Would love to hear thoughts on this which I also posted on lesswrong. Thanks.
Far UVC holds little interest for me when the vastly more effective, cost-efficient, and problem-free method of UV in ductwork (Near UV) gets pretty much zero attention, as far as I can find in my research. I’m flummoxed by this and would love to have people show me that I’m wrong about it. Cheers.
vastly more effective, cost-efficient, and problem-free method of UV in ductwork (Near UV) gets pretty much zero attention
The big problem is that ducts are relatively rare, something like 10% globally. While ducts are common in the US, Canada, and Australia, they’re rare elsewhere including Europe and Asia. [1]
You also need to tune your HVAC to recirculate a lot of air even when the system isn’t calling for heat or cooling, which people usually don’t.
And then if you do have ducts and are moving a lot of air you don’t need UV: if you’re running MERV-13 (typically the most the blower is able to handle) that’s removing worst case 50% of particles, and you can generally put out enough air to hit targets with the existing system. And then consider that in-duct UV systems fail invisibly and fail open.
[1] Around here the old houses are mostly radiators, new ones are mostly mini-splits, and only ones built or renovated in between have ducts. Older commercial buildings are also generally radiators, though that’s becoming less common. I asked Claude Opus 4.7, ChatGPT 5.5 Thinking, and Gemini 3.1 Pro “Approximately what fraction of indoor hours spent by humans around the world are in spaces with a ducted HVAC system? What’s your 50% confidence interval?” and got 9-13%, 10-20%, and 6-11%. The big factor here is that while ducts are common in the US, Canada, and Australia, they’re rare elsewhere including Europe and Asia.
Oh to add, on the “intermittent fan” issue. Here’s Claude on how frequently HVAC systems turn over all the air in a building (and so would sanitize all the building air with UV in ductwork):
Key Variable: Air Changes per Hour (ACH)
HVAC systems are typically sized and rated in ACH — how many times per hour the system moves a volume of air equal to the total space it serves.
Thanks for this meaty reply. Good info, much appreciated. But my basic reply, short form:
”X doesn’t solve everything (globally) and is not perfect” is never and nowhere a valid argument against doing X [as opposed to Y]. Or at least discussing X!
There’s a massive space where Near UV in ductwork could deliver very large and positive results, very efficiently relative to installation cost, energy use, maintenance and attention required, and probably other dimensions of “efficiency.” (Compared to not doing it, OR compared to doing Far UVC where ductwork UV is feasible.)
So I’m still just really curious, stuck with the same Q: why do I never find people talking about it?
My external post probably would have been better with some explicit comparisons, but my claim is that in-duct UVC (a) isn’t widely applicable, and so the overall potential benefit of pushing for it is low and (b) isn’t cost effective even where it’s applicable.
I think (b) is the more important one and where we most disagree. I’ve now added the cost-effectiveness calculation to the end of https://www.jefftk.com/p/against-in-duct-uv and it looks to me like even in the best case in-duct is much more expensive per CADR than filters or far-uvc.
I finally found some time to study your longer reply and study/research more. Main thing I didn’t know is how effective MERV-13 filters are (@ ~50%). (I built a box-fan filter as soon as I learned about it back when summer wildfire smoke started hitting Seattle some years back. Works great!) Given the high air-turnover in ducted systems, seems like that alone can do (almost?) all the necessary. Thanks.
Would love to hear thoughts on this which I also posted on lesswrong. Thanks.
Far UVC holds little interest for me when the vastly more effective, cost-efficient, and problem-free method of UV in ductwork (Near UV) gets pretty much zero attention, as far as I can find in my research. I’m flummoxed by this and would love to have people show me that I’m wrong about it. Cheers.
The big problem is that ducts are relatively rare, something like 10% globally. While ducts are common in the US, Canada, and Australia, they’re rare elsewhere including Europe and Asia. [1]
You also need to tune your HVAC to recirculate a lot of air even when the system isn’t calling for heat or cooling, which people usually don’t.
And then if you do have ducts and are moving a lot of air you don’t need UV: if you’re running MERV-13 (typically the most the blower is able to handle) that’s removing worst case 50% of particles, and you can generally put out enough air to hit targets with the existing system. And then consider that in-duct UV systems fail invisibly and fail open.
[1] Around here the old houses are mostly radiators, new ones are mostly mini-splits, and only ones built or renovated in between have ducts. Older commercial buildings are also generally radiators, though that’s becoming less common. I asked Claude Opus 4.7, ChatGPT 5.5 Thinking, and Gemini 3.1 Pro “Approximately what fraction of indoor hours spent by humans around the world are in spaces with a ducted HVAC system? What’s your 50% confidence interval?” and got 9-13%, 10-20%, and 6-11%. The big factor here is that while ducts are common in the US, Canada, and Australia, they’re rare elsewhere including Europe and Asia.
Wrote this up as a full post: https://www.jefftk.com/p/against-in-duct-uv
Oh to add, on the “intermittent fan” issue. Here’s Claude on how frequently HVAC systems turn over all the air in a building (and so would sanitize all the building air with UV in ductwork):
Key Variable: Air Changes per Hour (ACH)
HVAC systems are typically sized and rated in ACH — how many times per hour the system moves a volume of air equal to the total space it serves.
Typical ACH Ranges
Thanks for this meaty reply. Good info, much appreciated. But my basic reply, short form:
”X doesn’t solve everything (globally) and is not perfect” is never and nowhere a valid argument against doing X [as opposed to Y]. Or at least discussing X!
There’s a massive space where Near UV in ductwork could deliver very large and positive results, very efficiently relative to installation cost, energy use, maintenance and attention required, and probably other dimensions of “efficiency.” (Compared to not doing it, OR compared to doing Far UVC where ductwork UV is feasible.)
So I’m still just really curious, stuck with the same Q: why do I never find people talking about it?
Thanks,
Steve
My external post probably would have been better with some explicit comparisons, but my claim is that in-duct UVC (a) isn’t widely applicable, and so the overall potential benefit of pushing for it is low and (b) isn’t cost effective even where it’s applicable.
I think (b) is the more important one and where we most disagree. I’ve now added the cost-effectiveness calculation to the end of https://www.jefftk.com/p/against-in-duct-uv and it looks to me like even in the best case in-duct is much more expensive per CADR than filters or far-uvc.
I finally found some time to study your longer reply and study/research more. Main thing I didn’t know is how effective MERV-13 filters are (@ ~50%). (I built a box-fan filter as soon as I learned about it back when summer wildfire smoke started hitting Seattle some years back. Works great!) Given the high air-turnover in ducted systems, seems like that alone can do (almost?) all the necessary. Thanks.