Drainage Fixture Units & RV Plumbing

[TedO]

This is ultimately about a 1968 RV I'm restoring but hopefully has enough plumbing concepts to qualify the forum. Since I haven't found anything to tell me otherwise, I'm trying to follow residential codes/best practices as much as possible in redoing the plumbing. Due to space constraints and for other reasons that would take a lot of explaining, I need to run all of the "greywater" together into one pipe for a few feet. Unless I'm not understanding something, if this was a residential application, any combination other than the lavatory/shower or lavatory/kitchen sink would require 2'' drain pipe. It would be MUCH easier to use 1-1/2'' for all three combined. So, I'm hoping someone can advise me on whether I have the code requirements right and also what would be best in terms of how it functions. The 2'' version could be done. It would be great to have a valid reason for why I don't have to. Thanks in advance.

 

[James Henry]

You can only have 3 drainage fixture units on an 1-1/2" drain. A lav is 1 d.f.u. and a sink or shower is 2 d.f.u., which adds up to 3 d.f.u.'s. if you add another sink your over the limit.

 

[Sponsor]

 

[TedO]

OK - thanks. It seems pretty straightforward. I'm curious why a full bath group with <1.6 gal. WC is 5 f.u. Do they not include the lavatory? Otherwise seems like it would WC, shower and lav, which would be 3+2+1. I'm wondering how they arrive at caveats like that.

 

[Cjlambert]

It's based on the theory that it is very unlikely that more than two fixtures in a bathroom will be in use at one time.

 

[TedO]

OK - thanks. Families with little kids and only one bathroom might argue with that, but I get it.

 

[hj]

RVs, mobile homes, and manufactured homes have their own "codes", which are seldom the same as "residential codes".

 

[TedO]

Thanks hj. Agreed. You inspired me to find this (not sure why it's taken me so long): "NFPA 1192: Standard on Recreational Vehicles, 2018." It has a detailed section on plumbing. I haven't read it word for word, but so far the only reference I can find on drain pipe sizing related to my question is: "Drain pipe sizes shall be determined by the type of fixtures and the total number connected to each drain." Not very helpful by itself. When hooked up to "city water" I think the amount of water entering drains in a RV would be the same as in a residence. So unless I find something more specific, residential codes seem like the best guide.

 

[TedO]

Just found this under "Drainage Piping": "One and one-quarter inch (32 mm) minimum diameter piping shall be required for one and not more than three individually vented fixtures." If I'm reading that right, having three fixtures on a 1-1/4'' drain pipe just doesn't sound like a good idea to me. Does anyone else have an opinion on that?

 

[James Henry]

I looked all over hell online for information on RV plumbing and this is all I could come up with.

 

[TedO]

Where did you find that? That's a copy of my plan!