ASPE Open Forum

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  • 1.  DFU to GPM

    Posted 09-05-2019 08:58
    We constantly get asked by civil for a sanitary GPM flow from a building so they can determine if the existing main is adequate. 

    Does everyone else run into this? If so, how do you determine GPM flow?

    In the past we have provided GPD estimates based on occupancy type. I have heard of dividing DFU by 2 to get GPM. But, what is the most accurate method? Or is there better information to provide to civil?

    Layne Micek
    Vice President, Plumbing Engin
    Schnackel Engineers, Inc.

  • 2.  RE: DFU to GPM

    Posted 09-06-2019 08:15
    That is truly a unicorn in the plumbing industry.  I use GPD from EPA based on building type as well.  If I'm pressed for an answer I use WSFU converted to GPM, figuring that water-in equals water-out.  I have never found a silver-bullet answer to this question.

    David Texter
    Plumbing Engineer Manager
    KLH Engineers

  • 3.  RE: DFU to GPM

    Posted 09-06-2019 08:38
    We can go with below snip numbers. 

    Plumbing Engineer
    Setty & Associates

  • 4.  RE: DFU to GPM

    Posted 09-09-2019 10:12
    Pretty common in my experience. Both the IPC & UPC equate 1gpm = 2 dfu's.

    Reza Bahador, CPD
    Senior Plumbing Designer

  • 5.  RE: DFU to GPM

    Posted 09-06-2019 09:15
    I struggle with this as well. A while ago I found this website that helps with Hunter's Curve for WSFU ( I have never really gotten it to work but if anyone has or can please share. I attached my attempt at the spreadsheet. The paper attached was distributed at last years ASPE Convention and seems to indicate 1 DFU = 7.5 gpm? It seems pretty high, but I think it is referring to peak discharge rates. We size our pipes based on peak flow, but civils I have worked with are more interested in a volume load. I stumbled upon this empirical equation for estimated daily loads ( There was a chiller plant with 12 floor drains. If you use code DFUs for floor drains that would be 24 DFU * 7.5 gpm/DFU = 180 gpm. Extrapolate that out *60min*24h = 59178 gpd. Using the equation from ETB gives 11 gpm and 3616 gpd which seems much more reasonable. I've also considered using Manning's Formula for Uniform Flow. In a gravity flow regime, 4" cast iron pipe at 1/8" slope will flow 55 gpm half full and 110 gpm full. I know this probably doesn't answer the question, but it's my thought process because I run into the same issue. You can give them a peak and average number and let them take it from there.

    Andrew Peter
    Piping Engineer


    Hunter.xlsx   77K 1 version

  • 6.  RE: DFU to GPM

    Posted 09-06-2019 10:03
      |   view attached

    I found that exact website for Hunter's Curve and attempted it myself (also without success). See attached for my attempt.

    Interestingly, our numbers for binomial and normal flow rates match...but they're both obviously wrong when compared to the tables. I would love to find out what I'm doing incorrectly here, as having an excel equation could replace many cumbersome "table+interpolation" values in our master design spreadsheets.

    Timothy Witt
    Project Manager


    Hunter's Curve.xlsx   14K 1 version

  • 7.  RE: DFU to GPM

    Posted 09-06-2019 11:16
    Hi Andrew! Sounds like you saw our presentation on the Hunter Curve update with our creation of the Water Demand Calculator. Calculating peak drainage is also a statistical problem but substantially differs from calculating peak water demand. This is in the distant horizon for another IAPMO research project. David Texter's solution to use WSFU gpm is not altogether accurate either. The flow dynamic is different in a drainage system than it is for the water distribution that is under constant pressure. The drainage system has "surge discharges", "overlapping discharges", "uniform flows at terminal velocity", each having a different peak dynamic. That is why there are 3 different curves in Figure 5 of the article. Regarding 1 DFU = 7.5 gpm (or 1 cfm), that is the actual measured discharge rate of a lavatory. Table 1 in the article shows the measured highest mean discharge rate of the fixtures. The discharge rates were ordered from lowest to highest based on the cfm discharge rates and these values became the relative fixture units. So the WC is 6x the discharge rate of the lavatory (6x7.5 = 45). The discharge rates of these fixtures were measured before the theory of probability was used to determine peak discharge flows. To find the probable peak discharge rate in gpm, refer to Figure 5 in the article. For example, a total number of 800 fixture units on the x-axis, will have a probable peak discharge load of 92 fixture units on the y-axis (blue curve). Multiply 92 by 7.5gpm, which will result in approximately 690 gpm for an estimated peak load. To put this in visual terms, 800 FUs is equivalent to a total of 133 WCs. Out of the 133 WCs, only 15 WCs (actually 15.333) will have a probable overlapping peak discharge. So the peak demand in gpm will be based on the 15 WCs. Multiply 15 by the discharge rate of the WC (which was measured as 45gpm in the article), which is a peak discharge rate of 675gpm (would be 690 if multiplied by 15.333). This is actually how Hunter perceptually developed the discharge curves. 
    What is the implication of this? To get this kind of peak discharge rate is to assume that all 133 WCs are in battery, and based on the theory of probability assuming congested use from WCs discharging 5 gallons of water at a rate of 45gpm, there will be a constant simultaneous overlapping discharge rate of 690 gpm. The curves he developed would serve the worst case scenario, and anything less would be safe. You can see the oversizing potential of using these curves in every case. 
    Granted, Hunter scrapped this method after 1940 when he developed the more accurate curves for the peak water supply demand. At the end of that publication (BMS65 reproduced by ASPE) Hunter said with slight modification, the same curves could be used for the peak drainage estimate. What those modifications were, we will never know. Due to illness, Hunter left the National Bureau of Standards, retired in Florida and soon passed. Hence the need to reevaluate the method to estimate peak loads for the drainage system. 

