# DFU load when 2 dishwashers. ( = 3" pipe ? )

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by geniescience, Jun 3, 2010.

1. ### geniescienceHomeowner

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Is this right?

2 DFUs per dishwasher
2 DFUs per kitchen faucet

Total is 6 DFU when there are two DWs, and that requires a drain pipe diameter of 3"
Total is 4 DFU when there is one DW, and that requires a drain pipe diameter of 2"

Y / N ?

2. ### jimboPlumber

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Under CA Code (UPC) a kitchen sink WITH DW = 2 DFU. Additional DW = 2 DFU .

Is this one sink with 2 faucets and 2 DW? 2 Sinks each with one faucet and DW?
2" pipe horizontal accepts 8 DFU.

4. ### hjModerator & Master PlumberStaff Member

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The NEXT domestic sink drain with DW that I use 2" for will be the FIRST ONE, and the same would be for two DWs.

5. ### geniescienceHomeowner

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OK, I get it now.
One kitchen faucet AND one dishwasher makes two DFU's. Is this right? (Y / N ?)
Which takes a 1.5" P trap and drain.

Adding an extra dishwasher adds two DFU's, which then fits easily on a 2" diameter trap. (Y / N ?)

6. ### geniescienceHomeowner

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If I understand you right, you have never used 2" for a kitchen sink drain with DW. And the second part of your sentence seems to say the same thing, i.e. that there is no need for 2" even with a second dishwasher.

7. ### Doherty PlumbingJourneyman & Gas Fitter

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Don't listen to HJ he's old school

If you can you should ALWAYS run a 2" drain for your kitchen sink. The trap arm doesn't need to be 2" but the soil-or-waste stack should be and everything in the ground should be a min. of 2" as well.

But if you have 2 dishwashers it would seem that your local code would require you to have a 2" trap as well. Because there is no sense in having a 1.5" trap feed a 2" line if the reason you running 2" is because you need to handle the volume of water you're getting. You can only drain as much as the smallest pipe will allow. But I recommend 2" on the S-O-W because of clogging issues. I go into homes all the time that have plugged kitchen sinks. 2" won't prevent it but it will HELP in the long run!

8. ### geniescienceHomeowner

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I've 2" pipe "in the soil-or-waste stack and everything in the ground".
And 1.5" in the trap and arm.
What threw me was a chart I found showing 2 DFU for dishwashers, separate from faucets.

Huh? Is that right? I'm confused.
Now secondly I'm also curious.

What makes a DW a 2 DFU device? A dishwasher Tee has a 1/2" diameter for the dishwasher drain hose connection (!) because it doesn't _ need _ a large diameter to pass that drain water over to the P trap. This can be seen in all the products sold as dishwasher Tee's, without exception, and they are installed in millions of houses and have passed millions of inspections. So why would a DW be = 2 DFU's?

What about those folks who have several faucets (like filtered tap water, instant scalding hot, and a second full-blast faucet at a prep sink) and two dishwashers. Is this going to put the total beyond 8 DFU's? Do they need to upsize everything to 2.5" or 3" ? This is not like a bathroom with multiple shower heads and body sprays, because only two of the faucets are high-flow. Everything else is low-flow like a drinking fountain. Please somebody help me understand this!

9. ### nukemanNuclear Engineer

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In our code:

- kitchen sink = 2 DFU
- DW = 2 DFU
- kitchen sink w/ food waste grinder and/or dishwasher = 2 DFU

So, if you just had a sink or a DW, you would need a 1.5" line. With both together, you still only need a 1.5" line. Often, plumbing codes will consider probabilities when considering multiple common items working together. You probably won't be draining a full sink of water everytime the DW is draining. As an example, a bathroom group is counted as 5 DFU, but if you just added up the WC + shower + lav (3 + 2 + 1), you get 6 DFU (or more if you had double lavs, a separate tub, etc.). The thought is that you aren't likely to be using all of them at the same time.

For a DW, the discharge line may be small, but remember it is a pumped source (not gravity only). That is why is needs a drain line larger than just the dishwasher drain hose connection.

With your pair of DWs, are you planning on discharging them at the same point (kitchen sink) or is one at an island or some other part of the kitchen? If you discharge them at the same point, you may have to step up to 2". You should talk to the inspector to see what he thinks. Since it is not real likely that both DWs will be running and draining at the same time, he may allow for something smaller than 2".

10. ### hjModerator & Master PlumberStaff Member

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I DO run a 2" line to the sink LOCATION in the wall, but everything outside the wall is 1 1/2". I have NEVER used a 2" trap on a kitchen sink, regardless of how many sink bowls or dishwashers there are. But to clarify the statement about it being a "pumped" drain, once it enters the air gap OR disposer, it converts to a gravity drain. It would ONLY be a pumped drain if it were connected directly to its own "P" trap, which is SELDOM done because of sanitary reasons.

