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History Lesson:
Fresh Air Inlet & the House Trap

Letters To The Editor - July 2008


In addition to this, then, it would be proper to individually vent each fixture and keep such vent as close to the trap as possible so that all horizontal piping gets circulation and is protected from corrosion. If the distance between the trap and individual vent is greater then 2 feet, then that portion of horizontal pipe (fixture drain) will be more susceptible to corrosion. A running trap is placed after the FAI in order to prevent sewer gases from exiting the FAI from the sewer main.

So the FAI is only necessary when using ferrous plumbing materials, such as cast-iron. Since plastic is not susceptible to corrosion, a fresh air inlet is not necessary. If an FAI is not needed, then a house trap is not needed. If an FAI is required because of ferrous piping, then “local conditions would necessitate” the installation of a building trap.

(This also explains the 2-foot rule of New York City, where individual venting within 2 feet of every fixture is mandated. This is a material mandate because of the use of cast-iron and has nothing to do with preventing trap siphonage. When plastic is used for horizontal runs and fixture drains, this rule would be irrelevant and regular IPC venting rules could be taken advantage of in full. I have yet to find a NYC inspector or plumber who knows the reason behind the NYC 2-foot venting rule!)

The other good reason for a house trap — closely built houses on a hill all connected to a city sewer. In this condition, it’s possible that sewer gas will be noticed by houses higher then other houses’ vents and roof lines. In this application, an FAI is unnecessary if all the plumbing is plastic and the house trap would exist alone.

It would be a grand thing if the code and commentary explicitly stated the only main “local conditions” that would necessitate the installation of a house trap:

1. To protect an FAI.
2. When buildings are connected to a public sewer, and conditions would be present that would cause sewer gases from building vents to be noticed by higher-elevation houses nearby. (This would be irrelevant with private septic systems.)

It also would be nice if the reasons for prohibition of the building trap were stated in code or commentary:
They plug up and are a maintenance problem.
They are unsanitary to maintain.
They prevent the most effective discharge of a building drain into the sewer.
It would be nice if it were clearly stated in code or commentary the sole reason for an FAI:
To prevent the premature corrosion of ferrous DWV materials hung in the horizontal position. (Vertical position would not affected.)

FAIs should be prohibited when using plastic materials for this reason: they are simply not necessary. FAIs should be required only when using ferrous horizontal drain piping.
What the FAI and house trap are, and are not, should be clearly understood. This will help everyone to understand and install better plumbing systems.

I know Julius will be able to add so much more to this.

Aaron Ourada
Radiant Technology LLC
Walden, N.Y.

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