This section is from the book "Questions And Answers On The Practice And Theory Of Sanitary Plumbing", by R. M. Starbuck. Also available from Amazon: Questions and Answers on the Practice and Theory of Sanitary Plumbing.
The fresh air inlet is a line of pipe run from the house side of the main trap and opening into the outer air. The use of this pipe is to prevent air lock between the fixture traps and the main trap, to bring into the system a supply of fresh air, and to aid in creating a circulation of this air through the plumbing system.
Because it would not only fail to prevent air lock but would not create a circulation of air through the system, and would vent the sewer in a dangerous place.
Through the cellar wall at a point as far removed as possible from windows, doors, and cold air box to the furnace.
Fifteen or twenty feet if possible. This frequently necessitates running out into the yard or lawn before coming up through the ground.
It is not, for the reason that in such a case there is a circulation from the sewer through the roof, and no danger of air lock. Moreover, if used on systems having no main trap, it would ventilate the sewer in a dangerous place.
By bringing in air upon the seal it increases evaporation and tends to cool the water in the trap, so that freezing is more apt to occur.
They make the entrance of air upon the trap seal less direct and therefore lessen the liability of freezing.
It is often taken from the vent hub of the trap, sometimes from a T next to the trap, and sometimes from a Y and bend next to the trap, the latter being the better plan, as it brings the air in less directly. This course is fast being adopted in the principal cities. It is sometimes carried up through the roof.
If there is only one stack in the house and that is four inches, the weight of air in that stack and in the fresh air inlet may balance each other, with the result that there is a poor circulation or none at all.
Owing to air lock, syphonage would be very uncertain, and there would very likely be disagreeable gurgling sounds when there was a discharge from fixtures.