DFU Requirements

glenmbOctober 27, 2009


My inspector flagged my permit application for my master bath renovation and has given me a chance to resubmit

The basics

4x5 Shower

3/4" Thermostatic Valve going to seperate volume controls for:

Rainshower Head - 2.5GPM

Standard Head - 2.5GPM

Slidebar - 2.5GPM

3 Body Sprays - 5.4GPM total

2 inch drain

Inspector says based upon his DFU calculations he would fail - and my options (with the issues)

Put in a diverter valve so not all heads could be opened at once - but would limit what head could work with another

Put in a 2nd 2 inch drain - pain as the wedi pan is orderd and comes with one 2 inch in center

Go with a 3 inch drain - pan as above and additional plumbing would be a big undertaking

The inspector was very nice on the phone and worried about all valves being opened at the same time - I just want to better understand from the plainspeak on this great forum if his interpertation is correct and what at least national DFU code says - if pushed think I will be back to ask about installing that 2nd drain

Many thx in advance - Glenn

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"...least national DFU code says..."

You have to be able to get rid of all the water that can enter.
As simple as that.

    Bookmark   October 28, 2009 at 9:09PM
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In the plumbing industry the abbreviation "DFU" stands for "Drainage Fixture Units" and it is the official unit of measure for determining the size of all DWV (Drain, Waste & Vent" lines.

Although the actual numeric value may vary from one code to another, the end result is always the same.

In your code there is a table that lists every type of fixture which may be connected to the drainage system and it assigns a specific numeric value to that fixture.

By example, under the IRC(International Residential Code) a shower is assigned a value of 2dfu's, when the shower enclosure has more than one shower head we must assign the value to each shower head that may be used simultaneously. According to your list you have a total of 6 shower heads so the combined DFU load for your shower stall would be 6x2-12dfu's.

Once we have determined the total DFU load we then consult another table that lists the maximum number of DFU's which may be conveyed by the different sizes of pipe.

By example, code generally calls for a 2" drain for a shower however, under the IRC a horizontal 2" vented branch line would be limited to a maximum of 6dfu's and a horizontal 3" line is limited to a maximum of 20dfu's, therefore, just as your inspector has already informed you, your options would be two install two 2" drains for a combined total of 12dfu's or you could install one 3" drain that can handle up to 20dfu's.

The only other option is to install diverter valves that would limit the number of shower heads that can be used at any given time to insure the dfu load does not exceed 6dfu's.

In my jurisdiction they would not allow installing the diverter to limit the calculation because there is no way to insure the diverter would not be removed at some future date, We are required to provide drainage equal to or greater than the combined load of all shower heads. regardless of how the valves are configured.

Under both the IRC & UPC we gave another table that lists the maximum number of dfu's permitted on the pipe for the building "Main Drain" and "House sewer".

Under the UPC we have a 4th table that is used for sizing vents.

    Bookmark   October 29, 2009 at 9:55AM
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Your inspector is correct in that your 12.9 DFUs are too many for a 2 inch drain, but only marginally so.

Under the IRC plumbing provisions, a 2" shower drain can only carry a maximum of 12.3 DFUs....so you would have no choice but to install a 3" drain...

Two 2" drains 'might' work but only if designed and approved by an engineer and only if your downstream drains could still handle the flow.

Your inspector cannot approve a 2nd 2" drain without a design professional signing off on the plan, and that translates for you into $$$ spent hiring an engineer.

Either reduce your DFUs by eliminating some of the shower heads, or go with the 3" drain all the way to your building sewer.

    Bookmark   October 29, 2009 at 8:18PM
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I don't know where Manhattan is getting 12.3dfu's for a 2" line,

On IRC table 3005.4.1 for branches and stacks a 2" line is permitted a maximum of 6dfu on a horizontal run and 10dfu's on a vertical run.

    Bookmark   October 30, 2009 at 12:50AM
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The maximum flow in any size of pipe can be calculated without going to the DFU tables.
Make the total match the carrying capacity of a two inch drain.

What distance is between the shower drain p trap and the next bigger pipe? Downstream.
What distance is there until venting?

With fixtures that spray less than 2.5GPM each, and body sprays less than 1.8GPM each, you might fit a lot of them onto a 2" drain.


    Bookmark   November 1, 2009 at 8:26PM
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