In this day of tightly sealed structures with very little air infiltration I pose the following question: Bathroom exhaust fans are typically rated at 100 cfm while some large kitchen vent hoods are rated as high as 800 cfm. Central vacuum systems also "suck" on the house envelope but are not rated in cfm. Where are these devices supposed to get their "makeup" or return air in a well designed structure?? They can move any air if there is none available. Thanks in advance.