SylvanLMP
Hello Harry, You should be congratulated for having the knowledge to know decent Quality in workmanship and materials.
Even one of the manufactures here of both plastic (garbage) and Quality cast Iron says use Quiet cast Iron for drainage and if pressed use plastic for venting.
These folks pushing plastic are both lazy and very inconsiderate as to environmental issues plus they love getting the FREE HIGH off sniffling toxic glue and primmer.
Plastic piping is not recyclable for one thing and does give off some pretty nasty fumes in a fire.
LUCKY the fumes will kill most people long before the flames ever get to them so worrying about getting out a burning structure is not even an issue to concern themselves about.
Now lets consider the Fresh Air Inlet FAI as we call it in the trade.
The FAI should be half the size of the building house drain (trap) but never less then 3″ diameter
The FAI should be located within four feet of the building house trap (upstream)
The FAI should either have a cover plate affixed to the pipe inlet or return bends (180 degree angle fitting)
The FAI should be located at least 6″ above grade.
The cover should be full size If your plumber really likes dabbling in math ask him/her If the FAI is 3″ how many 5/8 holes are required to give the same area?
How many 3/8 holes and how many 1/2″ holes would also equal the same free area.
On the apprentice tests they mixed sizes and had rolling offset formulas also thrown in for good measure 
The Fresh air Inlet brings in the cold fresh air (cold air is heavier then warm air so thus the 6″ height would account for the colder air being this low correct)?
Now as this drainage / soil / waste system is inside the home the warmer SEWER GASES want to rise out to the vent terminals located on the roof BUT these warmer gases CANNOT rise as they are in a vacuum.
Now here comes the FAI to the rescue by allowing the cold FRESH air to replace the escaping warmer sewer gas going up the vent stacks This is constant circulation at its best with no moving parts or mechanical help what so ever.
Venting is a science BUT some stone aged mentality misfits cannot figure out how to properly design a system and thus these under rock dwellers rely on non professional venting means that rely on either moving parts or a piece of crap rubber to insure system integrity.
Venting is properly sized by developed length of pipe and fittings AND Fixture Units connected.
Vent terminals are sized to prevent closure (hoarfrost) and to make sure the escaping sewer fumes do not re enter the structure or an adjacent building.
There are a few other aspects regarding the FAI but to try to give all the information out on here is counter productive as this would only make the non knowing so called trades men feel bad about the lack of proper training they have received.
PLEASE feel free to E mail me and we can shoot back all kinds of mathematics relating to “plumbing”
By the way the answer to the 3/8 holes for a cover plate on a 3″ FAI is 64 holes
Now if you really want to have fun lets discuss roof drain inlets on a promenade deck.
Have a great one 
SylvanLMP
» This message has been edited by SylvanLMP on 28 October 2001
» This message has been edited by SylvanLMP on 02 November 2001
