Demand based sizing?

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JoeJee

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I am using Fixture Units to get a size of “x” pipe. When I start looking at demand and specifically the expected GPM required when certain fixtures are running things get a bit more confusing.

Can I discount the expected GPM b/c we don’t usually run hot or cold at 100%

What psi drop is tolerable when calculating friction loss?

Is there an equivalent length chart for PEX-A?

Looking at some of these I came up with 1.5-2” copper for hot trunk which I know is ridiculous so trying to figure out what I did wrong. Could just be the HVAC contractor side of me wants really low friction in the pipes…
 

wwhitney

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Looking at some of these I came up with 1.5-2” copper for hot trunk which I know is ridiculous so trying to figure out what I did wrong.
Are you designing the system to allow all the hot water fixtures in the building to be used at once? That would be overkill. You could try sizing the pipes as if only the two largest fixtures the pipe carries are being used simultaneously (but at most one tub filler).

On the other questions:

- For, say, a showerhead that uses 2 gpm, if you have your water heater set to 130F and your cold is 50F, and you know that the user will want 110F max at the showerhead, then yes I'd say the showerhead hot demand is only 1.5 gpm.

- Depends on the supply pressure and the minimum the fixture requires. If your supply pressure is 60 psi, and your showerhead, say, is rated at 40 psi, then you could drop 20 psi on the piping, end to end.

- The manufacturer will publish one.

Cheers, Wayne
 

Fitter30

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If a system is designed and all fitting,valves and devices are figgered in for fiction loss still would want to add another 10-20% for piping changes and design.
 

Terry

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I think this chart is for PEX

ipc_water_size_50.jpg


And a copper chart

water-sizing-chart-terrylove-40-60.jpg
 

JoeJee

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Are you designing the system to allow all the hot water fixtures in the building to be used at once? That would be overkill. You could try sizing the pipes as if only the two largest fixtures the pipe carries are being used simultaneously (but at most one tub filler).

On the other questions:

- For, say, a showerhead that uses 2 gpm, if you have your water heater set to 130F and your cold is 50F, and you know that the user will want 110F max at the showerhead, then yes I'd say the showerhead hot demand is only 1.5 gpm.

- Depends on the supply pressure and the minimum the fixture requires. If your supply pressure is 60 psi, and your showerhead, say, is rated at 40 psi, then you could drop 20 psi on the piping, end to end.

- The manufacturer will publish one.

Cheers, Wayne
This is helpful, thank you. I am not expecting all but did add up several that could be on at the same time. That is how I ended up with what I did. I also used 3-5 psi drop. Maybe I can go to 10-15? For a rough in, is there not a general rule for fixtures since you won’t know what the final manufacture will be?
 

Terry

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For a plumbing inspection, we work the numbers backwards on the chart.
It's not considered good plumbing if you're asking people to wait to use a kitchen sink or run a load of wash while someone is showering.
Inspectors will count up the fixture units according to their charts and either pass or fail the job.
 

wwhitney

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I took the OP's question to mean this:

From first principles, you can determine the pressure loss through water pipes based on the sizes and lengths (including fittings) and flow rate, and look at fixture demands (flow vs psi for the fixture) and model how a given system will perform. Much the way it is done for gas piping.

But for water we also have the notion of fixture units and the associated charts. Presumably those were designed using the above engineering method based on some assumptions. What are those assumptions? How many GPM per fixture unit? How many of the fixtures do you need to consider operating simultaneously (that could mean that the translation from fixture units to GPM is not proportional)? What is the allowable frictional loss in psi?

I think the OP did one design using the fixture units method, and then did another design using the pressure loss computation method. For the latter, he had to make some assumptions about the answers to the questions in the previous paragraph. And apparently the assumption made were very conservative, compared to the tables, as he says he got answers there were way too big.

So another way of putting the question is: how do you recreate the fixture units tables from engineering principles? How did the code writers come up with those tables?

Cheers, Wayne
 

Terry

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I'm taking a shower in a 35" x 35" space. Someone is at the kitchen sink, and now the water is either too hot or too cold. The space is so small that I can't get out of the way of the spray without opening the shower door and waiting for whoever is at the kitchen sink to finish up. They have no awareness of the discomfort they're causing me.
 

JoeJee

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I took the OP's question to mean this:

From first principles, you can determine the pressure loss through water pipes based on the sizes and lengths (including fittings) and flow rate, and look at fixture demands (flow vs psi for the fixture) and model how a given system will perform. Much the way it is done for gas piping.

But for water we also have the notion of fixture units and the associated charts. Presumably those were designed using the above engineering method based on some assumptions. What are those assumptions? How many GPM per fixture unit? How many of the fixtures do you need to consider operating simultaneously (that could mean that the translation from fixture units to GPM is not proportional)? What is the allowable frictional loss in psi?

I think the OP did one design using the fixture units method, and then did another design using the pressure loss computation method. For the latter, he had to make some assumptions about the answers to the questions in the previous paragraph. And apparently the assumption made were very conservative, compared to the tables, as he says he got answers there were way too big.

So another way of putting the question is: how do you recreate the fixture units tables from engineering principles? How did the code writers come up with those tables?

Cheers, Wayne

Mostly, yes, that is what I am getting at. I get Equivalent Length, I get static pressure, I get friction loss. I don’t understand Fixture units outside of rough estimates. I get that they give me a distance but it still doesn’t tell me anything. To me Fixture Units is more of a wild A$$ guess than the other ways to size pipe for fluid flow without having an engineering background.

So Terry has the correct idea. Essentially I am trying to avoid the unnecessary pressure loss when multiple fixtures are utilized at different start/end times. Not all but the most common ones. A good EL chart for Pex-A would help a lot if anyone knows of one.
 

JoeJee

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Finally found one. I’ll do some calculations with EL/10-15psi drop and report back to see how it differs from FU sizing.
 

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JoeJee

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Quick update:
Spending very little time I came up with a developed length of 217.4 ft at my furthest fixture (not counting hot water return line).

This calculates to an 1-1/4 PEX using Fixture Units at the 250’ mark.

This Calculates to a roughly 1” Copper pipe based on Figure E103.3.

I haven’t found a good calculator for friction loss and distance but this is a start. Looks like 1-1/4 Pex for part of the trunk might be needed….
 

John Gayewski

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Mostly, yes, that is what I am getting at. I get Equivalent Length, I get static pressure, I get friction loss. I don’t understand Fixture units outside of rough estimates. I get that they give me a distance but it still doesn’t tell me anything. To me Fixture Units is more of a wild A$$ guess than the other ways to size pipe for fluid flow without having an engineering background.

So Terry has the correct idea. Essentially I am trying to avoid the unnecessary pressure loss when multiple fixtures are utilized at different start/end times. Not all but the most common ones. A good EL chart for Pex-A would help a lot if anyone knows of one.
A fixture unit is 1gpm. Until you get to 7. As you get higher fixture units and gpm become less linear. This is a never fail 99 percent design calc.
 

John Gayewski

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I think the engineering toolbox has a wsfu to gpm converter. There's no reason to make it more complicated as the wsfu is designed to size water piping at 99 percent of probable use scenarios. There are updated calculators available through IAPMO with more stringent water use considerations built into it.
 
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