True, but they don’t keep the air clean nearly as fast as putting room air purifiers near people (closer to source) unless you have an HVAC that is so powerful you feel wind inside..
It's really not related to wind speed or hvac power. If you have a well insulated house then a normal hvac system can maintain positive pressure and keep the air inside clean. Assuming it's designed for that purpose.
Alternatively just having a bunch of box fans forcing air through filters is also good. That's essentially what a air purifier is.
How is positive pressure maintained? It seems like there would be continual loss of air to the environment that would have to be replaced. Ideally the replacement air would come through a filter, but at least in a central hvac system I don't think that happens. I suppose the new air coming in wouldn't be a big deal if the average air molecule doors a full cycle through the hvac many times before being lost again to the outside environment.
