And if you're cycling the air much faster then you need much more UV in the duct to get enough exposure time. And then you have to maintain the lamps inside a dusty vent...has its uses but it's operationally tricky and there are tradeoffs.

An upper-room 254nm system with the required expert installation is not going to be cheap or accessible. The bulbs are cheap--the systems are not, because the safety margins are much tighter.

You can use 222nm lamps as an upper-room system just fine--you can set it up yourself without worrying too much about overexposure. A lot of people do just because it's simpler to stick the lamp on a high bookshelf instead of mounting it on a swivel head and tilting it downward. It works fine! Makes the system a bit more dependent on vertical air currents to work well but the efficacy hit is pretty minor. There aren't zero risks but I sure prefer it to a 254nm overexposure.

(Disclosure: I am a cofounder and part-owner of Aerolamp)