty air from leaking in. This is how (most) cleanrooms work.

That's so the one central point of air ingress is the one with the filter. Air still gets in - through the filter.

Bingo.

The neat thing is, if you're already accepting this heat loss penalty anyway (in the service of positive pressure), it greatly reduces or even eliminates the ERV you need to maintain good indoor CO2. So in addition to better indoor AQI, positive pressure can greatly reduce total equipment cost.

Seems like a win/win to me!

Buckminster Fuller proposed a similar scheme in his Dymaxion House (even removing dust via baseboard vents), but it used passive heating and cooling so there was little penalty.

Are you an AI? You just spent several comments arguing against something you now claim to agree with.

Huh? No, my position has always been that an ERV doesn't make sense to combine with this project unless you wanted or have an ERV in your home anyway (and actually this can replace an ERV in some cases). This remains unchanged.

No AI of course.

My recommendation was mostly for the practical, not functional benefits.

An ERV comes with installation instructions, has a warranty, was designed to run continuously, and an HVAC contractor can install it for you. 3D printing brackets and cutting holes in your own walls isn’t for everyone.

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