How do I convert Water Supply Fixture Units to GPM?

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Seattle

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Need some assistance at understanding how to convert WSFU's to GPM. I have a total of 29.0 WSFU in a single-family residence computed as follows:

1 Bathtub 4.0
2 Clothes Washers 8.0
1 Dishwasher 1.5
2 Hose Bibbs 3.5
2 Lavatories 2.0
2 Kitchens 3.0
1 Shower 2.0
2 Water closets 5.0
TOTAL 29.0

How do I find out the total GPM for this group of residential fixtures?

My jurisdiction uses the UPC 2009.

Thanks for your help.

Water pipe sizing
 
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Jadnashua

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From the Engineering Toolbox website:
The Water Supply Fixture Units - WFSU - are used to determine the water demand in water supply systems. One WFSU for a singel unit corresponds to one GPM.
  • 1 WSFU = 1 GPM
This conversion can only be used for one or a few fixtures. When the total amount for many fixtures are added up, the number must be compensated due to the intermittent use of the fixtures. This is normal taken care of in the tables available for sizing supply pipe lines.

Since you rarely would ever have all fixtures turned on at the same time, the sizing tables take care of this. Note also, that the bathtub valve can easily supply 6 or more gpm, and if you run a 3/4" valve, maybe in the high teens, so those values are nominal. If you have a large garden tub, you may want or need more capacity than the minimum. A hose bib could also provide lots more than listed...those values are minimums.
 

Jimbo

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2 kitchens/2 clothes washers makes it sound like a 2 family residence. certainly all 4 of those things could be in use at the same time. don't scrimp, and don't forget to "upsize" if using pex or cpvc
 

Seattle

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I knew that 1 WSFU was equal to 1 GPM but only if you have a few fixtures. Given that I have 29 WSFU's, how would I come up with the correct GPM?

The property is one single-family house that has a little converted garage unit behind it.

Thanks.
 
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hj

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You have to use the tables listed in your local plumbing code, which compensate for the "load factor" depending on the building's occupancy. And even they seldom convert to gpm, they convert it to the size pipe needed based on distance from the meter to the furthest fixture, pressure at the main/meter, and building height.
 
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