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Old 10-22-2019, 08:01 PM   #1
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Good resource for calculating H20 supply / drain pipe size?

Need to figure out what size (ID) pipe to use for feeding fresh water system (sink, shower, & external shower), and for the grey-water drain (sink, shower). No black.

I thought this would be easy, but from a quick google search, maybe not so much. Not looking for a handout - just a straightforward guide to figure this out on my own.

Also, what are y'all running for slope on the drain? Do you run more slope than you would in a residential application to make up for the fact you might not be level?
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Old 10-22-2019, 08:19 PM   #2
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Need to figure out what size (ID) pipe to use for feeding fresh water system (sink, shower, & external shower), and for the grey-water drain (sink, shower). No black.

I thought this would be easy, but from a quick google search, maybe not so much. Not looking for a handout - just a straightforward guide to figure this out on my own.

Also, what are y'all running for slope on the drain? Do you run more slope than you would in a residential application to make up for the fact you might not be level?
Whatís the GPM of your pump? What are your fixtures?
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Old 10-22-2019, 08:38 PM   #3
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3.5 gpm pump, 1.5 gpm shower head, 1.5gpm sink faucet, outdoor multi-use water fixture 2 gpm.
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Old 10-22-2019, 09:13 PM   #4
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A 1-1/4" drain pipe can gravity flow 25gpm.
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Old 10-22-2019, 09:52 PM   #5
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That's good to know, Marc. Many thanks.



I'm trying to get it as small as practical. I've relatively narrow channel in which I can run it & not bump into other stuff either already there or planned, so the smaller the diameter the more slope I have to work w/.
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Old 10-22-2019, 10:02 PM   #6
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An 1 1/2” would be your standard for a fixture in a house. A bus/RV has such low water usage it seems to me that you can run 1 1/4” if you have to, but I think you’ll be limiting your fitting selection.

How long are your runs to the grey tank?

You’re setting up for street water too?

I think I’d go with 1/2” water pipe.
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Old 10-23-2019, 06:29 AM   #7
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I have 1 1/4 for drains, and 1/2 pex for water. My old camper used 3/8 pex and that seemed to work just fine.
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Old 10-23-2019, 10:29 AM   #8
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An 1 1/2” would be your standard for a fixture in a house. A bus/RV has such low water usage it seems to me that you can run 1 1/4” if you have to, but I think you’ll be limiting your fitting selection.

How long are your runs to the grey tank?

You’re setting up for street water too?

I think I’d go with 1/2” water pipe.
~10' of grey water drain. Yes on city water, but it will be regulated down to pump pressure.

The more I think about it, it's unlikely 1/4" of diameter will make or break me being able to achieve my desired slope. So if 1.5 won't cut it, I probably need to try to figure out another way.
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Old 10-23-2019, 11:35 AM   #9
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3.5 gpm pump, 1.5 gpm shower head, 1.5gpm sink faucet, outdoor multi-use water fixture 2 gpm.
What pump are you using?
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Old 10-23-2019, 11:48 AM   #10
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Shurflo 2088
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Old 10-24-2019, 01:12 PM   #11
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In response to the "what's a good resource" question: "pressure loss table" is the magic phrase to search for. You'll find data such as this showing volume of water contained in a 100 foot length of various diameters of PEX as well as the pressure loss incurred at various flows with various diameters.

Based on those tables 3/8 PEX is likely adequate on account of the short distances and low flows involved in an RV. I do like that it has a small interior volume -- that means hot water arrives at the faucet faster. But maybe the pressure loss is a bit much in the RV context where water pressure is already rather low. It's also a little challenging to find 3/8 fittings, but that's a one-time problem.

In any case the tables show that 1/2 PEX is abundantly adequate for the water supply job. There's definitely no need to go larger, with one minor exception. If your fresh water tanks fill directly from the city pressure connection it might be worthwhile to build that section with 3/4 to help the tanks fill just a little faster.
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Old 10-26-2019, 11:20 AM   #12
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Muchas Gracias!
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