Supply Pipe Sizing FAQ Clarification

[jjohanson]

I had a few questions to clarify the water pipe sizing information that is on the FAQ: https://terrylove.com/watersize.htm

1)
The first item on chart at the bottom says that that for 6 fixture units, you should use a 1/2" supply in building, then says for 19 units you should use 3/4". It the number of units listed the max for that value or the min? IE - does this mean that between 6 and 19 units you should use 1/2" or 3/4"? Where is the line drawn?

2)
Is there are rule of thumb for conversion from copper to pex sizing? For example, I believe 1/2" L-Type copper is 0.545", where 1/2" Uponor Pex is 0.475". I know pex has less friction, though. So for example, if I had 12 fixture units, should I use the size given for Pex, or should I move up one size.

3)
Is there a problem oversizing supply pipe? Obviously, if pipe is undersized, you can have too little flow. Is there an issue with oversizing, IE - too little flow?

Thanks preemptively for the help!

 

[hj]

chart

1. The chart is a maximum fixtures. Once you go over 6, which happens very quickly, you have to go to 3/4". If you exceed the 3/4 fixture units, then you need 1".

2. PEX is not that much smoother than copper so I would upsize it. There is no downside to oversizing water lines.

 

[Sponsor]

 

[Basement_Lurker]

1. The line sizes listed are for a maximum # of FUs
2. The general rule is that you should upsize the run of pex, but it really depends on a number of factors. In reality, if all you are doing is replacing or adding on to a section of copper tubing, then 1/2" pex is fine and you won't notice any difference in my opinion. You are right about pex having less fricitonal loss, but that really only becomes a factor over an entire system...such as if you went with a home run system and didn't use any fittings at all except for the connection to the manifold and the wall stubout.
3. Oversizing your piping is better than undersizing it. Oversizing the piping will result in increased volume of water (do not read that as higher water pressure)

 

[Jadnashua]

A downside of large supply lines is on the hot side. After a long period of disuse, you'll have more cold water to flush out of the line to get hot to the fixture. Good insulation and possibly recirculation can shorten that time to nearly zero.

 

[jjohanson]

Thank you for the clarifications!
My questions were answered to a tee!

 

[Terry]

A downside of large supply lines is on the hot side.

If the hot side is downsized, then there will be temperature fluctuation when other fixtures are used in the home.
I prefer to size the hot and the cold evenly.
There are more cold fixtures, and so the cold branch will start off bigger and wind up bigger.
For a bathroom set, I typically run 3/4" for the cold and 1/2" for the hot.
If it's two bathrooms back to back, it's 3/4" for the cold and the hot before it branches off for the individuals.

It kind of sucks if someone washes their hands in the next bathroom and the water in the shower turns cold.

I like recirc lines if you have a large run, you will have more usable hot water and less will go down the drain while you are waiting.