Most portable vacuum cleaners have fairly poor filters. Sure most of the real "dirt" gets caught in the bag, but much of the very fine stuff just blows out the back end. A central vac has the same problem, but because it is placed in one place, it can have a much stronger motor to allow it to draw through those vacuum tubes and to fight against pretty good filters. In fact, only about 2% of the fine dust gets past their filters.
Just how do they channel those tubes through old lathe and plaster walls. Well we sent our cameras right inside the walls to follow the work of a Beam Vacuum installation. One of the tricks is to glue two sections together, lift them up into the wall, glue on another section, lift it further up, etc. until you get to the second floor. That was a great job guys. Usually it is a lot easier than that to install.
I would however like to talk about that 2% of dust that still comes out the other end of a central vac. With the barrel in the basement or the garage, that dust may not bother you. But all of the companies that install these things have the capacity to install an outdoor exhaust. They never even offer it to you because it means more fittings and more labour and hence a higher cost, probably about $60 to $100 more. So if one company says you should do that, and the other says that it is not necessary (even if they have the same filter system), you will go with the cheaper one. The reality is that if anyone in your household is sensitive to fine particles, or you don't want to collect that fine dust someplace in your house where you might track it back in, you should install an outside exhaust. I would not have an installation in my house that did not exhaust outdoors.
In fact, if you do exhaust outdoors, then you have just made a significant step forward in the health of the air in your house. You are now using a powerful motor to remove the dust from the house. You don't even need such good filters on the motor so it can work harder at pulling stuff up from the rugs and less against those super filters. Health authorities recognise the value of outside exhaust. I got a hospital to accept rugs for sick kids, on the basis that we would install a central vac with an outdoor exhaust.
One new reason for having an outside exhaust is that some cases have been showing up where carbon monoxide from garages are finding their way into the house through the idle central vac tubes. We are only learning about this route for garage gasses because people are now installing CO detectors inside the house, and some of them are going off at strange times, times when even the heating system is not on, and it has been traced down to the vac pipes.
These are great systems, and the guys that did the installation did a great job -- but do tell them to add an outside exhaust to the order. It will cost you a bit more, but it is worth it. For more details follow this link.