    If you have any further questions feel free to ask.

    Dan Cole
    Sr. Director of Technical Services

  • 8.  RE: DFU to GPM

    Posted 09-10-2019 10:05
    Dan - for now, until the research catches up, could the plumbing engineering/designer not to just use the peak calculated domestic cold water GPM as the peak sanitary flow rate?  What goes in = what goes out?

    Christoph Lohr
    Plumbing Engineer
    Henderson Engineers, Inc.

  • 9.  RE: DFU to GPM

    Posted 09-09-2019 08:47
    This is indeed a very interesting conversation​. Many have referenced converting using 2 DFU = 1 GPM, though I'd argue that is incorrect. That conversion rate seems to be based on the ICC base model code for "Values for continuous and semi-continuous flow", found in section 709.3 of the 2015 & 2018 IPC. The reason I am asserting the conversion is incorrect can be found in the code commentary,

    Equipment that discharges either continuously or semi-continuously, such as pumps and ejectors, is assigned two fixture units for each  gallon per minute (gpm) of discharge rate.

    That conversation rate was assigned in order to accommodate not only the waste that is being discharged, but the air that will be pushed in front of it as well. This type of discharge is very disruptive to a gravity drainage system and if not accommodated for properly can result in a waster closet being turned into a bidet at a very un-opportune time, or result in the discharge stopping the gravity flow upstream leading to stoppages.

    As far as converting to a GPM value, I believe Andrew Peter is more on a correct path. The building drainage system is designed to flow at no more than 1/2 full, so in using his example of 4 inch cast iron at 1/8 inch slope, 55 GPM is a reasonable number. A 4 inch building drain run at 1/8 inch slope is able to accommodate 180 DFU. If you convert that using the 1 GPM = 2 DFU then you end up with 90 GPM. The building just will not flow like that. Even at peak demand it will never be at a full flow condition.

    I believe it is reasonable to provide the GPM value based on the size of the building drain using Manning's Formula for Uniform Flow, you will be providing them a number for the maximum possible discharge possible for the building drain installed even if it isn't carrying the maximum DFU allowable. Public sewers are almost always flowing at a 1/2 full condition and in many cases will have surcharges during peak hours throughout the day, using the maximum flow rate possible based on the building drain size should help jurisdictions properly size their sewer mains and avoid surcharging conditions as much as possible. If on the other hand you provide a number based on full flow conditions, this could inadvertently lead to oversizing which is never a good idea in a gravity drainage situation when you consider the fixtures are using less and less water. Eventually that could lead to a situation where "there isn't enough water in the pond to float the boat", which again will lead to stoppages.

    James Richardson
    Plumbing Inspection Supervisor
    City of Columbus

  • 10.  RE: DFU to GPM

    Posted 09-06-2019 09:26
      |   view attached
    I developed the attached spreadsheet to answer just that question for an assisted living facility that takes into account usage from residents, employees, visitors, and meals served per person per day.  The result seems to be much more reasonable than the "water in = water out" method.

    The subtotal from the IAPMO source comes from another spreadsheet I developed using data extrapolated from their water savings tables in the referenced document.

    I'd share the IAPMO-based spreadsheet also, but I'm not sure if I'd be violating any copyright laws (probably not, since I modified their tables so greatly that it barely resembles the original).  If anyone can convince me otherwise, I'll attach that in another reply.

    Randle David Eagan

    David Eagan, CPD, CPE
    Plumbing Designer and Estimator
    RJ Mechanical, Inc.


  • 11.  RE: DFU to GPM

    Posted 09-06-2019 15:52
    In my opinion, until proven and documented otherwise, design engineers must use 1 gpm = 2 dfu (per IPC 709.3) or any other number defined per local codes, no matter if it's realistic or not. Doing otherwise is a code violation, no one wants to commit. 
    Approach "water in = water out" is not entirely accurate. Water out is 80-90% of water in, depending on how much water get lost in evaporations (cooling towers, pools, fountains, irrigation, wash-downs etc.). 
    Thank you

    Gregory Shvartsman
    IGS Consulting
    Plumbing and Fire Protection Design Optimization, QC, VE.

  • 12.  RE: DFU to GPM

    Posted 09-09-2019 08:45
    Can someone explain how the humble lavatory, with a maximum allowable flow rate of 0.5gpm somehow develops a discharge rate of 7.5gpm?  Did 1940's faucets really flow that much water?  If so, then IAPMO needs to get the proverbial lead out and generate some new data.  Toilets no longer flush 6gpf.

