UV doesn't know good bacteria from bad.

That said, UV death for bacteria requires a certain exposure time, and it's unlikely that any bacteria flying around in your airstream will get killed by a single light. Bacteria on surfaces exposed to the UV light most likely will get killed, but if they're stuck on a surface, then they're not likely to harm you anyway.

On the third hand, what good bacteria are you expecting to be flying around in your HVAC?

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This was discussed a while back for covid.

https://www.eng-tips.com/viewthread.cfm?qid=466759

Basically it is possible but not practical as you need 40 to 50 seconds exposure to UV-C light.

HEPA filters are much better.

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One thing that stuck bacteria can do is reproduce - one source of Legionaire's Disease was dampness in a ventilation system that allowed a large number to collect.

I am unsure of studies about helpful bacteria that are airborne. Gut bacteria for sure but nothing for lungs.

There are good bacteria on your skin and in your body (actually more than you have cells). That is why the overuse of disinfectants is so bad for you (because after killing good bacteria, there is room for bad bacteria).

There are no air-borne bacteria the human body needs (is in symbiosis with). So this is a moot request. There also is no selective killing method. I suspect larger bacteria with more robust skin will take more exposure to kill than others... but there is no correlation of the one being bad or good for humans. If air is a concern, you want to totally kill as much as possible.

Thank you for the great responses.

Quote:

One thing that stuck bacteria can do is reproduce - one source of Legionaire's Disease was dampness in a ventilation system that allowed a large number to collect.

I thought about that, but I think it was more complicated than that, i.e., it wasn't just "moisture," but actually standing water. Moreover, there' must also have been sufficient nutrients for the bacteria to multiply. If such a situation exists, then a UV system would be effective, but one can't economically line an entire HVAC system with UV lights, on the off-chance there's a pocket of Legionnaire's bacteria lurking in one's HVAC.

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I don't think Legionaire bacteria grow in just air. They need water. Unless we talk about a damper downstream of a humidifier mister or so. Wet cooling towers, and "old" water in infrequently used domestics water pipes is where those are a problem. A damper that is wet enough to allow growth is called a valve and would be in a water pipe

UV only works in the area that is radiated. It doesn't have a residual disinfecting properties like ozone or other ionizing methods would have. So even if that damper story is true, it only could work with UV if the UV shines on the damper (ideally both faces, but there may be some reflection).

UV also is an energy hog and maintenance hassle. So yo may spend a lot of $ for not much outcome. You also need safety devices that shut off the UV BEFORE a door is opened. Those UV tubes are very harsh on eyes.

Thanks for the thoughts. I'm thinking the UV lights aren't going to do much in an HVAC system due to the exposure time required.

I think you're right.

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I pretty much gave up on that during the height of COVID; the exposure time required was the "killer" That, or the kilowatts required to generate the UV power density required to kill instantaneously.

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UV also doesn't work for COVID because that goes from person to person within a space. It doesn't go through the AHU.
By the time the virus goes from one room (via return air) through the AHU to other rooms, it is too diluted to still be a concern (at least compared to other pathways). If you really are concerned like in a lab or hospital, you just don't recirculate air from one room to the next room to begin with.

2+ years ago, no one knew the longevity of SARS-COV2 in free air and on surfaces, and UV was used to sanitize living spaces in some cases

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But now we know more.... so no need to repeat the wrong. And sanitizing the space with UV was pretty stupid back then also. You can't have people in an UV-lit space. And once people are allowed back in, the UV isn't working anymore and the virus coming from one person still goes to the next. You don't need to know specifics about COVID, you only need to know how UV works. So not knowing wasn't a good excuse to do that. it was a thing restaurants (that violate every health code there is) to tell customers "hey, we did the UV thing last week, so all come back now".

That UV sanitizing was done to give people (that don't know science) a piece of mind. And it was clear from the beginning this was an airborne disease. So all the grocery washing exercises were pointless as well and was known to be so. Just because people today still wear a mask outside when no one is around, doesn't mean there was ever a scientific reason to do so.

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And it was clear from the beginning this was an airborne disease.

That was not 100% true; there was still concern about touch transmission.

And where in this thread is there wrong information?

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Maybe the first 3 weeks it wasn't known. But right in the beginning of the pandemic every single (then zoom) ASHRAE meeting I attended was about COVID. It was all about transmission in air and and about how being outside is safe. There was that study about the split AC unit in a Chinese restaurant that was chewed through multiple times. There were multiple CFD studies that showed how viruses transfer within a a space (and not through the AHU in any meaningful number). And there were the studies that showed that being outside on a beach is safe. That literally was in the very beginning of the pandemic. The Science was there right away. Politicians then, for one reason or another, came up with the rules to wear a mask outside, and to tell people to wash hands and so on. Obviously politicians had their own agenda, and have no scientific education. And obviously those rules were only meant to give people a coping blanket, politicians never adhered to the rules.

And the fact that UV doesn't have a residual disinfecting value was maybe known.. .. since UV was discovered. It is light, why would people think it disinfects after air or surfaces are not exposed anymore?

Quote:

It is light, why would people think it disinfects after air or surfaces are not exposed anymore?

Who here said anything like that? Attached is medRxiv preprint from March of 2020, which is when COVID was declared to be a pandemic in the US, and obviously, scientists, who apparently weren't as smart as your ASHRAE members, were diligently studying touch transmission.

https://files.engineering.com/getfile.aspx?folder=...

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Yes a virus can survive on a surface. But that is not the transmission path unless you lick the surface.

And if you UV a space, then let people back in (and turn the UV off), the people still will infect each other via airborne particles. No surface treatment or treatment in air handler will prevent that.

That UV idea almost sounds like the former president's idea of exposing humans to UV or ingesting disinfectants to kill COVID.

Quote:

But that is not the transmission path unless you lick the surface.

Nor is licking the transmission path for flu or colds, and yet, touch transmission can occur with both, since people touch surface and then touch their faces or eyes or noses.

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Just to stay on topic .. how would UV help then? Radiate doorknobs and tables with UV after every touch?

No one is disputing UV can work. But you would have to apply it in the space where humans can infect each other while those humans are in that space. That likely is more harmful than the actual virus.

Using UV in the space only before humans enter will not do anything. Same for applying UV in the AHU.

You would basicaly create a huge tanning box. Possible? Yes. But not practical or healthy.

Again, no one has suggested that here.

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Covid gave the opportunity to a lot of snake oil sellers to flog useless equipment/liquids at a premium.
The number experts" that I met with that were trying to sell us the most absurd "solutions" to Covid and that went out almost tail between the legs when I dismantled their half truths and outright lies and misconceptions made the entire Covid experience much more bearable.