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Comment by lifeisstillgood | Hacker News Reader

UV-C lights that irradiate an air duct is literally the most surprising (and in retrospect obvious) thing I learnt today.

Is this something a household HVAC could be upgraded to. Do HNers in areas that have air con think it's a "yeah people will want that" idea?

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If DIYing, choose your UV wavelength wisely. UVC (around 254nm wavelength) is germicidal and doesn't produce ozone (it actually dissociates ozone). "Vacuum UV" (in the range 185nm) converts oxygen in the air to ozone, which is highly toxic.

[1] http://www.uvresources.com/blog/the-ultraviolet-germicidal-i...

Also the whole skin cancer thing.

The point of the parents' posts is that you have these issues with ozone even if you just irradiated the air ducts without direct contact to UVC.

Hopefully the UV light is blocked from escaping the device.

I always wondered just how much UV-C do you need to effectively kill the virus (power vs duct diameter and air speed).... because, instead of traditional masks, you could take a small fan, blow air into a pipe with a UV-C bulb inside, and pump the sterilized air to the human... basically something like this: https://www.youtube.com/watch?v=fzGrh2io9Ds&app=desktop

I tried looking into that this spring. I suspect the answer is for this virus not much. Nothing to indicate that any part of it's life cycle involves hanging around latent.

Interesting bit on how UV-C works to sterilize.

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

Interesting thing bacteria and some viruses have DNA repair enzymes that can fix damage from UV-C. Medically you're always bedeviled by bacterial and fungal spoors.

Limitations with UV-C sterilization I think prevents wider use. Not a magic bullet. Though it probably works well for influenza, colds, and this bat virus.

The real question is is there a safe wavelength and low enough power where you could swap out fluorescent tubes and effectively manage the virus.

I was imagining some kind of doorway device - but yeah, that's a really interesting approach.

Again, what is safe .. but it the infrastructure is there ...

Yes, far UVC, 222 nm, won't penetrate your skin far enough to get to living cells and can be used around humans safely. It's a new technology, though, and not widely deployed.

Also light has very complex interactions with various of our biological symptoms, and I personally would be wary of unintended consequences without long term testing. e.g. it could interfere with sleep, or have other subtle effects

Any HVAC company can install this for you right now, or you can buy a kit and do it yourself. A UV light bulb to install in your central air system is an off the shelf product. "Whole house UV air purifier" as a search term will pull these up.

Are these products actually effective though? I mean the duration and intensity required to kill viruses in moving air must be quite high? Or is it just snake oil, like how a lot of hand dryers have bluish/purple LEDs (which emit no UV) that shine on your hands?

I can’t speak to their effectiveness in air, but they work very well for e.g. bacteria in rapidly flowing water. Aquariums have been using them for as long as I can remember.

Yes, they're effective. But near-UVC is dangerous to your skin and eyes, so it absolutely must be shielded. Far-UVC is quite safe to humans, and just as effective against coronaviruses, but it's rather expensive at the moment.

https://www.researchsquare.com/article/rs-25728/v1

It's important to remember that your immune system is improved by coming in contact with common viruses. This is even more important for kids.

There's evidence that people that have been exposed to previous, weaker, coronaviruses are more likely to have mild responses to Covid-19, for example.

https://science.sciencemag.org/content/370/6512/89

It isn't very practical, as long as you don't worry about getting the virus from the outside in, which would be an unusual thing to worry about for home owners.

Just vent it out in the open air and not on people.

>Just vent it out in the open air and not on people

Terrible for energy efficiency

That's what an ERV or HRV is for: https://en.wikipedia.org/wiki/Energy_recovery_ventilation https://en.wikipedia.org/wiki/Heat_recovery_ventilation

Et voilà, now it's no longer terrible for efficiency since you're recovering 70-90% of the energy. For fixed-plate exchangers there's no cross-contamination, either.

There are plenty of products for both HVAC and plumbing that use UV-C. The new hotness is 405nm because it’s safe around humans and there are already under cabinet lights for kitchens and overhead recessed lights for bathrooms, etc.

What magnitude of irradiance is required?

If the required power is,say, 500w continues, or more, that's going to get quite expensive to run.

For example, where I live running a 500w device 24/7 is going to add an extra $1100 a year to the power bill.

Looking at germicidal bulbs at 1000bulbs.com, the highest wattage bulb they have is 65 watts [1]. Most are well under that.

Looking at random in-duct systems, it looks like they range from 1 to 10 or so bulbs with the later being for commercial buildings. Home systems seem to lean toward 1 or 2 bulbs of maybe 20ish watts or so total. This site [2] says typically $15-30 per year energy cost

[1] https://www.1000bulbs.com/product/113917/AU-LG64T5L.html

[2] https://www.pickhvac.com/faq/hvac-uv-lights/

Nope. I would much rather have a good filter than mechanically removes infectious particles, wildfire smoke, and particulates in general.

For a residential recirculating system, the relevant parameter is flow rate times fraction of particles removed in one pass. An AC or heater generally has a high flow rate and needs a filter to keep the heat exchanger and fan clean regardless. The added material and energy cost of a good filter is low.

On the other hand, if you are moving air from one space to another and don’t want to contaminate the space you’re moving air to, the absolute filter / sterilizer efficiency may matter. Things like HEPA may be useful here.

I would be surprised if a UV lamp is cost effective for new installation. For retrofit in a system that can’t handle a better filter, maybe.

Germicidal irradiation via UV light on HVAC is just not possible. Due to the amount of energy it requires to be effective is ridiculously high.

https://www.climask.com.tr is interesting. Like a face mask on AC unit. Nano-silver is effective against bacterias and viruses. And having a filter with nano-silver coated surface is just a simple yet an effective approach. Cost-wise changing a good quality air filter every 2-3 months is much cheaper than having a UVGI installed and paying higher electricity bill every month.

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