    Victoria Johnson
    Plumbing Engineer
    Bass, Nixon & Kennedy Engineers

  • 13.  RE: DFU to GPM

    Posted 09-09-2019 08:53

          The DFU values are not based entirely on the GPM flow from the faucet for a given fixture. The rates also take into consideration the average size of the vessel and the length of time it take to empty the vessel when discharged from a full capacity. The commentary does provide some insight for the DFU values:

    The conventional method of designing a sanitary drainage system is based on drainage fixture unit (dfu) load values. The fixture unit approach takes into consideration the probability of load on a drainage system. The dfu is an arbitrary loading factor assigned to each fixture relative to its impact on the drainage system. The dfu values are determined based on:

    •Average rate of water discharge by a fixture;

    •Duration of a single operation; and

    •Frequency of use or interval between each operation.

    Because dfu values have a built-in probability factor, they cannot be directly translated into flow rates or discharge rates. A dfu is not the same as a water supply fixture unit (wsfu) in Table E103.3(2), Appendix E.

    James Richardson
    Plumbing Inspection Supervisor
    City of Columbus

  • 14.  RE: DFU to GPM

    Posted 09-10-2019 10:08
    Victoria - it's not just IAPMO but also ICC that uses the same 1940's Hunter's Curve - problem is that our industry needs a bunch of research to update these kinds of things, and we just haven't had that kind of investment from society for a long while.  If you want to help generate new data, advocating for research funding (both ASPE, IAPMO, and others) is what we all need to do to push much needed updates to our code.

    Christoph Lohr
    Plumbing Engineer
    Henderson Engineers, Inc.

  • 15.  RE: DFU to GPM

    Posted 09-09-2019 08:54
    I have always used:
    Average Flow = GPD/(60 x normal bldg. hrs of operation)
    Peak Flow = Avg flow x 2.5

    The above comes from an ASPE design guide book by James C Church​

    Richard Snow

  • 16.  RE: DFU to GPM

    Posted 09-09-2019 12:31
      |   view attached
    Aijan Lu wrote a great piece on this for the ASPE Journal a while back. It exceeds the code min of 2:1. I use his method all the time.

    Manuel Manzano
    Senior Associate
    Michaud, Cooley, Erickson
    Minneapolis MN

  • 17.  RE: DFU to GPM

    Posted 09-10-2019 15:29
      |   view attached
    I was doing research on some old papers and found Nomograph 31.  It included all of the equations we learned in CPD class.  Plus it included a conversion of DFU's to GPM's and used that as a method to size vent stacks and stack vents.  (I considered that Robert S Wyly and Herbert N Eaton where either acquaintances with Dr. Hunter or were privy to his notes or fellow researchers.)  Interestingly, the results of Nomograph 31, very closely match the Table ​906.1 of the IPC.  So I came to the conclusion that if the DFU to GPM conversion was good for stacks, the same flows would be encountered in building drains, branch drains, etc.  I have included the results in Table 1-8 of Chapter 1 of Volume 2 of the 2018 ASPE PEDH.  This similar conversion is also shown in Figure 11-18 of Chapter 11 of Volume 3 of the 2015 ASPE PEDH

    Interestingly, on the buildings where we estimate the incoming and outgoing flow the incoming is about 8 to 10% higher than the outgoing flow.  Keep in mind that the incoming flow if flowing in a pipe that is full.  so when you flush the WC, that 27 gpm/4 second is instantly flowing through the line.  On the receiving end, the WC is filling up with water, and then siphoning out over a longer period of time.  The Sanitary pipe is not full.  By the time that flush volume travels several hundred feet, the flow is more evenly distributed to an "average" flow.

    Watch if the civil engineer asks you about sizing a lift pump on site.  Keep in mind, "in the building," we need to keep the pipes open so air can flow and protect trap seals.  Once you get outside the building, there are no traps to protect.  One civil engineer told me that they use a 50% diversity on the DFU's they get from the Plumbing Designer.

    So if you look at the Hunter Curve for drainage, the curve on the small number of DFU's is very steep, likely gpm/2 = DFU.  Since we are sizing pipe on the small end where we are making connections to system with continuous flow units, likely a very good conversion.  We still size the drainage piping for 1 DFU for the lav per plumbing code.

    Concerning flows from 0.5 gpm lav faucets . . . James Church in his 1979 Practical Plumbing Design Guide suggests using 0.5 WSFU's for 0.5 gpm faucets.

    Paul Baker
    Senior Mechanical Engineer
    Jacobs Engineering Group, Inc.

  • 18.  RE: DFU to GPM

    Posted 09-16-2019 09:00

    In our State, Maryland,  peak flow is estimated per the plumbing code. Maryland has a guide for predicting GPD, based on the 10 States Code.

    See link below:





    Michael J. Purtell, PE, CPD, LEED AP, CxA

    Senior Vice-President


    Gipe Associates, Inc.



    1220 East Joppa Road, Suite 223

    Building A, Radio Park

    Towson, Maryland 21286

    Office:     410-832-2420

    Mobile:   410-371-3949

    Going BEYOND the Expected


    W.O. #: