Kitchen Sink "and/or" Dishwasher DFU UPC Clarification

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Jeff in Seattle

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Hello,

I have a 3" sewer main with a number of branch waste lines that enter it. 3" lines are rated for 35 DFU maximum. I have added a few fixtures in my basement and elsewhere and am approaching the maximum DFU rating for my main. Recently, I was doing some careful calculation of every fixture and waste line that entered the sewer main. My question comes when you look at a DFU table where it states:

  • Kitchen sink, domestic, with or without food waste grinder and/or dishwasher: 2 DFUs
Now, I assume this entry means when you have the dishwasher discharge hose going into the waste disposal like you commonly see most all the time (and like my kitchen was originally before my remodel).

However, the table continues....
  • Standalone Kitchen sinks: 2 DFUs
  • Standalone Dishwashers: 2 DFUs
Okay - DFUs and pipe sizing are supposedly based on the the possibility that every fixture could be drained simultaneously. Since you could feasibly drain a full kitchen sink while the dishwasher was discharging at the same time, wouldn't that count as 4 DFUs needed volume-wise? THAT's my main question. WHY the "and/or" was ever included in this one entry of the DFU chart is confusing. And if for whatever reason you get a zero DFU rating for dishwashers hooked to the kitchen sink, then can you explain why?

It should be noted that I have seen efficiency dishwasher units that you hook to the kitchen faucets. That would make sense to count the sink and dishwasher together since while the dishwasher is being used, the sink cannot. Kind of like the notation on diverters on shower heads versus multiple shower heads. Diverters do not add DFU since the water is going one way or another (not increasing the amount of water down the drain. While piped multiple heads beyond the first showerhead adds 1 DFU per (they can all be on at the same time increasing water down the drain).

Now, I just recently remodeled my kitchen. The UPC demands an air gap device for the dishwasher (not a high loop for the hose). This means you have to have a not so pretty air gap device on the sink (above the flood stage). During the remodel, my wife used up all 5 holes of the new sink with various fixtures leaving no room for the air gap. So, I bought a Johnston tee (an air gap device that can be used on the dishwasher as long as you are on an exterior wall (which I am). So the water discharges up the wall over the flood stage of the sink (this is where the Johnston Tee is installed with a vent on the exterior wall) then down to a wall trap which drains into the kitchen waste and vent line in the wall. This makes clearer sense to me for the standalone kitchen and dishwasher (each 2 DFUs) entries. BUT - depending on the answers from the original entry, does this still count as 2 DFUs?

YES, I know that all fixtures draining at the same time is near impossible and not practicable. Depending on this answer and some of the fixtures I might be installing, I might exceed the 35 DFUs (I am that close). I will likely have to up the sewer main to 4" in the last 10' before it exits the house crawlspace (which goes to a 4" sanitary ground line heading to the septic tank.

Thanks!
 

Cool Blue Harley

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A fixture unit is a numerical load factor. He idea originated with Dr. Roy Hunter, a physicist, in the early 1920's.

The fixture units represent a load factor based upon the laws of probability, owing to he fact that not all discharges will occur simultaneously nor will all discharging flows be coincident. Only a portion of the discharges will actually overlap in he piping system.

A kitchen sink is rated at 2 drainage fixture units with or without a DW.
 

Jeff in Seattle

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A fixture unit is a numerical load factor. He idea originated with Dr. Roy Hunter, a physicist, in the early 1920's.

The fixture units represent a load factor based upon the laws of probability, owing to he fact that not all discharges will occur simultaneously nor will all discharging flows be coincident. Only a portion of the discharges will actually overlap in he piping system.

A kitchen sink is rated at 2 drainage fixture units with or without a DW.

So that withstanding...the kitchen sink is 2DFU's -period. So by extrapolation, does that mean the dishwasher is 0DFU's when attached to a kitchen sink but rated at 2DFU's when standalone (or additional DWs)? And can I have your opinion - do you think they wrote that "and/or" line when the dishwasher is hooked under the sink (before the trap - as they usually are). And further, if, (like in my case), now the dishwasher has it's own trap and drains into the kitchen sink waste/vent line (after the sink's trap), how would you rate the DFU's? Thanks so much Cool Blue Harley for your insight.
 

Cool Blue Harley

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Jeff. Your DW is zero DFU when discharging into your kitchen sink. The same would be true of your clothes washer when discharging into a laundry sink.

