Water Supply Pipe sizing

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Polecatt

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Hello,

Looking for some help with a few general water supply pipe size questions.

I'm wondering what the impact on a supply system is of changes in pipe size. I am on Municipal supply fed by 1/2" copper from the main about 25 feet out through the slab. what happens if I run 3/4" as a main trunk off the water meter? Is there any reason to go from the smaller supply pipe size to larger?

Also wondering what the impact of the reductions in pipe size that are introduced with PEX 1/2" fittings which are considerably smaller than the ID of the 1/2" PEX pipe itself. Do these impact pressure and flow rate differently than the standard 1/2" copper fittings that go on the outside of the pipe instead of the inside with PEX?

Any thoughts or comments would be appreciated!!

Thanks

Water pipe sizing

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SacCity

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1/2 water service is no longer valid,
current code reqires a minimum of 3/4 inch.
Upgrading from 1/2 inch to 3/4 inch will basically double the area of the supply to the home...

If only one person takes a shower at a time you may not notice the improvement. But with two fixtures going you will be impressed by the improvement.

I generally will not install less than 1 inch supply and many homes have enough bathrooms that 1-1/4 inch is required...

But the pipe size should be determined by how many fixtures you have...
 

Gary Swart

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You probably learned this in high school, but we all tend to forget what we don't use. To get an idea of the difference in pipe sizes, use the formula: Area = Pi times radius squared. You can use 3.14 for Pi, and radius is 1/2 the diameter. Do this math for 1/2", 3/4", and 1" pipe and you will quickly see there is a huge difference in the areas even when the diameters are only increased 1/4". This is what the fixture units are based on. Using Pex does compound the problem even more because the diameter of Pex is smaller than the diameter of pipe.
 

Jimbo

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You can never make the pressure and/or volume better than the supply will deliver. BUT using larger pipes will MINIMIZE any further losses. There is pressure loss caused by friction, and it is directly related to the diameter of the pipe, and the gallons per minute used.
 

Tom Sawyer

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.7854 x D squared is less math.

Yes, 1/2" PEX fittings cause a considerable reduction in flow.
 
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hj

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Even LESS math. When comparing sizes, all you have to do is square the radius of the pipes to see how the areas compare. In this case Pi is a constant which factors/cancels out. As for the original question, he has two things going against him. First the PEX tubing is already a lot smaller than copper tubing because of that same math that says a small reduction in diameter means a LARGE reduction in area, and then the "insert" fittings reduce the size even further. A 1/2" insert PEX fitting, especially if it is plastic, may only have a 1/4" diameter opening in it.
 
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Polecatt

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I appreciate the math that highlights the difference in pipe capacity but I'm also interested in how these combine and impact eachother. Jimbo above came the closest to answering the question in regard to the impact of smaller pipe on larger pipe. To use my specific example... I have 1/2" copper supply to my house and I have no intention of changing that. Perhaps it's more physics than math I am looking for a refresher on? Is there any reason to put new lines in that are larger than the smallest upstream size which is the 1/2" supply?

Water pipe sizing
 
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Jadnashua

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Smaller means more friction, which means less volume that can be delivered. Pex that uses the expansion fitting method is (slightly) bigger in ID, but 1/2" pex is closer to 3/8" copper pipe in the volume it can carry. Minimum restrictions may not be noticeable, depending on what you are doing...watering the lawn would be a big issue...filling a tub, and maybe (not all) washing machines. A typical shower probably wouldn't care UNLESS someone else turned water on as well. Larger supply pipes have less restrictions, provide more volume, and have frewer impacts when someone else turns water on.
 

Terry

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You can plumb two fixtures with 1/2" pipe.
You can plumb a toilet and a tub together.
If you have a sink, a washer or an outside hosebib, then you need to increase pipe size to 3/4"

The larger the pipe, the less friction loss.
Increasing pipe size reduces friction.

The longer the pipe, the more friction loss there is.

pipe_size_1.jpg



pipe_size_2.jpg



Water pipe sizing
 
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Polecatt

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Thanks for the redirect to the excellent water pipe size summary page Terry.

Looks like this is written for copper. Will pex change the supported fixture units table?

