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1. ## fixture units

Twinpeaksr's post and Terry's reply brought something to a head for me. I don't understand how to use fixture units for supply or DWV. I only know they help calculate pipe sizing requirements based on fixtures served. Can someone here help please? Thanks

2. You get drainage fixture units and that relates to the pipe size (somewhat) and more to the venting pie sizes.
Water supply fixture units relates to water pipe sizes.

What are you trying to understand?

3. I'm starting with the assumption that there are charts for these units, and that x number of units requires a certain size pipe, be it supply or DWV. A lav sink would seem to have less Fixture Units than a shower, for example. But that's the extent of what I think I know.

4. i wish i could help but if it was easy everyone would be doing it.

the dialogue would fill this page and more on how to properly size dwv(drain waste and vent), that being said if you had a more specific question i might be able to help you out.

5. Each plumbing code has charts of drain and supply FU numbers. The Wisconsin code happens to be available on line. Go to page 8 of this pdf ( it is page 26 of the document) http://www.legis.state.wi.us/rsb/code/comm/comm082.pdf

6. ## Dfu

Drainage sizes are dependent on the number of fixtures connected to the pipe and the pipe lengths. Water fixture units are also based on the number of faucets, BUT, there is also a factor applied because they know that not every faucet and toilet is going to be used at the same time. Therefore, water supply tables vary according to the building's usage and number of occupants.

A fixture unit is a fictitious number (from a code which is based on years of collective experience) assigned to a fixture (just a number assigned to a fixture), be it waste, vent, water supply, etc.
It provides a diversity to sizing, in other words if you had two 2" lines from a fixture, you would not size the collector pipe for twice the area of one 2" pipe. This is done because it not highly likely that all the fixtures would be turned on at the same time.
Some codes like the NPC or the IPC have examples on sizing in their appendix.

8. ## another Good answer HJ

9. On the DWV side, is there a down side to oversizing the vent or drain? I can't think of any but thought I would put it out there.

Thanks!

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