Seattle Home Inspector's Blog


This is NOT a trap!

     "S"-traps, "P"-traps, "R"-traps---what is the big deal?  With the exception of the "R"-trap we are talking plumbing----I'll leave the Rat traps for another "year" sunsmile.  There are oodles of different kinds of traps.  In modern homes we are primarily talking about "P"-traps.  S-traps were more common in older buildings where the plumbing was added post construction and the walls weren't torn apart to install drains inside the walls.  In modern installations we want to see P-traps. 

     Every once in a while a homeowner will install a drain that essentially resembles an S-trap by "function"---if not by design.  In this first picture the drain functions as an S-trap.  Follow the drain shape----see how it forms an "S"?  The issue with an S-trap is that the velocity of water when it goes over the last hill and down the drain can result in the plug of water pulling the water out of the trap and down the drain with it.  These traps often go glug-glug-glug when they drain.  Siphoned traps are not good because they can allow sewer gases into the home----and as mentioned in a previous blog we try to minimize the similarities between our bathrooms and our outhouses.trap
























     I don't want to get into the technical details of proper drain installation and what else is problematic with this installation---I just want to talk about S-traps and P-traps.  The chrome trap pictured is "technically" a P-trap, but because it is installed into a 90 degree elbow instead of a "T" it will function as an S-trap and siphoning is a possibility.  What a "T" does is allow for the connection of the drain "vent" which allows air to be pulled down the drain with the water instead of the water that is supposed to stay in the trap.

     Often times we have situations where we want to install a drain but installation of the vent is too difficult or not practical.  Enter----"The Air-Admittance Valve", or AAV.  This device allows us to introduce air to the drain without having to run a pipe up through the roof.  For a long time these devices were not approved by the plumbing codes.  Now they are approved and accepted by most jurisdictions.  This next picture shows a typical AAV installed in new plumbing.trap














     In this installation the "T" for the sink drain P-trap is hidden in the wall and the AAV is connected to it but left accessible under the sink. Visualize how when the sink drains and enters the T in the wall, the AAV allows air to enter the drain.  These Air Admittance Valves must always be accessible.  All AAV's have a flap that keeps water and gasses from backing out of the vent.



Charles Buell 

     PS, for those of you that are new to my blog (or for some other "unexplained" reason have never noticed)sunsmile all  pictures and smiley-face inserts (emoticons) have messages that show up when you point at them with your cursor.

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Comment balloon 12 commentsCharles Buell • April 08 2008 08:54AM
This is NOT a trap!
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