Against In-Duct UV
Worldwide, just 10% of indoor time occurs where in-duct UV could work.
A new deep-dive challenges the common pitch for in-duct UV (placing UV lights inside HVAC ducts). The biggest issue is limited applicability: worldwide, only about 10% of indoor hours are spent in spaces with ducted systems (based on estimates from Claude Opus, ChatGPT, and Gemini). Ducted HVAC is common in the US/Canada/Australia but rare elsewhere; even in those regions, older homes (radiators) and newer builds (mini-splits) often lack ducts.
Even in compatible spaces, in-duct UV has major drawbacks. The blower only runs when heating or cooling is needed—a small fraction of the time—so effective air cleaning requires continuous recirculation, which increases power costs and humidity issues. MERV-13 filters already remove >50% of viral droplets and bacteria; in-duct UV might push that to near 100% but at best doubles CADR. Moreover, UV bulbs fail invisibly: you'd stop getting clean air without noticing. The article recommends standalone air purifiers, far-UVC (for large/quiet spaces), or upper-room UVC (for high ceilings) as superior options.
- Only ~10% of global indoor hours are in ducted spaces (AI consensus from three models).
- Ducted systems run blowers infrequently; continuous recirculation is power-hungry and can worsen humidity.
- In-duct UV fails open (no warning when bulb dies) and adds minimal benefit over MERV-13 filters.
Why It Matters
Overreliance on in-duct UV may miss 90% of indoor environments; professionals should shift to proven alternatives.