I also noticed that the Meter and street service are either 3/4" or 1" where the building supplies can be larger than that. I assume that aligns with the comments above that a larger pipe will cause less friction regardless of what other pipe sizes exist in the system.

My current setup with the 1/2" copper supply to the meter and 1/2" copper through the rest of the house seems fine. It is a small house and I have not had problems with pressure even when multiple fixtures are in use.

The reason I'm asking all the questions is to get a better understanding of what the impact of adding new fixtures in a basement reno might be and the pipe size I would use to remain efficient without decreasing performance.

I have come across a lot of conflicting info out there as to what will work...
 

Chad Schloss

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you are making a little thing into something big.. if you use pex, just upsize to 3/4".. if you use copper, use 1/2" for your setup. you say 1/2" copper worked with your setup, go with it, or 3/4" pex. either way will produce the same thing. you have 1/2" copper supply and are content with that, so leave it alone. if you want pex, upsize, because the I/D. is smaller. that's it. nothing to it. I am not a plumber, but can see this clear as mud :p

No, you are not a plumber, and your advice is wrong, wrong, wrong
 
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Polecatt

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Thanks for the straightforward view Chad.

This was how I was hoping the question would resolve but thought it would be a good idea to check in with the experts to get a few different points of view!

After looking at my meter more closely I tihnk it might be 5/8" but I suspect that doesn't really change anything.
 

Terry

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If you are looking for a dumb answer, then Chad has it for you.

If you want to ask a plumbing question, then ask a plumber.
So far, you have had a one bath home?
And then you want to add more? Whey wouldn't you change the pipe size to take in the added fixtures you will be supplying? One of the other posters suggested keeping a water bucket near a toilet to flush with.

Is this how you guys do remodel work?
Wow!

http://darwinawards.com/

A standard one-bath home with kitchen sink, dishwasher, water heater, clothes-washer, 1.6 tank toilet, lavatory, tub/shower combo and two hose bibs would be counted as 18 fixture units.

Most standard two bath homes consisting of kitchen sink, dishwasher, water heater, clothes-washer, two 1.6 tank toilet, two lavatories, one shower, one tub/shower combo, and two hose bibs would be counted as 23.5 fixture units.

You can have maybe 7 fixture units on 1/2"
19 fixture units on 3/4", so a two bath home needs 1" pipe from the meter.

Can you put your finger in a pencil sharpener?
Yes you can.
Is it a good idea?
 
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Polecatt

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Definitely not looking for a dumb answer! Would prefer a good answer and to learn something along the way... Which is exactly why I came here.

Is there an assumption that as the number of fixture units increases, the number of simultaneous uses will increase as well? To present an extreme example, if I had 10 toilets all connected along a 1/2" branch of pipe representing 25 fixture units would each one operate sufficiently when run independantly? If this is true, the branch pipe would need to be upgraded to 1" to allow all the toilets to flush simultaneously?
 

Terry

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As long as you only plan on using one or two things at a time, then the 1/2" would work.
But homes are plumbed assuming that people don't live alone. They assume that there will be someone in the kitchen making dinner, and perhaps you are watering the lawn outside. You need to wash clothes for work the next day, so that starts to load. Someone takes a shower and in the second bathroom, someone goes in a flushes a toilet.
Now you can always holler around the house and ask everyone to just knock it off, but in most homes, since they are plumbed correctly, there's no need to be yelling.
I did see a flipped home where the carpenter did the plumbing. It was a three bath home, all undersized. The homeowner was not happy, but it was too late to fix it. It takes the same amount of time to do it right the first time. It saves on the lungs too.
 

Polecatt

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I suppose this may partially explain why I haven't noticed any major performance issues with my existing 1/2" service. My usage patterns happen to not place high enough demand to require the additional flow volume. Basically I have been lucky... but this 1/2" is clearly not ideal or anywhere near sufficient for a heavier load.

I will take some time to map it all out and calculate the branch sizes based on the supported fixture units.

Thanks again to everyone for the great contributions.
 

Chad Schloss

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I was making a funny and I guess it came out wrong. No, don't listen to me, I was being a goof.
 
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