Last edited: Jun 4, 2010
11. ### geniescienceHomeowner

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That makes sense. I've always seen it that way.
After reading what Nukeman said about "groups", I think I've learned something new.
It's clearer than ever now what a group is and why.
Recently I heard about upsizing beyond 2". I guess that was a mistake.

12. ### nukemanNuclear Engineer

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Right, hj. I may not have been clear on my post. He was wondering how the DW could be 2 DFU if the discharge line is only 1/2" and was wondering why it needed a 1 1/2" drain if it was only receiving a 1/2" supply. Since the 1/2" line is pumped, that is the reason why the smaller line needs to be increased to something larger when it goes from being a pumped situation to a gravity one.

As far as the bath groups go, I have seen codes that spell out 1 bath = 5/6 DFU, 2 baths = something less than 10/12 DFUs, etc. The code that I am under doesn't spell that out (at least not that I've seen). Again, the reason is that it is not likely that you will flush every toilet in the house, while having every shower, lav, etc. going at the same time.

Sounds like 2" in the wall will do the trick. You just have to figure out what you want to do about the traps and trap arms. The 1.5" stuff is much easier to come by. I don't recall ever seeing DW connections, baffle tees, etc. in 2", but that doesn't mean they don't exist. I'm just a DIY'r like yourself, so I've just seen what is sold in the home improvement stores. Maybe the pros can point out if plumbing supply houses typically have these fittings in sizes > 1.5".

Once you have a plan, run it by your inspector and see what he thinks. What you can actually do depends on your local code and how the local inspector interprets that code.

13. ### geniescienceHomeowner

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DFU and confusion

In a homeowner's forum, a plumber who knows how to quote Code wrote that UPC Table 7-5 says
• 1-1/2" pipe is rated 2 DFU on a vertical run but only 1 DFU on a horizontal run.
• a kitchen sink is 3 DFU

I can't find Table 7-5 on the web so I'm unable to go to his sources and see for myself what he may be misinterpreting.
He has said that a 2" pipe is required everywhere in the kitchen.

14. ### hjModerator & Master PlumberStaff Member

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quote; 1-1/2" pipe is rated 2 DFU on a vertical run but only 1 DFU on a horizontal run. a kitchen sink is 3 DFU

In that case ALL the inspectors I have EVER worked with were incompetent, because they have ALL allowed a 1 1/2" horizontal arm to the sink location, AND 1 1/2" piping inside the cabinets regardless of the pipe size in the wall.

15. ### jimboPlumber

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His reference to table 7-5 is correct. But a kitchen sink is 2 dfu, not 3.

There is nothing wrong with using 1Â½ for a kitchen sink, up to the point where anything else ties into it. It also would not be unusual to use 2" in the wall,, "just because".

16. ### geniescienceHomeowner

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1 DFU when horizontal, means the arm has to be 2" (downstream from the P trap and before the 'wall').
This (as HJ said) has not been required before.

Did something change while I was away on vacation last month?

17. ### nukemanNuclear Engineer

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From what I have seen online:

- UPC Table 7-3 lists the trap size for different fixtures (1.5" for kitchen sink)
- UPC Table 7-5 lists the DFU capacity for horizontal/vertical DWV (1.5" has 1 DFU capacity horizontal and 2 DFU vertical)

However, Table 7-5 excludes the trap arm (section from trap to vent). The trap arm is the same size as the p-trap (1.5" in this case).

So, under UPC, a kitchen sink can use 1.5" to the vent. Typically, there is a santee and the flow goes downward at that point, so that could also be 1.5". If you had the situation where the plumbing remained horizontal after the vent, then you would have to step up to 2" at that point.

Clear as mud?

Now, there can be local changes to the code and maybe the info that I am seeing is outdated, but from the pros on here (and from what I could find), it looks like 1.5" is all that is needed in a typical kitchen sink installation. The only question would be what your local AHJ thinks about the 2 DW setup.

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Kitchens have 1-1/2" trap arms and p-traps.
It's been that way forever.

After the trap arm it becomes 2"
That would be in the wall at the point that it's vented.

19. ### geniescienceHomeowner

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Clear. Great information too!

20. ### geniescienceHomeowner

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Clear. This confirms it!

21. ### nukemanNuclear Engineer

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Terry: I suppose the reason (besides just being a good idea) to use the 2" in the wall is that you'll probably have to go horizontal at some point before hitting the stack. If you used 2", then you could run horizontal or vertical and still meet code.