Fresh Air Inlet & the House Trap
Letters To The Editor - July 2008
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
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:
The FAI is the vent for the house trap. (Wrong. It’s on the
wrong side to be a vent. Why is it called “fresh air inlet”
and not “house trap vent” then?)
The FAI is to equalize sewer pressures. (False. How can it do this
with a running trap in the way?)
The house trap is a secondary “safety net” to prevent
sewer gases from entering the house. (False. Individual fixture
trap seals and proper venting is the key to preventing sewer gases
from entering a building.)
The FAI is there to equalize pressure, allowing air to push out
as water moves through. (Wrong. It’s called “fresh air
inlet,” not “fresh air outlet.” Proper venting
equalizes pressure within a plumbing system.)
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
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.
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
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
I know Julius will be able to add so much more to this.
Radiant Technology LLC