We all use the term “DFU” (drainage fixture unit) when referring to a load value assigned to a particular piece of pipe, but where did the DFU come from?
Dr. Roy B. Hunter coined the term DFU. Instead of plumbers trying to figure out exactly how full each pipe is, Hunter based the DFU on the smallest load value of a fixture, which was the lavatory. The lavatory with a concave bowl shape entering the drain discharged at a value of about 7.5 gallons per minute. That amount is really close to a cubic foot of water, so he coined that 1 DFU.
However, you can’t just start at the smallest, you also have to see the fixture with the largest demand, which is the water closet. A water closet discharged at about 45 gpm, which would load the pipe six times more, making that 6 DFU. There is more information on how to precisely obtain a DFU load for each fixture, but this article is not about how to load a pipe properly and you can find that reading about Hunter.
STILL THE SAME
I want to talk about why most of the DFU values haven’t changed in over 80 years! How can this even be possible? The first thing I hear about changing the DFU value is for the water closet. I know it was five to seven gallons per flush many years ago, and now it’s down to 1.6 gpf. I understand that the flush has greatly diminished in the volume of water. However, even though it’s much less water, it’s significantly more of a violent flush. So keeping the same DFU value for a water closet still makes sense to me.
When it comes to the load value in a plumbing system, I feel it’s more important to talk about the current material used compared to 80 years ago when Hunter gave us these DFU values. When I look at older plumbing systems, metallic piping makes up a majority of the material, whether it’s galvanized steel or cast iron. How are those pipes connected? Since so much of the galvanized steel is threaded, and I know that some of the waste will travel over the female threads. Between flowing over the threads and the roughness coefficient of the metallic piping, a smaller waste load can travel much farther and faster inside a smoother PVC pipe then it could with the older metallic piping.
We can also look at the fittings, have you ever seen a kitchen sink with galvanized material threaded into a short radius fitting going from vertical to horizontal? In the past, that was a very typical installation. We know now that this is a prime spot for a clogged drain. It is important to maintain the flow velocity throughout that fitting.
So, with the materials of the piping and the fittings directly impacting the flow velocity of waste inside the drain system, we may need to either change the DFU value of the fixtures or change the value of what can be installed on a particular size of the drain system.
Whether we change the load value per fixture or the maximum amount of load per specific pipe size, the one thing that we need to do is find consistency while implementing the correct DFU values that Hunter did not give us because they are not a specific fixture.
DIFFERENT SCENARIOS
What about a sanitary sump with a pump discharging into a gravity drain? How do we load that pipe now? We do have two different code citations that may help us answer these questions. In Wisconsin we have intermittent flow and continuous flow. Most other codes are continuous and semicontinuous. Our intermittent flow stops at a timed duration of two minutes, and if the duration exceeds two minutes, it’s now continuous flow.
We then have to convert the gpm load of the pump into a DFU load once it enters the gravity drain. Here’s an example: A 30 gpm pump that runs for only one minute would have a DFU value of 30 once it enters the gravity drain. If that same 30 gpm pump ran for five minutes, it would have a DFU value of 60 once it enters the gravity.
This is where the clarity of the code becomes fuzzy. Our load values for this install have changed multiple times in the last 70 years. In 1956 for every gpm that a pump produced, we had to assign 2 DFU to the piping. In 1970 it changed, we had to assign 1 DFU to the piping per gpm. In 1985, for every 2 gpm, we had to assign 1 DFU to the piping. In 2009 for every gpm, we had to assign 2 DFU to the piping. That means the same 30 gpm pump produced a DFU load value of 60 DFU, 30 DFU, 15 DFU and 60 DFU since 1956. See what I mean by fuzzy!
Now let’s discuss putting a load on a receptor that serves a water softener.
In Wisconsin we have a dedicated load value for each size receptor. For example, a 1 1/2-inch receptor will have a DFU load value of 2 DFU, a 2-inch receptor will have a DFU load value of 3 DFU, and so on. That means if we install the water softener into the 1 1/2-inch receptor, it will put an additional 2 DFU load onto the drain system. Is that load enough? Should we be loading the receptor the same way as we would the sanitary sump with pump? If we load the receptor this way, we will be adding DFU to the drain system.
We could also throw another wrinkle into this, what if we don’t install a dedicated receptor for the water softener. We can discharge the water softener into a laundry tray or even a floor drain. When we install it this way, we just disregard the DFU value produced by the water softener. I don’t think that we need a complete overhaul on DFU load values associated with the drain piping system.
I do think we may need a slight adjustment and definitely some consistency and clarity.
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Ken Dickerson teaches plumbing apprenticeship at Waukesha County Technical College. He is the 2022 Associated Builders and Contractors of Wisconsin Instructor of the Year, a U.S. Air Force Veteran and proud husband and father of two. To reach Ken, email editor@plumbermag.com
