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
| Setting | Typical ACH | Full Air Turnover |
|---|---|---|
| Residential (fan on intermittently) | 0.5 – 1.0 ACH | Every 1 – 2 hours |
| Residential (fan running continuously) | 2 – 4 ACH | Every 15 – 30 minutes |
| Light commercial / office | 4 – 6 ACH | Every 10 – 15 minutes |
| Hospitals / clean rooms | 15 – 60+ ACH | Every 1 – 4 minutes |
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.