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
 

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.