Minimum DFUs and drainline carry
By Timothy Allinson, P.E.,
Murray Co., Long Beach, Calif.
It’s amazing how, after being in this business for 27 years, there are still code issues that come up that I’ve never heard of, even once, in all those years of designing projects. Granted, 15 of those 27 years were spent in NYC using an entirely different code from the one I have used for the past 12 years, but it still amazes me when new issues arise. Sometimes issues are a function of code change or an interpretation on the part of the local authority; that is to be expected. But when an issue is raised based on a code provision that has existed for many years, unnoticed by yours truly, well, that just shines my shoes.
Case in point: At the end of Chapter 7 of the UPC (Sanitary Drainage) is Part II — Building Sewers. By definition, the building sewer starts two feet outside the building and extends to the public sewer. I find it curious that the break point between the building drain and the building sewer, which should coincide with the break point between the plumbing system and the civil utility, is a two-foot dimension per code, whereas all the construction contracts are based on a five-foot dimension. Regardless, I have never paid much attention to Part II of Chapter 7, because it really applies to the civil engineer more than it does to you and me. In fact, it makes me wonder why Part II of Chapter 7 even exists, since it addresses civil scope rather than plumbing scope. Interestingly, there is no similar section in the IPC.
Anyway, the other day an associate here in my office came back from a meeting and asked me where in the code it defines the minimum number of fixture units required for a drainage line; I answered that, “It doesn’t.” Well, in due course, that same engineer pointed out to me Table 7-8 (in Part II of Chapter 7) that defines maximum and minimum fixture unit loading for building sewers size 8" through 12". Oddly, the maximum fixture unit values should, but don’t, agree with the values in Table 7-5, the sanitary waste and vent sizing table. For example, Table 7-5 limits the fixture units of an 8" horizontal drain at 2% pitch to 2,640, whereas Table 7-8 allows 3,900 fixture units outside the building. Why the difference?
As mentioned, Table 7-8 also defines the minimum DFUs for drain lines while 7-5 does not. The 8" drain example above would require at least 275 DFUs connected to it, otherwise it is considered oversized and, in this age of attention to dry drains, this aspect of the code seems to be getting more attention. Table 7-8 also addresses pipe slopes of 1% and 0.5%; the flatter the pipe, the greater the minimum number of DFUs, which makes sense.
On the project where this was raised as an issue, there was an existing 8" building sewer that was to be utilized for a lesser “post-renovation” DFU load than previously served. The inspector picked up on this and suggested that the 8" pipe would have to be dug up in the street and replaced with a smaller line. This represents a great deal of work for the sake of reducing a pipe size. Note that all that was required by code, based on the renovated building’s load, was a 4" building sewer.
An idea came to mind for a solution to this troublesome problem. The existing 8" building sewer made a straight shot from the basement of the building to the public sewer connection. My suggestion, unconventional as it may be, was to jet the existing 8” pipe clean and use it as a conduit to run a new 4" drain rather than dig up the street. In theory, this could be done by sliding 10-foot lengths of no-hub down the length of the 8" pipe. Rollers might be needed to get it all the way through, and the void between the two pipes would have to be sealed in some fashion at the point of connection to the sewer, but this would surely be easier than digging up a very busy major street for a considerable distance.
I was excited about this creative solution to an unusual problem, but as it turned out, it proved to be unnecessary. The design engineer was able to adjust the drainage fixture units by increasing the ejector pump flow rate and associated fixture units in order to get the DFU load above the minimum requirement for an 8" pipe. So, in the end, this oddball code section was circumvented out of necessity; but this begs a bigger issue.
Taking this concept one step further, neither the UPC nor the IPC dictate minimum number of fixture units for drains within a building, but maybe they should.
Let’s use the IPC as an example. If there is a private toilet room (WC and lav) sitting on a grade slab far from any other fixtures, most of us would run the buried drain as 4" at 1% slope for myriad reasons, even though the code allows it to be 3". With today’s 1.28 gpf water closets, the 4" drain is guaranteed to have frequent blockages, because the carry capacity of the pipe (in terms of distance) isn’t very far with so little drainage flowing through it.
If the code were to be reinterpreted to say that the pipe size maxima listed in Table 710.1 also represent the minima for the next larger pipe size, then the drain in the example above would have to be 3" rather than 4".
Reducing drain pipe size does increase drainline carry. Studies have been done by ASPE, EPA Water Sense and others, but the most informative data I could find on the subject was a report by the Canada Mortgage and Housing Corporation. In general terms, reducing the drain serving a water closet from 4" to 3" will increase drainline carry by 25%. But since the 4" cast iron drain serving a 1.28 gpf toilet was found to have a drainline carry of just 12 feet or so, we are only talking about an increase to 15 feet. This, in and of itself, will not make a world of difference.
Drainline slope has a more dramatic effect on carry distance than diameter. Increasing the slope of a given pipe from 1% to 2% will increase carry distance approximately 100%. The combined effect of changing a 4” pipe at 1% to a 3" pipe at 2% will increase carry distance about 200%. Further, changing the pipe from cast iron to PVC will increase carry distance another 200%. So, while I could not find any data specifically stating this, the net effect of changing a drain from 4” cast iron at 1% slope to 3" PVC at 2% slope should increase carry distance from about 12 feet to a theoretical 96 feet, give or take. This is a significant difference, and something worth considering for unique design applications, especially for the small toilet room out in left field that will be subject to frequent blockages.
Timothy Allinson is a senior professional engineer with Murray Co., Mechanical Contractors, in Long Beach, Calif. He holds a bsme from Tufts University and an mba from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed accredited professional. Allinson is a past-president of aspe, both the New York and Orange County Chapters. He can be reached at laguna_tim@yahoo.com.