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Question On Water Flow

 
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Old 07-06-2019, 03:32 PM   #1
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Question On Water Flow


A thought experiment, not based on any actual scenario:

If I have a long run of 3/4" diameter water supply line (PEX, Copper, whatever), and somewhere in the middle of that run, the diameter briefly drops down to a 1/2" fitting, then right back up to 3/4....would the volume or pressure of the entire run of pipe be limited by the presence of just the one smaller fitting? Does it bottleneck the entire system after that fitting, in other words?
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Old 07-06-2019, 04:03 PM   #2
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Re: Question On Water Flow


stretch out a garden hose and turn the water on.

now get a pair of vise grips and partially clamp the hose mid way.

what happened????

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Old 07-06-2019, 06:39 PM   #3
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Re: Question On Water Flow


In reality, not so much. A 3/4" pex coupler is about 1/2" ID, but it slows flow as much as 5 feet of pipe.
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Old 07-06-2019, 08:05 PM   #4
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Re: Question On Water Flow


Quote:
Originally Posted by griz View Post
stretch out a garden hose and turn the water on.

now get a pair of vise grips and partially clamp the hose mid way.

what happened????
The visual is a good one. Thanks.

So, having the increased volume from larger diameter pipe behind the fitting does not compensate for the smaller fitting? Would, for example, a 1" pipe going down to a 1/2" coupling flow faster on the other side than a 3/4" down to 1/2"?
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Old 07-06-2019, 08:40 PM   #5
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Re: Question On Water Flow


Quote:
Originally Posted by onmywayup View Post
The visual is a good one. Thanks.

So, having the increased volume from larger diameter pipe behind the fitting does not compensate for the smaller fitting? Would, for example, a 1" pipe going down to a 1/2" coupling flow faster on the other side than a 3/4" down to 1/2"?

Golden has the approach you're looking for. Each fitting has a resistance to flow equivalent to X many feet if pipe. You're looking to have a system resistance equivalent to however many feet of pipe.

Yes, you can compensate for a restriction by upsizing the piping. There are limits.
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Old 07-06-2019, 08:43 PM   #6
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Re: Question On Water Flow


Quote:
Originally Posted by onmywayup View Post
The visual is a good one. Thanks.

So, having the increased volume from larger diameter pipe behind the fitting does not compensate for the smaller fitting? Would, for example, a 1" pipe going down to a 1/2" coupling flow faster on the other side than a 3/4" down to 1/2"?
You'll get a higher gpm with the larger hose. The flow rate is the same no matter what side of the restriction you're on.
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Old 07-06-2019, 11:50 PM   #7
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Re: Question On Water Flow


Flow is how much water per GPM,

Pressure, static or dynamic
Velocity is how much force.


Add up all of the devices like aerators and showerheads downstream. If you have a full bath with two sinks downstream figure around 5gpm (high end) not counting the toilet. Typical household pressure should be around 65. You're gonna lose a pound for every 10ft of head (rise) plus a little due to friction loss inside of pipes (differs with type of pipe) as well as 90's and such but figure a couple of pounds tops going from the POC, through and upstairs to supply a bathroom, reducing it to a 1/2 shouldn't be an issue for one bathroom but I wouldn't want to supply a whole house if I can plumb all 3/4 till the last tee. If you can fill a bucket in less that 60 seconds on that reduced flow you're golden for at least one bathroom. How fast that fills depends on your pressure.

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