If your DW has its own trap and vent, and is combined downstream from the trap and vent from your kitchen sink, you would then have a total of four drainage fixture units on that pipe.

My 2013 UPC/California code book doesn't show the and/or you are referring to and my 2016 is in the office.

Your conclusion regarding the and/or is correct.

Hope this helps!
 

Kreemoweet

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There's only so much water that can be discharged from a single trap, of a particular size. It doesn't matter if the trap serves a sink only, a
sink and DW, a sink and disp., or a sink and DW and disp. (the "and/or" is used merely to shorten that last phrase).
For example, sink plus DW discharge volume will not generally equal sink only volume PLUS DW only volume.

That said, these rules are not explainable solely by the physics involved. Other things, such as tradition and economic interest,
play their part. That's why different plumbing codes are sometimes very different. I note that my (oldish) copy of the UPC
gives a kitchen sink plus DW a 3 DFU rating, rather than the 2 you mention.

And bye the bye, I don't think a Johnson tee requires an exterior vent. I've seen, and installed, the vents in an interior wall surface many times. It's no different in that respect from any other air gap.
 
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Jeff in Seattle

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There's only so much water that can be discharged from a single trap, of a particular size. It doesn't matter if the trap serves a sink only, a
sink and DW, a sink and disp., or a sink and DW and disp. (the "and/or" is used merely to shorten that last phrase).
For example, sink plus DW discharge volume will not generally equal sink only volume PLUS DW only volume.

That said, these rules are not explainable solely by the physics involved. Other things, such as tradition and economic interest,
play their part. That's why different plumbing codes are sometimes very different. I note that my (oldish) copy of the UPC
gives a kitchen sink plus DW a 3 DFU rating, rather than the 2 you mention.

And bye the bye, I don't think a Johnson tee requires an exterior vent. I've seen, and installed, the vents in an interior wall surface many times. It's no different in that respect from any other air gap.


Wow, thanks so much kreenoweet! For grins, I had a spreadsheet with the 2DFUs and the 4DFUs - then a 3DFU for average-sake! As for the Johnston tee, yeah I guess you could put the ugly pvc vent cover on the inside, but it's pretty hideous. Too bad someone doesn't make a fancy, decorative cover that would fit in that o-ring connection!
 

Jeff in Seattle

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Jeff. Your DW is zero DFU when discharging into your kitchen sink. The same would be true of your clothes washer when discharging into a laundry sink.

If your DW has its own trap and vent, and is combined downstream from the trap and vent from your kitchen sink, you would then have a total of four drainage fixture units on that pipe.

My 2013 UPC/California code book doesn't show the and/or you are referring to and my 2016 is in the office.

Your conclusion regarding the and/or is correct.

Hope this helps!

Oh yes Cool Blue Harley - it does. I have come to the conclusion that my 3" sewer main will be at 35 or 36 when it reaches the footing where I will eventually be adding the final waste line (with 2 to 5 DFUs). Since the Kitchen Sink and Dishwasher were actually the very first thing on my sewer line, this answer was somewhat important as it skewed every DFU all the way down the line. I believe just on the other side of the footing there is green 4" PVC line outdoors underground. I will extend that just under the footing and the where the new waste line will connect. At the wye, I will increase to 4" at the transition from ABS to the sanitary piping. The good thing is that I won't have to up the existing sewer line in my crawl since I will be at max DFUs.

One more question - a kitchen sink or laundry tub is rated at 2DFUs, while most other sinks are 1DFU. Do you happen to know where I can look up the volume of the sink before it goes from 1DFU to 2? The ground floor over the crawl will be a mother-in-law's quarter with one bed, bath, common room, kitchenette and a laundry closet (hence where most of the new waste lines will be added). I wasn't going to put in a large, standard kitchen sink with two basins since this will be more like an efficiency space (or bonus/home theater for us). It's not a big deal since I have to up the final connection anyway, but I was wondering how they rated the DFUs.

Jeff
 

Jeff in Seattle

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Picture back from kitchen remodel with Johnston tee moving the air gap off the sink and outdoors. This also meant adding a trap to the dishwasher and into the sink waste/vent line. It was much cleaner afterwards. Second pic is the ugly Johnston Tee on the exterior wall. You can see the sink was filled on the third pic, and the backsplash was tiled and pretty busy. The high loop was just to the left of the electrical box, so it ended up better to be outside. Finished kitchen is the last picture.
 

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