A few weeks ago I asked some plumbing questions, including how to
plumb back-to-back water closets. The upshot was that UPC 704.2
(operative in my area) requires the use of a "double fixture fitting"
(DFF), not a double sanitary tee or a double combo. Subsequently, two
issues arose that have made me reconsider using a DFF. The first one
is that the biggest ABS DFF I can find is 3", so I would have to
reduce my closet drains to 3".
Second, I read the following article:
It claims that modern toilets develop enough flushing pressure that
the DFF pattern does not adequately protect one closet from having its
trap siphoned by flushing the other closet. The author suggests
instead using a double combo or double wye, which I can find in 4".
One thing the author doesn't mention is whether he is refering only to
pressure-assisted toilets; I'm going to use the gravity Wellworth.
So, plumbing mavens, should I just go ahead with the 3" DFF, or should
I try to talk my inspector into allowing the 4" double combo? I have
plenty of altitude for using a double combo. Does the article above
make any sense? It seems like, in addition to contradicting UPC
704.2, the double combo would also violate the "takeoff above weir"
rule (UPC 905.5).