History LessonIt would be a tremendous value if Julius Ballanco would write an article on the history and reasons behind the fresh air inlet and the house trap.
My biggest beef in all my years of plumbing has been the ignorance around these two items. I’ve had master plumbers and worst of all, inspectors, say some really wild and ignorant things about the inlet and trap:
I’m sure there are other stupid things you’ve heard, too. Anyway, since the code and commentary do not explicitly comment on the reasons behind the building (house) trap and FAI, here is what I’ve found over the years through research of old plumbing manuals and text books.
What came first - the house trap or the FAI? That depends. I haven’t found a definitive answer, but I do know that an FAI should never exist without a building trap in front of it to prevent sewer gases from spilling out all over the lawn or sidewalk out front.
That is one purpose of the house trap - to prevent sewer gases from exiting the FAI. Perhaps the old-timers first installed the FAI and then realized a stinky problem, which was subsequently solved with a running trap in front of it.
The IPC commentary says that the house trap was invented to prevent rats from entering the building. I believe its invention is deeper than that.
The IPC in Section 1002.6 prohibits the use of building traps “except where local conditions necessitate.” What are those local conditions? It’s very rare to find anyone who knows. It’s not written anywhere in the code or commentary what those special conditions would be. When inspectors don’t know, then a “prohibited” device often gets installed, just because they said so.
Additionally, nowhere in the code and commentary that I’ve found explicitly states the reasons for installing an FAI.
Digging in old plumbing manuals, here’s the only reason I’ve found for an FAI:
The FAI is installed at the lowest part of the plumbing system, either just before the building drain exits the building or just after. The FAI’s purpose is to allow fresh air to enter the plumbing system and exit the vent through the roof, in order to prevent the premature corrosion and failure of ferrous plumbing materials such as cast-iron, galvanized and copper piping. The corrosion is most prevalent at the top of all horizontal piping runs, where sewer gases combine with condensate to form carbonic acid. This acidic condensate clings to the top of horizontal runs and will eventually eat through the pipe, causing premature failure. The proper application of an FAI to ferrous DWV piping materials will ensure a long service life.
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.)
- 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.
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.
Radiant Technology LLC
On The MarkI loved Dan Holohan’s column in the May issue ofPMabout Mark (“This One’s For Mark”). We come across such projects a lot of times. I have learned how to handle them, and we will not undertake the project at all if we cannot revise it or redesign it. I would have ended the article: “This is for all the Marks out there!”
ThanksI wanted to say “thank-you” for the first-place award on the Tool Tips page in the April issue. It was flattering, mainly since I am such an avid admirer of your magazine; I read it from front cover to back.
I am heading towards my retirement years (if there is such a thing), but have a well-known and established plumbing shop with two sons who didn’t want to take over the family business. My oldest son, a college graduate, swore he would never be a plumber and wanted nothing to do with it.
Well, he recently started to work with me since the financial perks of plumbing were far better than his other options. I didn’t want to tell him about the first prize and just wanted to wait until he read it himself. (I have him hooked on PM already.)
After his issue arrived, he called me and said “Dad, you sure make being a plumber exciting!”
So you both made my day! Thanks again.
The Faucet Doctor
Harrison City, Pa.
Report Abusive Comment